Add new filmstrip mode for PhotoView.

Change-Id: I9da9896303ced8d63a3557d5e6e9bc06fb366cf5

1 files changed, 1137 insertions, 485 deletions

diff --git a/src/com/android/gallery3d/ui/PositionController.java b/src/com/android/gallery3d/ui/PositionController.java
index 625505f49..d190e04ec 100644
--- a/src/com/android/gallery3d/ui/PositionController.java
+++ b/src/com/android/gallery3d/ui/PositionController.java
@@ -18,352 +18,415 @@ package com.android.gallery3d.ui;
 
 import android.content.Context;
 import android.graphics.Rect;
-import android.graphics.RectF;
-import android.util.FloatMath;
+import android.util.Log;
+import android.widget.Scroller;
 
 import com.android.gallery3d.common.Utils;
-import com.android.gallery3d.data.MediaItem;
 import com.android.gallery3d.util.GalleryUtils;
+import com.android.gallery3d.util.RangeArray;
+import com.android.gallery3d.util.RangeIntArray;
 
 class PositionController {
+    private static final String TAG = "PositionController";
+
     public static final int IMAGE_AT_LEFT_EDGE = 1;
     public static final int IMAGE_AT_RIGHT_EDGE = 2;
     public static final int IMAGE_AT_TOP_EDGE = 4;
     public static final int IMAGE_AT_BOTTOM_EDGE = 8;
 
-    private long mAnimationStartTime = NO_ANIMATION;
+    // Special values for animation time.
     private static final long NO_ANIMATION = -1;
     private static final long LAST_ANIMATION = -2;
 
-    private int mAnimationKind;
-    private float mAnimationDuration;
-    private final static int ANIM_KIND_SCROLL = 0;
-    private final static int ANIM_KIND_SCALE = 1;
-    private final static int ANIM_KIND_SNAPBACK = 2;
-    private final static int ANIM_KIND_SLIDE = 3;
-    private final static int ANIM_KIND_ZOOM = 4;
-    private final static int ANIM_KIND_FLING = 5;
+    private static final int ANIM_KIND_SCROLL = 0;
+    private static final int ANIM_KIND_SCALE = 1;
+    private static final int ANIM_KIND_SNAPBACK = 2;
+    private static final int ANIM_KIND_SLIDE = 3;
+    private static final int ANIM_KIND_ZOOM = 4;
+    private static final int ANIM_KIND_OPENING = 5;
+    private static final int ANIM_KIND_FLING = 6;
 
     // Animation time in milliseconds. The order must match ANIM_KIND_* above.
-    private final static int ANIM_TIME[] = {
+    private static final int ANIM_TIME[] = {
         0,    // ANIM_KIND_SCROLL
         50,   // ANIM_KIND_SCALE
         600,  // ANIM_KIND_SNAPBACK
         400,  // ANIM_KIND_SLIDE
         300,  // ANIM_KIND_ZOOM
+        600,  // ANIM_KIND_OPENING
         0,    // ANIM_KIND_FLING (the duration is calculated dynamically)
     };
 
     // We try to scale up the image to fill the screen. But in order not to
     // scale too much for small icons, we limit the max up-scaling factor here.
     private static final float SCALE_LIMIT = 4;
-    private static final int sHorizontalSlack = GalleryUtils.dpToPixel(12);
 
-    private static final float SCALE_MIN_EXTRA = 0.6f;
+    // For user's gestures, we give a temporary extra scaling range which goes
+    // above or below the usual scaling limits.
+    private static final float SCALE_MIN_EXTRA = 0.7f;
     private static final float SCALE_MAX_EXTRA = 1.4f;
 
-    private PhotoView mViewer;
-    private EdgeView mEdgeView;
-    private int mImageW, mImageH;
-    private int mViewW, mViewH;
-
-    // The X, Y are the coordinate on bitmap which shows on the center of
-    // the view. We always keep the mCurrent{X,Y,Scale} sync with the actual
-    // values used currently.
-    private int mCurrentX, mFromX, mToX;
-    private int mCurrentY, mFromY, mToY;
-    private float mCurrentScale, mFromScale, mToScale;
-
-    // The focus point of the scaling gesture (in bitmap coordinates).
-    private int mFocusBitmapX;
-    private int mFocusBitmapY;
-    private boolean mInScale;
-
-    // The minimum and maximum scale we allow.
-    private float mScaleMin, mScaleMax = SCALE_LIMIT;
+    // Setting this true makes the extra scaling range permanent (until this is
+    // set to false again).
     private boolean mExtraScalingRange = false;
 
-    // This is used by the fling animation
-    private FlingScroller mScroller;
+    // Film Mode v.s. Page Mode: in film mode we show smaller pictures.
+    private boolean mFilmMode = false;
+    private static final float FILM_MODE_SCALE_FACTOR = 0.7f;
 
-    // The bound of the stable region, see the comments above
-    // calculateStableBound() for details.
-    private int mBoundLeft, mBoundRight, mBoundTop, mBoundBottom;
+    // The scaling factor in current mode.
+    private float mScaleFactor = mFilmMode ? FILM_MODE_SCALE_FACTOR : 1.0f;
 
-    // Assume the image size is the same as view size before we know the actual
-    // size of image.
-    private boolean mUseViewSize = true;
+    // In addition to the focused box (index == 0). We also keep information
+    // about this many boxes on each side.
+    private static final int BOX_MAX = PhotoView.SCREEN_NAIL_MAX;
 
-    private RectF mTempRect = new RectF();
-    private float[] mTempPoints = new float[8];
+    public static final int IMAGE_GAP = 96;
+    private static final int HORIZONTAL_SLACK = GalleryUtils.dpToPixel(12);
 
-    public PositionController(PhotoView viewer, Context context,
-            EdgeView edgeView) {
-        mViewer = viewer;
-        mEdgeView = edgeView;
-        mScroller = new FlingScroller();
-    }
+    private Listener mListener;
+    private volatile Rect mOpenAnimationRect;
+    private int mViewW = 640;
+    private int mViewH = 480;;
 
-    public void setImageSize(int width, int height) {
+    // A scaling guesture is in progress.
+    private boolean mInScale;
+    // The focus point of the scaling gesture, relative to the center of the
+    // picture in bitmap pixels.
+    private float mFocusX, mFocusY;
 
-        // If no image available, use view size.
-        if (width == 0 || height == 0) {
-            mUseViewSize = true;
-            mImageW = mViewW;
-            mImageH = mViewH;
-            mCurrentX = mImageW / 2;
-            mCurrentY = mImageH / 2;
-            mCurrentScale = 1;
-            mScaleMin = 1;
-            mViewer.setPosition(mCurrentX, mCurrentY, mCurrentScale);
-            return;
-        }
+    // This is used by the fling animation (page mode).
+    private FlingScroller mPageScroller;
 
-        mUseViewSize = false;
+    // This is used by the fling animation (film mode).
+    private Scroller mFilmScroller;
 
-        float ratio = Math.min(
-                (float) mImageW / width, (float) mImageH / height);
+    // The bound of the stable region that the focused box can stay, see the
+    // comments above calculateStableBound() for details.
+    private int mBoundLeft, mBoundRight, mBoundTop, mBoundBottom;
 
-        // See the comment above translate() for details.
-        mCurrentX = translate(mCurrentX, mImageW, width, ratio);
-        mCurrentY = translate(mCurrentY, mImageH, height, ratio);
-        mCurrentScale = mCurrentScale * ratio;
+    //
+    //  ___________________________________________________________
+    // |   _____       _____       _____       _____       _____   |
+    // |  |     |     |     |     |     |     |     |     |     |  |
+    // |  | Box |     | Box |     | Box*|     | Box |     | Box |  |
+    // |  |_____|.....|_____|.....|_____|.....|_____|.....|_____|  |
+    // |          Gap         Gap         Gap         Gap          |
+    // |___________________________________________________________|
+    //
+    //                       <--  Platform  -->
+    //
+    // The focused box (Box*) centers at mPlatform.mCurrentX
 
-        mFromX = translate(mFromX, mImageW, width, ratio);
-        mFromY = translate(mFromY, mImageH, height, ratio);
-        mFromScale = mFromScale * ratio;
+    private Platform mPlatform = new Platform();
+    private RangeArray<Box> mBoxes = new RangeArray<Box>(-BOX_MAX, BOX_MAX);
+    // The gap at the right of a Box i is at index i. The gap at the left of a
+    // Box i is at index i - 1.
+    private RangeArray<Gap> mGaps = new RangeArray<Gap>(-BOX_MAX, BOX_MAX - 1);
 
-        mToX = translate(mToX, mImageW, width, ratio);
-        mToY = translate(mToY, mImageH, height, ratio);
-        mToScale = mToScale * ratio;
+    // These are only used during moveBox().
+    private RangeArray<Box> mTempBoxes = new RangeArray<Box>(-BOX_MAX, BOX_MAX);
+    private RangeArray<Gap> mTempGaps = new RangeArray<Gap>(-BOX_MAX, BOX_MAX - 1);
 
-        mFocusBitmapX = translate(mFocusBitmapX, mImageW, width, ratio);
-        mFocusBitmapY = translate(mFocusBitmapY, mImageH, height, ratio);
+    // The output of the PositionController. Available throught getPosition().
+    private RangeArray<Rect> mRects = new RangeArray<Rect>(-BOX_MAX, BOX_MAX);
 
-        mImageW = width;
-        mImageH = height;
+    public interface Listener {
+        void invalidate();
+        boolean isDown();
 
-        mScaleMin = getMinimalScale(mImageW, mImageH);
+        // EdgeView
+        void onPull(int offset, int direction);
+        void onRelease();
+        void onAbsorb(int velocity, int direction);
+    }
 
-        // Start animation from the saved rectangle if we have one.
-        Rect r = mViewer.retrieveOpenAnimationRect();
-        if (r != null) {
-            // The animation starts from the specified rectangle; the image
-            // should be scaled and centered as the thumbnail shown in the
-            // rectangle to minimize janky opening animation. Note: The below
-            // implementation depends on how thumbnails are drawn and placed.
-            float size = MediaItem.getTargetSize(
-                    MediaItem.TYPE_MICROTHUMBNAIL);
-            float scale = (size / Math.min(width, height)) * Math.min(
-                    r.width() / size, r.height() / size);
-
-            mCurrentX = Math.round((mViewW / 2f - r.centerX()) / scale) + mImageW / 2;
-            mCurrentY = Math.round((mViewH / 2f - r.centerY()) / scale) + mImageH / 2;
-            mCurrentScale = scale;
-            mViewer.openAnimationStarted();
-            startSnapback();
-        } else if (mAnimationStartTime == NO_ANIMATION) {
-            mCurrentScale = Utils.clamp(mCurrentScale, mScaleMin, mScaleMax);
-        }
-        mViewer.setPosition(mCurrentX, mCurrentY, mCurrentScale);
+    public PositionController(Context context, Listener listener) {
+        mListener = listener;
+        mPageScroller = new FlingScroller();
+        mFilmScroller = new Scroller(context);
+
+        // Initialize the areas.
+        initPlatform();
+        for (int i = -BOX_MAX; i <= BOX_MAX; i++) {
+            mBoxes.put(i, new Box());
+            initBox(i);
+            mRects.put(i, new Rect());
+        }
+        for (int i = -BOX_MAX; i < BOX_MAX; i++) {
+            mGaps.put(i, new Gap());
+            initGap(i);
+        }
     }
 
-    public void zoomIn(float tapX, float tapY, float targetScale) {
-        if (targetScale > mScaleMax) targetScale = mScaleMax;
+    public void setOpenAnimationRect(Rect r) {
+        mOpenAnimationRect = r;
+    }
 
-        // Convert the tap position to image coordinate
-        int tempX = Math.round((tapX - mViewW / 2) / mCurrentScale + mCurrentX);
-        int tempY = Math.round((tapY - mViewH / 2) / mCurrentScale + mCurrentY);
+    public void setViewSize(int viewW, int viewH) {
+        if (viewW == mViewW && viewH == mViewH) return;
 
-        calculateStableBound(targetScale);
-        int targetX = Utils.clamp(tempX, mBoundLeft, mBoundRight);
-        int targetY = Utils.clamp(tempY, mBoundTop, mBoundBottom);
+        mViewW = viewW;
+        mViewH = viewH;
+        initPlatform();
 
-        startAnimation(targetX, targetY, targetScale, ANIM_KIND_ZOOM);
-    }
+        for (int i = -BOX_MAX; i <= BOX_MAX; i++) {
+            setBoxSize(i, viewW, viewH, true);
+        }
 
-    public void resetToFullView() {
-        startAnimation(mImageW / 2, mImageH / 2, mScaleMin, ANIM_KIND_ZOOM);
+        updateScaleAndGapLimit();
+        snapAndRedraw();
     }
 
-    public float getMinimalScale(int w, int h) {
-        return Math.min(SCALE_LIMIT,
-                Math.min((float) mViewW / w, (float) mViewH / h));
-    }
+    public void setImageSize(int index, int width, int height) {
+        if (width == 0 || height == 0) {
+            initBox(index);
+        } else {
+            setBoxSize(index, width, height, false);
+        }
 
-    // Translate a coordinate on bitmap if the bitmap size changes.
-    // If the aspect ratio doesn't change, it's easy:
-    //
-    //         r  = w / w' (= h / h')
-    //         x' = x / r
-    //         y' = y / r
-    //
-    // However the aspect ratio may change. That happens when the user slides
-    // a image before it's loaded, we don't know the actual aspect ratio, so
-    // we will assume one. When we receive the actual bitmap size, we need to
-    // translate the coordinate from the old bitmap into the new bitmap.
-    //
-    // What we want to do is center the bitmap at the original position.
-    //
-    //         ...+--+...
-    //         .  |  |  .
-    //         .  |  |  .
-    //         ...+--+...
-    //
-    // First we scale down the new bitmap by a factor r = min(w/w', h/h').
-    // Overlay it onto the original bitmap. Now (0, 0) of the old bitmap maps
-    // to (-(w-w'*r)/2 / r, -(h-h'*r)/2 / r) in the new bitmap. So (x, y) of
-    // the old bitmap maps to (x', y') in the new bitmap, where
-    //         x' = (x-(w-w'*r)/2) / r = w'/2 + (x-w/2)/r
-    //         y' = (y-(h-h'*r)/2) / r = h'/2 + (y-h/2)/r
-    private static int translate(int value, int size, int newSize, float ratio) {
-        return Math.round(newSize / 2f + (value - size / 2f) / ratio);
+        updateScaleAndGapLimit();
+        startOpeningAnimationIfNeeded();
+        snapAndRedraw();
     }
 
-    public void setViewSize(int viewW, int viewH) {
-        boolean needLayout = mViewW == 0 || mViewH == 0;
+    private void setBoxSize(int i, int width, int height, boolean isViewSize) {
+        Box b = mBoxes.get(i);
 
-        mViewW = viewW;
-        mViewH = viewH;
+        // If we already have image size, we don't want to use the view size.
+        if (isViewSize && !b.mUseViewSize) return;
+        b.mUseViewSize = isViewSize;
 
-        if (mUseViewSize) {
-            mImageW = viewW;
-            mImageH = viewH;
-            mCurrentX = mImageW / 2;
-            mCurrentY = mImageH / 2;
-            mCurrentScale = 1;
-            mScaleMin = 1;
-            mViewer.setPosition(mCurrentX, mCurrentY, mCurrentScale);
+        if (width == b.mImageW && height == b.mImageH) {
             return;
         }
 
-        // In most cases we want to keep the scaling factor intact when the
-        // view size changes. The cases we want to reset the scaling factor
-        // (to fit the view if possible) are (1) the scaling factor is too
-        // small for the new view size (2) the scaling factor has not been
-        // changed by the user.
-        boolean wasMinScale = (mCurrentScale == mScaleMin);
-        mScaleMin = getMinimalScale(mImageW, mImageH);
+        // The ratio of the old size and the new size.
+        float ratio = Math.min(
+                (float) b.mImageW / width, (float) b.mImageH / height);
+
+        b.mCurrentScale *= ratio;
+        b.mFromScale *= ratio;
+        b.mToScale *= ratio;
 
-        if (needLayout || mCurrentScale < mScaleMin || wasMinScale) {
-            mCurrentX = mImageW / 2;
-            mCurrentY = mImageH / 2;
-            mCurrentScale = mScaleMin;
-            mViewer.setPosition(mCurrentX, mCurrentY, mCurrentScale);
+        b.mImageW = width;
+        b.mImageH = height;
+
+        if (i == 0) {
+            mFocusX /= ratio;
+            mFocusY /= ratio;
         }
     }
 
-    public void stopAnimation() {
-        mAnimationStartTime = NO_ANIMATION;
+    private void startOpeningAnimationIfNeeded() {
+        if (mOpenAnimationRect == null) return;
+        Box b = mBoxes.get(0);
+        if (b.mUseViewSize) return;
+
+        // Start animation from the saved rectangle if we have one.
+        Rect r = mOpenAnimationRect;
+        mOpenAnimationRect = null;
+        mPlatform.mCurrentX = r.centerX();
+        b.mCurrentY = r.centerY();
+        b.mCurrentScale = Math.max(r.width() / (float) b.mImageW,
+                r.height() / (float) b.mImageH);
+        startAnimation(mViewW / 2, mViewH / 2, b.mScaleMin, ANIM_KIND_OPENING);
     }
 
-    public void skipAnimation() {
-        if (mAnimationStartTime == NO_ANIMATION) return;
-        mAnimationStartTime = NO_ANIMATION;
-        mCurrentX = mToX;
-        mCurrentY = mToY;
-        mCurrentScale = mToScale;
+    public void setFilmMode(boolean enabled) {
+        if (enabled == mFilmMode) return;
+        mFilmMode = enabled;
+        mScaleFactor = enabled ? FILM_MODE_SCALE_FACTOR : 1.0f;
+
+        updateScaleAndGapLimit();
+        stopAnimation();
+        snapAndRedraw();
     }
 
-    public void beginScale(float focusX, float focusY) {
-        mInScale = true;
-        mFocusBitmapX = Math.round(mCurrentX +
-                (focusX - mViewW / 2f) / mCurrentScale);
-        mFocusBitmapY = Math.round(mCurrentY +
-                (focusY - mViewH / 2f) / mCurrentScale);
+    public void setExtraScalingRange(boolean enabled) {
+        if (mExtraScalingRange == enabled) return;
+        mExtraScalingRange = enabled;
+        if (!enabled) {
+            snapAndRedraw();
+        }
     }
 
-    // Returns true if the result scale is outside the stable range.
-    public boolean scaleBy(float s, float focusX, float focusY) {
+    // This should be called whenever the scale range of boxes or the default
+    // gap size may change. Currently this can happen due to change of view
+    // size, image size, and mode.
+    private void updateScaleAndGapLimit() {
+        for (int i = -BOX_MAX; i <= BOX_MAX; i++) {
+            Box b = mBoxes.get(i);
+            b.mScaleMin = getMinimalScale(b.mImageW, b.mImageH);
+            b.mScaleMax = getMaximalScale(b.mImageW, b.mImageH);
+        }
 
-        // We want to keep the focus point (on the bitmap) the same as when
-        // we begin the scale guesture, that is,
-        //
-        // mCurrentX' + (focusX - mViewW / 2f) / scale = mFocusBitmapX
-        //
-        s *= getTargetScale();
-        int x = Math.round(mFocusBitmapX - (focusX - mViewW / 2f) / s);
-        int y = Math.round(mFocusBitmapY - (focusY - mViewH / 2f) / s);
+        for (int i = -BOX_MAX; i < BOX_MAX; i++) {
+            Gap g = mGaps.get(i);
+            g.mDefaultSize = getDefaultGapSize(i);
+        }
+    }
 
-        startAnimation(x, y, s, ANIM_KIND_SCALE);
-        return (s < mScaleMin || s > mScaleMax);
+    // Returns the default gap size according the the size of the boxes around
+    // the gap and the current mode.
+    private int getDefaultGapSize(int i) {
+        if (mFilmMode) return IMAGE_GAP;
+        Box a = mBoxes.get(i);
+        Box b = mBoxes.get(i + 1);
+        return IMAGE_GAP + Math.max(gapToSide(a), gapToSide(b));
     }
 
-    public void endScale() {
-        mInScale = false;
-        startSnapbackIfNeeded();
+    // Here is how we layout the boxes in the page mode.
+    //
+    //   previous             current             next
+    //  ___________       ________________     __________
+    // |  _______  |     |   __________   |   |  ______  |
+    // | |       | |     |  |   right->|  |   | |      | |
+    // | |       |<-------->|<--left   |  |   | |      | |
+    // | |_______| |  |  |  |__________|  |   | |______| |
+    // |___________|  |  |________________|   |__________|
+    //                |  <--> gapToSide()
+    //                |
+    // IMAGE_GAP + MAX(gapToSide(previous), gapToSide(current))
+    private int gapToSide(Box b) {
+        return (int) ((mViewW - getMinimalScale(b) * b.mImageW) / 2 + 0.5f);
     }
 
-    public void setExtraScalingRange(boolean enabled) {
-        mExtraScalingRange = enabled;
-        if (!enabled) {
-            startSnapbackIfNeeded();
+    // Stop all animations at where they are now.
+    public void stopAnimation() {
+        mPlatform.mAnimationStartTime = NO_ANIMATION;
+        for (int i = -BOX_MAX; i <= BOX_MAX; i++) {
+            mBoxes.get(i).mAnimationStartTime = NO_ANIMATION;
+        }
+        for (int i = -BOX_MAX; i < BOX_MAX; i++) {
+            mGaps.get(i).mAnimationStartTime = NO_ANIMATION;
         }
     }
 
-    public float getCurrentScale() {
-        return mCurrentScale;
+    public void skipAnimation() {
+        if (mPlatform.mAnimationStartTime != NO_ANIMATION) {
+            mPlatform.mCurrentX = mPlatform.mToX;
+            mPlatform.mAnimationStartTime = NO_ANIMATION;
+        }
+        for (int i = -BOX_MAX; i <= BOX_MAX; i++) {
+            Box b = mBoxes.get(i);
+            if (b.mAnimationStartTime == NO_ANIMATION) continue;
+            b.mCurrentY = b.mToY;
+            b.mCurrentScale = b.mToScale;
+            b.mAnimationStartTime = NO_ANIMATION;
+        }
+        for (int i = -BOX_MAX; i < BOX_MAX; i++) {
+            Gap g = mGaps.get(i);
+            if (g.mAnimationStartTime == NO_ANIMATION) continue;
+            g.mCurrentGap = g.mToGap;
+            g.mAnimationStartTime = NO_ANIMATION;
+        }
+        redraw();
     }
 
-    public boolean isAtMinimalScale() {
-        return isAlmostEquals(mCurrentScale, mScaleMin);
+    public void up() {
+        snapAndRedraw();
     }
 
-    private static boolean isAlmostEquals(float a, float b) {
-        float diff = a - b;
-        return (diff < 0 ? -diff : diff) < 0.02f;
+    ////////////////////////////////////////////////////////////////////////////
+    //  Start an animations for the focused box
+    ////////////////////////////////////////////////////////////////////////////
+
+    public void zoomIn(float tapX, float tapY, float targetScale) {
+        Box b = mBoxes.get(0);
+
+        // Convert the tap position to distance to center in bitmap coordinates
+        float tempX = (tapX - mPlatform.mCurrentX) / b.mCurrentScale;
+        float tempY = (tapY - b.mCurrentY) / b.mCurrentScale;
+
+        int x = (int) (mViewW / 2 - tempX * targetScale + 0.5f);
+        int y = (int) (mViewH / 2 - tempY * targetScale + 0.5f);
+
+        calculateStableBound(targetScale);
+        int targetX = Utils.clamp(x, mBoundLeft, mBoundRight);
+        int targetY = Utils.clamp(y, mBoundTop, mBoundBottom);
+        targetScale = Utils.clamp(targetScale, b.mScaleMin, b.mScaleMax);
+
+        startAnimation(targetX, targetY, targetScale, ANIM_KIND_ZOOM);
     }
 
-    public void up() {
-        startSnapback();
-    }
-
-    //             |<--| (1/2) * mImageW
-    // +-------+-------+-------+
-    // |       |       |       |
-    // |       |   o   |       |
-    // |       |       |       |
-    // +-------+-------+-------+
-    // |<----------| (3/2) * mImageW
-    // Slide in the image from left or right.
-    // Precondition: mCurrentScale = 1 (mView{W|H} == mImage{W|H}).
-    // Sliding from left:  mCurrentX = (1/2) * mImageW
-    //              right: mCurrentX = (3/2) * mImageW
-    public void startSlideInAnimation(int direction) {
-        int fromX = (direction == PhotoView.TRANS_SLIDE_IN_LEFT) ?
-                mImageW / 2 : 3 * mImageW / 2;
-        mFromX = Math.round(fromX);
-        mFromY = Math.round(mImageH / 2f);
-        mCurrentX = mFromX;
-        mCurrentY = mFromY;
-        startAnimation(
-                mImageW / 2, mImageH / 2, mCurrentScale, ANIM_KIND_SLIDE);
+    public void resetToFullView() {
+        Box b = mBoxes.get(0);
+        startAnimation(mViewW / 2, mViewH / 2, b.mScaleMin, ANIM_KIND_ZOOM);
+    }
+
+    public void beginScale(float focusX, float focusY) {
+        Box b = mBoxes.get(0);
+        Platform p = mPlatform;
+        mInScale = true;
+        mFocusX = (int) ((focusX - p.mCurrentX) / b.mCurrentScale + 0.5f);
+        mFocusY = (int) ((focusY - b.mCurrentY) / b.mCurrentScale + 0.5f);
+    }
+
+    // Scales the image by the given factor.
+    // Returns an out-of-range indicator:
+    //   1 if the intended scale is too large for the stable range.
+    //   0 if the intended scale is in the stable range.
+    //  -1 if the intended scale is too small for the stable range.
+    public int scaleBy(float s, float focusX, float focusY) {
+        Box b = mBoxes.get(0);
+        Platform p = mPlatform;
+
+        // We want to keep the focus point (on the bitmap) the same as when we
+        // begin the scale guesture, that is,
+        //
+        // (focusX' - currentX') / scale' = (focusX - currentX) / scale
+        //
+        s *= getTargetScale(b);
+        int x = mFilmMode ? p.mCurrentX : (int) (focusX - s * mFocusX + 0.5f);
+        int y = mFilmMode ? b.mCurrentY : (int) (focusY - s * mFocusY + 0.5f);
+        startAnimation(x, y, s, ANIM_KIND_SCALE);
+        if (s < b.mScaleMin) return -1;
+        if (s > b.mScaleMax) return 1;
+        return 0;
+    }
+
+    public void endScale() {
+        mInScale = false;
+        snapAndRedraw();
     }
 
     public void startHorizontalSlide(int distance) {
-        scrollBy(distance, 0, ANIM_KIND_SLIDE);
+        Box b = mBoxes.get(0);
+        Platform p = mPlatform;
+        startAnimation(getTargetX(p) + distance, getTargetY(b),
+                b.mCurrentScale, ANIM_KIND_SLIDE);
     }
 
-    private void scrollBy(float dx, float dy, int type) {
-        startAnimation(getTargetX() + Math.round(dx / mCurrentScale),
-                getTargetY() + Math.round(dy / mCurrentScale),
-                mCurrentScale, type);
+    public void startScroll(float dx, float dy) {
+        boolean hasPrev = hasPrevImages();
+        boolean hasNext = hasNextImages();
+
+        Box b = mBoxes.get(0);
+        Platform p = mPlatform;
+
+        int x = getTargetX(p) + (int) (dx + 0.5f);
+        int y = getTargetY(b) + (int) (dy + 0.5f);
+
+        if (mFilmMode) {
+            scrollToFilm(x, y, hasPrev, hasNext);
+        } else {
+            scrollToPage(x, y, hasPrev, hasNext);
+        }
     }
 
-    public void startScroll(float dx, float dy, boolean hasNext,
-            boolean hasPrev) {
-        int x = getTargetX() + Math.round(dx / mCurrentScale);
-        int y = getTargetY() + Math.round(dy / mCurrentScale);
+    private void scrollToPage(int x, int y, boolean hasPrev, boolean hasNext) {
+        Box b = mBoxes.get(0);
 
-        calculateStableBound(mCurrentScale);
+        calculateStableBound(b.mCurrentScale);
 
         // Vertical direction: If we have space to move in the vertical
         // direction, we show the edge effect when scrolling reaches the edge.
         if (mBoundTop != mBoundBottom) {
             if (y < mBoundTop) {
-                mEdgeView.onPull(mBoundTop - y, EdgeView.TOP);
+                mListener.onPull(mBoundTop - y, EdgeView.BOTTOM);
             } else if (y > mBoundBottom) {
-                mEdgeView.onPull(y - mBoundBottom, EdgeView.BOTTOM);
+                mListener.onPull(y - mBoundBottom, EdgeView.TOP);
             }
         }
 
@@ -371,23 +434,51 @@ class PositionController {
 
         // Horizontal direction: we show the edge effect when the scrolling
         // tries to go left of the first image or go right of the last image.
-        if (!hasPrev && x < mBoundLeft) {
-            int pixels = Math.round((mBoundLeft - x) * mCurrentScale);
-            mEdgeView.onPull(pixels, EdgeView.LEFT);
-            x = mBoundLeft;
-        } else if (!hasNext && x > mBoundRight) {
-            int pixels = Math.round((x - mBoundRight) * mCurrentScale);
-            mEdgeView.onPull(pixels, EdgeView.RIGHT);
+        if (!hasPrev && x > mBoundRight) {
+            int pixels = x - mBoundRight;
+            mListener.onPull(pixels, EdgeView.LEFT);
             x = mBoundRight;
+        } else if (!hasNext && x < mBoundLeft) {
+            int pixels = mBoundLeft - x;
+            mListener.onPull(pixels, EdgeView.RIGHT);
+            x = mBoundLeft;
+        }
+
+        startAnimation(x, y, b.mCurrentScale, ANIM_KIND_SCROLL);
+    }
+
+    private void scrollToFilm(int x, int y, boolean hasPrev, boolean hasNext) {
+        Box b = mBoxes.get(0);
+
+        // Horizontal direction: we show the edge effect when the scrolling
+        // tries to go left of the first image or go right of the last image.
+        int cx = mViewW / 2;
+        if (!hasPrev && x > cx) {
+            int pixels = x - cx;
+            mListener.onPull(pixels, EdgeView.LEFT);
+            x = cx;
+        } else if (!hasNext && x < cx) {
+            int pixels = cx - x;
+            mListener.onPull(pixels, EdgeView.RIGHT);
+            x = cx;
         }
 
-        startAnimation(x, y, mCurrentScale, ANIM_KIND_SCROLL);
+        startAnimation(x, y, b.mCurrentScale, ANIM_KIND_SCROLL);
     }
 
     public boolean fling(float velocityX, float velocityY) {
+        int vx = (int) (velocityX + 0.5f);
+        int vy = (int) (velocityY + 0.5f);
+        return mFilmMode ? flingFilm(vx, vy) : flingPage(vx, vy);
+    }
+
+    private boolean flingPage(int velocityX, int velocityY) {
+        Box b = mBoxes.get(0);
+        Platform p = mPlatform;
+
         // We only want to do fling when the picture is zoomed-in.
-        if (viewWiderThanScaledImage(mCurrentScale) &&
-            viewHigherThanScaledImage(mCurrentScale)) {
+        if (viewWiderThanScaledImage(b.mCurrentScale) &&
+            viewTallerThanScaledImage(b.mCurrentScale)) {
             return false;
         }
 
@@ -402,202 +493,443 @@ class PositionController {
             (velocityY < 0 && (edges & IMAGE_AT_BOTTOM_EDGE) != 0)) {
             velocityY = 0;
         }
-        if (isAlmostEquals(velocityX, 0) && isAlmostEquals(velocityY, 0)) {
-            return false;
-        }
 
-        mScroller.fling(mCurrentX, mCurrentY,
-                Math.round(-velocityX / mCurrentScale),
-                Math.round(-velocityY / mCurrentScale),
+        if (velocityX == 0 && velocityY == 0) return false;
+
+        mPageScroller.fling(p.mCurrentX, b.mCurrentY, velocityX, velocityY,
                 mBoundLeft, mBoundRight, mBoundTop, mBoundBottom);
-        int targetX = mScroller.getFinalX();
-        int targetY = mScroller.getFinalY();
-        mAnimationDuration = mScroller.getDuration();
-        startAnimation(targetX, targetY, mCurrentScale, ANIM_KIND_FLING);
+        int targetX = mPageScroller.getFinalX();
+        int targetY = mPageScroller.getFinalY();
+        ANIM_TIME[ANIM_KIND_FLING] = mPageScroller.getDuration();
+        startAnimation(targetX, targetY, b.mCurrentScale, ANIM_KIND_FLING);
         return true;
     }
 
-    private void startAnimation(
-            int targetX, int targetY, float scale, int kind) {
-        mAnimationKind = kind;
-        if (targetX == mCurrentX && targetY == mCurrentY
-                && scale == mCurrentScale) {
-            onAnimationComplete();
-            return;
+    private boolean flingFilm(int velocityX, int velocityY) {
+        boolean hasPrev = hasPrevImages();
+        boolean hasNext = hasNextImages();
+
+        Box b = mBoxes.get(0);
+        Platform p = mPlatform;
+
+        // If we are already at the edge, don't start the fling.
+        int cx = mViewW / 2;
+        if ((!hasPrev && p.mCurrentX >= cx) || (!hasNext && p.mCurrentX <= cx)) {
+            return false;
         }
 
-        mFromX = mCurrentX;
-        mFromY = mCurrentY;
-        mFromScale = mCurrentScale;
+        if (velocityX == 0) return false;
 
-        mToX = targetX;
-        mToY = targetY;
-        mToScale = Utils.clamp(scale, SCALE_MIN_EXTRA * mScaleMin,
-                SCALE_MAX_EXTRA * mScaleMax);
+        mFilmScroller.fling(p.mCurrentX, 0, velocityX, 0,
+                Integer.MIN_VALUE, Integer.MAX_VALUE, 0, 0);
+        int targetX = mFilmScroller.getFinalX();
+        ANIM_TIME[ANIM_KIND_FLING] = mFilmScroller.getDuration();
+        startAnimation(targetX, b.mCurrentY, b.mCurrentScale, ANIM_KIND_FLING);
+        return true;
+    }
 
-        // If the scaled height is smaller than the view height,
-        // force it to be in the center.
-        // (We do for height only, not width, because the user may
-        // want to scroll to the previous/next image.)
-        if (!mInScale && viewHigherThanScaledImage(mToScale)) {
-            mToY = mImageH / 2;
+    ////////////////////////////////////////////////////////////////////////////
+    //  Redraw
+    //
+    //  If a method changes box positions directly, redraw()
+    //  should be called.
+    //
+    //  If a method may also cause a snapback to happen, snapAndRedraw() should
+    //  be called.
+    //
+    //  If a method starts an animation to change the position of focused box,
+    //  startAnimation() should be called.
+    //
+    //  If time advances to change the box position, advanceAnimation() should
+    //  be called.
+    ////////////////////////////////////////////////////////////////////////////
+    private void redraw() {
+        layoutAndSetPosition();
+        mListener.invalidate();
+    }
+
+    private void snapAndRedraw() {
+        mPlatform.startSnapback();
+        for (int i = -BOX_MAX; i <= BOX_MAX; i++) {
+            mBoxes.get(i).startSnapback();
+        }
+        for (int i = -BOX_MAX; i < BOX_MAX; i++) {
+            mGaps.get(i).startSnapback();
         }
+        redraw();
+    }
 
-        mAnimationStartTime = AnimationTime.get();
-        if (mAnimationKind != ANIM_KIND_FLING) {
-            mAnimationDuration = ANIM_TIME[mAnimationKind];
+    private void startAnimation(int targetX, int targetY, float targetScale,
+            int kind) {
+        boolean changed = false;
+        changed |= mPlatform.doAnimation(targetX, kind);
+        changed |= mBoxes.get(0).doAnimation(targetY, targetScale, kind);
+        if (changed) redraw();
+    }
+
+    public boolean advanceAnimation() {
+        boolean changed = false;
+        changed |= mPlatform.advanceAnimation();
+        for (int i = -BOX_MAX; i <= BOX_MAX; i++) {
+            changed |= mBoxes.get(i).advanceAnimation();
         }
-        advanceAnimation();
+        for (int i = -BOX_MAX; i < BOX_MAX; i++) {
+            changed |= mGaps.get(i).advanceAnimation();
+        }
+        if (changed) redraw();
+        return changed;
     }
 
-    public void advanceAnimation() {
-        if (mAnimationStartTime == NO_ANIMATION) {
-            return;
-        } else if (mAnimationStartTime == LAST_ANIMATION) {
-            onAnimationComplete();
-            return;
+    ////////////////////////////////////////////////////////////////////////////
+    //  Layout
+    ////////////////////////////////////////////////////////////////////////////
+
+    // Returns the display width of this box.
+    private int widthOf(Box b) {
+        return (int) (b.mImageW * b.mCurrentScale + 0.5f);
+    }
+
+    // Returns the display height of this box.
+    private int heightOf(Box b) {
+        return (int) (b.mImageH * b.mCurrentScale + 0.5f);
+    }
+
+    // Returns the display width of this box, using the given scale.
+    private int widthOf(Box b, float scale) {
+        return (int) (b.mImageW * scale + 0.5f);
+    }
+
+    // Returns the display height of this box, using the given scale.
+    private int heightOf(Box b, float scale) {
+        return (int) (b.mImageH * scale + 0.5f);
+    }
+
+    // Convert the information in mPlatform and mBoxes to mRects, so the user
+    // can get the position of each box by getPosition().
+    //
+    // Note the loop index goes from inside-out because each box's X coordinate
+    // is relative to its anchor box (except the focused box).
+    private void layoutAndSetPosition() {
+        // layout box 0 (focused box)
+        convertBoxToRect(0);
+        for (int i = 1; i <= BOX_MAX; i++) {
+            // layout box i and -i
+            convertBoxToRect(i);
+            convertBoxToRect(-i);
         }
+        //dumpState();
+    }
 
-        long now = AnimationTime.get();
-        float progress;
-        if (mAnimationDuration == 0) {
-            progress = 1;
-        } else {
-            progress = (now - mAnimationStartTime) / mAnimationDuration;
+    private void dumpState() {
+        for (int i = -BOX_MAX; i < BOX_MAX; i++) {
+            Log.d(TAG, "Gap " + i + ": " + mGaps.get(i).mCurrentGap);
         }
 
-        if (progress >= 1) {
-            progress = 1;
-            mCurrentX = mToX;
-            mCurrentY = mToY;
-            mCurrentScale = mToScale;
-            mAnimationStartTime = LAST_ANIMATION;
-        } else {
-            float f = 1 - progress;
-            switch (mAnimationKind) {
-                case ANIM_KIND_SCROLL:
-                case ANIM_KIND_FLING:
-                    progress = 1 - f;  // linear
-                    break;
-                case ANIM_KIND_SCALE:
-                    progress = 1 - f * f;  // quadratic
-                    break;
-                case ANIM_KIND_SNAPBACK:
-                case ANIM_KIND_ZOOM:
-                case ANIM_KIND_SLIDE:
-                    progress = 1 - f * f * f * f * f; // x^5
-                    break;
+        dumpRect(0);
+        for (int i = 1; i <= BOX_MAX; i++) {
+            dumpRect(i);
+            dumpRect(-i);
+        }
+
+        for (int i = -BOX_MAX; i <= BOX_MAX; i++) {
+            for (int j = i + 1; j <= BOX_MAX; j++) {
+                if (Rect.intersects(mRects.get(i), mRects.get(j))) {
+                    Log.d(TAG, "rect " + i + " and rect " + j + "intersects!");
+                }
             }
-            if (mAnimationKind == ANIM_KIND_FLING) {
-                flingInterpolate(progress);
+        }
+    }
+
+    private void dumpRect(int i) {
+        StringBuilder sb = new StringBuilder();
+        Rect r = mRects.get(i);
+        sb.append("Rect " + i + ":");
+        sb.append("(");
+        sb.append(r.centerX());
+        sb.append(",");
+        sb.append(r.centerY());
+        sb.append(") [");
+        sb.append(r.width());
+        sb.append("x");
+        sb.append(r.height());
+        sb.append("]");
+        Log.d(TAG, sb.toString());
+    }
+
+    private void convertBoxToRect(int i) {
+        Box b = mBoxes.get(i);
+        Rect r = mRects.get(i);
+        int y = b.mCurrentY;
+        int w = widthOf(b);
+        int h = heightOf(b);
+        if (i == 0) {
+            int x = mPlatform.mCurrentX;
+            r.left = x - w / 2;
+            r.right = r.left + w;
+        } else if (i > 0) {
+            Rect a = mRects.get(i - 1);
+            Gap g = mGaps.get(i - 1);
+            r.left = a.right + g.mCurrentGap;
+            r.right = r.left + w;
+        } else {  // i < 0
+            Rect a = mRects.get(i + 1);
+            Gap g = mGaps.get(i);
+            r.right = a.left - g.mCurrentGap;
+            r.left = r.right - w;
+        }
+        r.top = y - h / 2;
+        r.bottom = r.top + h;
+    }
+
+    // Returns the position of a box.
+    public Rect getPosition(int index) {
+        return mRects.get(index);
+    }
+
+    ////////////////////////////////////////////////////////////////////////////
+    //  Box management
+    ////////////////////////////////////////////////////////////////////////////
+
+    // Initialize the platform to be at the view center.
+    private void initPlatform() {
+        mPlatform.mCurrentX = mViewW / 2;
+        mPlatform.mAnimationStartTime = NO_ANIMATION;
+    }
+
+    // Initialize a box to have the size of the view.
+    private void initBox(int index) {
+        Box b = mBoxes.get(index);
+        b.mImageW = mViewW;
+        b.mImageH = mViewH;
+        b.mUseViewSize = true;
+        b.mScaleMin = getMinimalScale(b.mImageW, b.mImageH);
+        b.mScaleMax = getMaximalScale(b.mImageW, b.mImageH);
+        b.mCurrentY = mViewH / 2;
+        b.mCurrentScale = b.mScaleMin;
+        b.mAnimationStartTime = NO_ANIMATION;
+    }
+
+    // Initialize a gap. This can only be called after the boxes around the gap
+    // has been initialized.
+    private void initGap(int index) {
+        Gap g = mGaps.get(index);
+        g.mDefaultSize = getDefaultGapSize(index);
+        g.mCurrentGap = g.mDefaultSize;
+        g.mAnimationStartTime = NO_ANIMATION;
+    }
+
+    private void initGap(int index, int size) {
+        Gap g = mGaps.get(index);
+        g.mDefaultSize = getDefaultGapSize(index);
+        g.mCurrentGap = size;
+        g.mAnimationStartTime = NO_ANIMATION;
+    }
+
+    private void debugMoveBox(int fromIndex[]) {
+        StringBuilder s = new StringBuilder("moveBox:");
+        for (int i = 0; i < fromIndex.length; i++) {
+            int j = fromIndex[i];
+            if (j == Integer.MAX_VALUE) {
+                s.append(" N");
             } else {
-                linearInterpolate(progress);
+                s.append(" ");
+                s.append(fromIndex[i]);
             }
         }
-        mViewer.setPosition(mCurrentX, mCurrentY, mCurrentScale);
-        mViewer.invalidate();
+        Log.d(TAG, s.toString());
     }
 
-    private void onAnimationComplete() {
-        mAnimationStartTime = NO_ANIMATION;
-        if (mViewer.isInTransition()) {
-            mViewer.notifyTransitionComplete();
-        } else {
-            if (startSnapbackIfNeeded()) mViewer.invalidate();
-        }
-    }
-
-    private void flingInterpolate(float progress) {
-        mScroller.computeScrollOffset(progress);
-        int oldX = mCurrentX;
-        int oldY = mCurrentY;
-        mCurrentX = mScroller.getCurrX();
-        mCurrentY = mScroller.getCurrY();
-
-        // Check if we hit the edges; show edge effects if we do.
-        if (oldX > mBoundLeft && mCurrentX == mBoundLeft) {
-            int v = Math.round(-mScroller.getCurrVelocityX() * mCurrentScale);
-            mEdgeView.onAbsorb(v, EdgeView.LEFT);
-        } else if (oldX < mBoundRight && mCurrentX == mBoundRight) {
-            int v = Math.round(mScroller.getCurrVelocityX() * mCurrentScale);
-            mEdgeView.onAbsorb(v, EdgeView.RIGHT);
-        }
-
-        if (oldY > mBoundTop && mCurrentY == mBoundTop) {
-            int v = Math.round(-mScroller.getCurrVelocityY() * mCurrentScale);
-            mEdgeView.onAbsorb(v, EdgeView.TOP);
-        } else if (oldY < mBoundBottom && mCurrentY == mBoundBottom) {
-            int v = Math.round(mScroller.getCurrVelocityY() * mCurrentScale);
-            mEdgeView.onAbsorb(v, EdgeView.BOTTOM);
-        }
-    }
-
-    // Interpolates mCurrent{X,Y,Scale} given the progress in [0, 1].
-    private void linearInterpolate(float progress) {
-        // To linearly interpolate the position on view coordinates, we do the
-        // following steps:
-        // (1) convert a bitmap position (x, y) to view coordinates:
-        //     from: (x - mFromX) * mFromScale + mViewW / 2
-        //     to: (x - mToX) * mToScale + mViewW / 2
-        // (2) interpolate between the "from" and "to" coordinates:
-        //     (x - mFromX) * mFromScale * (1 - p) + (x - mToX) * mToScale * p
-        //     + mViewW / 2
-        //     should be equal to
-        //     (x - mCurrentX) * mCurrentScale + mViewW / 2
-        // (3) The x-related terms in the above equation can be removed because
-        //     mFromScale * (1 - p) + ToScale * p = mCurrentScale
-        // (4) Solve for mCurrentX, we have mCurrentX =
-        // (mFromX * mFromScale * (1 - p) + mToX * mToScale * p) / mCurrentScale
-        float fromX = mFromX * mFromScale;
-        float toX = mToX * mToScale;
-        float currentX = fromX + progress * (toX - fromX);
-
-        float fromY = mFromY * mFromScale;
-        float toY = mToY * mToScale;
-        float currentY = fromY + progress * (toY - fromY);
-
-        mCurrentScale = mFromScale + progress * (mToScale - mFromScale);
-        mCurrentX = Math.round(currentX / mCurrentScale);
-        mCurrentY = Math.round(currentY / mCurrentScale);
-    }
-
-    // Returns true if redraw is needed.
-    private boolean startSnapbackIfNeeded() {
-        if (mAnimationStartTime != NO_ANIMATION) return false;
-        if (mInScale) return false;
-        if (mAnimationKind == ANIM_KIND_SCROLL && mViewer.isDown()) {
-            return false;
+    // Move the boxes: it may indicate focus change, box deleted, box appearing,
+    // box reordered, etc.
+    //
+    // Each element in the fromIndex array indicates where each box was in the
+    // old array. If the value is Integer.MAX_VALUE (pictured as N below), it
+    // means the box is new.
+    //
+    // For example:
+    // N N N N N N N -- all new boxes
+    // -3 -2 -1 0 1 2 3 -- nothing changed
+    // -2 -1 0 1 2 3 N -- focus goes to the next box
+    // N-3 -2 -1 0 1 2 -- focuse goes to the previous box
+    // -3 -2 -1 1 2 3 N -- the focused box was deleted.
+    public void moveBox(int fromIndex[]) {
+        //debugMoveBox(fromIndex);
+        RangeIntArray from = new RangeIntArray(fromIndex, -BOX_MAX, BOX_MAX);
+
+        // 1. Get the absolute X coordiates for the boxes.
+        layoutAndSetPosition();
+        for (int i = -BOX_MAX; i <= BOX_MAX; i++) {
+            Box b = mBoxes.get(i);
+            Rect r = mRects.get(i);
+            b.mAbsoluteX = r.centerX();
         }
-        return startSnapback();
-    }
 
-    private boolean startSnapback() {
-        boolean needAnimation = false;
-        float scale = mCurrentScale;
+        // 2. copy boxes and gaps to temporary storage.
+        for (int i = -BOX_MAX; i <= BOX_MAX; i++) {
+            mTempBoxes.put(i, mBoxes.get(i));
+            mBoxes.put(i, null);
+        }
+        for (int i = -BOX_MAX; i < BOX_MAX; i++) {
+            mTempGaps.put(i, mGaps.get(i));
+            mGaps.put(i, null);
+        }
+
+        // 3. move back boxes that are used in the new array.
+        for (int i = -BOX_MAX; i <= BOX_MAX; i++) {
+            int j = from.get(i);
+            if (j == Integer.MAX_VALUE) continue;
+            mBoxes.put(i, mTempBoxes.get(j));
+            mTempBoxes.put(j, null);
+        }
 
-        float scaleMin = mExtraScalingRange ?
-                mScaleMin * SCALE_MIN_EXTRA : mScaleMin;
-        float scaleMax = mExtraScalingRange ?
-                mScaleMax * SCALE_MAX_EXTRA : mScaleMax;
+        // 4. move back gaps if both boxes around it are kept together.
+        for (int i = -BOX_MAX; i < BOX_MAX; i++) {
+            int j = from.get(i);
+            if (j == Integer.MAX_VALUE) continue;
+            int k = from.get(i + 1);
+            if (k == Integer.MAX_VALUE) continue;
+            if (j + 1 == k) {
+                mGaps.put(i, mTempGaps.get(j));
+                mTempGaps.put(j, null);
+            }
+        }
 
-        if (mCurrentScale < scaleMin || mCurrentScale > scaleMax) {
-            needAnimation = true;
-            scale = Utils.clamp(mCurrentScale, scaleMin, scaleMax);
+        // 5. recycle the boxes that are not used in the new array.
+        int k = -BOX_MAX;
+        for (int i = -BOX_MAX; i <= BOX_MAX; i++) {
+            if (mBoxes.get(i) != null) continue;
+            while (mTempBoxes.get(k) == null) {
+                k++;
+            }
+            mBoxes.put(i, mTempBoxes.get(k++));
+            initBox(i);
         }
 
-        calculateStableBound(scale, sHorizontalSlack);
-        int x = Utils.clamp(mCurrentX, mBoundLeft, mBoundRight);
-        int y = Utils.clamp(mCurrentY, mBoundTop, mBoundBottom);
+        // 6. Now give the recycled box a reasonable absolute X position.
+        //
+        // First try to find the first and the last box which the absolute X
+        // position is known.
+        int first, last;
+        for (first = -BOX_MAX; first <= BOX_MAX; first++) {
+            if (from.get(first) != Integer.MAX_VALUE) break;
+        }
+        for (last = BOX_MAX; last >= -BOX_MAX; last--) {
+            if (from.get(last) != Integer.MAX_VALUE) break;
+        }
+        // If there is no box has known X position at all, make the focused one
+        // as known.
+        if (first > BOX_MAX) {
+            mBoxes.get(0).mAbsoluteX = mPlatform.mCurrentX;
+            first = last = 0;
+        }
+        // Now for those boxes between first and last, just assign the same
+        // position as the previous box. (We can do better, but this should be
+        // rare). For the boxes before first or after last, we will use a new
+        // default gap size below.
+        for (int i = first + 1; i < last; i++) {
+            if (from.get(i) != Integer.MAX_VALUE) continue;
+            mBoxes.get(i).mAbsoluteX = mBoxes.get(i - 1).mAbsoluteX;
+        }
 
-        if (mCurrentX != x || mCurrentY != y || mCurrentScale != scale) {
-            needAnimation = true;
+        // 7. recycle the gaps that are not used in the new array.
+        k = -BOX_MAX;
+        for (int i = -BOX_MAX; i < BOX_MAX; i++) {
+            if (mGaps.get(i) != null) continue;
+            while (mTempGaps.get(k) == null) {
+                k++;
+            }
+            mGaps.put(i, mTempGaps.get(k++));
+            Box a = mBoxes.get(i);
+            Box b = mBoxes.get(i + 1);
+            int wa = widthOf(a);
+            int wb = widthOf(b);
+            if (i >= first && i < last) {
+                int g = b.mAbsoluteX - a.mAbsoluteX - wb / 2 - (wa - wa / 2);
+                initGap(i, g);
+            } else {
+                initGap(i);
+            }
         }
 
-        if (needAnimation) {
-            startAnimation(x, y, scale, ANIM_KIND_SNAPBACK);
+        // 8. offset the Platform position
+        int dx = mBoxes.get(0).mAbsoluteX - mPlatform.mCurrentX;
+        mPlatform.mCurrentX += dx;
+        mPlatform.mFromX += dx;
+        mPlatform.mToX += dx;
+        mPlatform.mFlingOffset += dx;
+
+        snapAndRedraw();
+    }
+
+    ////////////////////////////////////////////////////////////////////////////
+    //  Public utilities
+    ////////////////////////////////////////////////////////////////////////////
+
+    public float getMinimalScale(int imageW, int imageH) {
+        float s = Math.min(mScaleFactor * mViewW / imageW,
+                mScaleFactor * mViewH / imageH);
+        return Math.min(SCALE_LIMIT, s);
+    }
+
+    public float getMaximalScale(int imageW, int imageH) {
+        return mFilmMode ? getMinimalScale(imageW, imageH) : SCALE_LIMIT;
+    }
+
+    public boolean isAtMinimalScale() {
+        Box b = mBoxes.get(0);
+        return isAlmostEqual(b.mCurrentScale, b.mScaleMin);
+    }
+
+    public int getImageWidth() {
+        Box b = mBoxes.get(0);
+        return b.mImageW;
+    }
+
+    public int getImageHeight() {
+        Box b = mBoxes.get(0);
+        return b.mImageH;
+    }
+
+    public float getImageScale() {
+        Box b = mBoxes.get(0);
+        return b.mCurrentScale;
+    }
+
+    public int getImageAtEdges() {
+        Box b = mBoxes.get(0);
+        Platform p = mPlatform;
+        calculateStableBound(b.mCurrentScale);
+        int edges = 0;
+        if (p.mCurrentX <= mBoundLeft) {
+            edges |= IMAGE_AT_RIGHT_EDGE;
         }
+        if (p.mCurrentX >= mBoundRight) {
+            edges |= IMAGE_AT_LEFT_EDGE;
+        }
+        if (b.mCurrentY <= mBoundTop) {
+            edges |= IMAGE_AT_BOTTOM_EDGE;
+        }
+        if (b.mCurrentY >= mBoundBottom) {
+            edges |= IMAGE_AT_TOP_EDGE;
+        }
+        return edges;
+    }
+
+    ////////////////////////////////////////////////////////////////////////////
+    //  Private utilities
+    ////////////////////////////////////////////////////////////////////////////
+
+    private float getMinimalScale(Box b) {
+        return getMinimalScale(b.mImageW, b.mImageH);
+    }
 
-        return needAnimation;
+    private float getMaxmimalScale(Box b) {
+        return getMaximalScale(b.mImageW, b.mImageH);
+    }
+
+    private static boolean isAlmostEqual(float a, float b) {
+        float diff = a - b;
+        return (diff < 0 ? -diff : diff) < 0.02f;
     }
 
     // Calculates the stable region of mCurrent{X/Y}, where "stable" means
@@ -615,111 +947,431 @@ class PositionController {
     // An extra parameter "horizontalSlack" (which has the value of 0 usually)
     // is used to extend the stable region by some pixels on each side
     // horizontally.
-    private void calculateStableBound(float scale) {
-        calculateStableBound(scale, 0f);
-    }
+    private void calculateStableBound(float scale, int horizontalSlack) {
+        Box b = mBoxes.get(0);
 
-    private void calculateStableBound(float scale, float horizontalSlack) {
-        // The number of pixels between the center of the view
-        // and the edge when the edge is aligned.
-        mBoundLeft = (int) FloatMath.ceil((mViewW - horizontalSlack) / (2 * scale));
-        mBoundRight = mImageW - mBoundLeft;
-        mBoundTop = (int) FloatMath.ceil(mViewH / (2 * scale));
-        mBoundBottom = mImageH - mBoundTop;
+        // The width and height of the box in number of view pixels
+        int w = widthOf(b, scale);
+        int h = heightOf(b, scale);
+
+        // When the edge of the view is aligned with the edge of the box
+        mBoundLeft = (mViewW - horizontalSlack) - w / 2;
+        mBoundRight = mViewW - mBoundLeft;
+        mBoundTop = mViewH - h / 2;
+        mBoundBottom = mViewH - mBoundTop;
 
         // If the scaled height is smaller than the view height,
         // force it to be in the center.
-        if (viewHigherThanScaledImage(scale)) {
-            mBoundTop = mBoundBottom = mImageH / 2;
+        if (viewTallerThanScaledImage(scale)) {
+            mBoundTop = mBoundBottom = mViewH / 2;
         }
 
         // Same for width
         if (viewWiderThanScaledImage(scale)) {
-            mBoundLeft = mBoundRight = mImageW / 2;
+            mBoundLeft = mBoundRight = mViewW / 2;
+        }
+    }
+
+    private void calculateStableBound(float scale) {
+        calculateStableBound(scale, 0);
+    }
+
+    private boolean hasNextImages() {
+        for (int i = 1; i <= BOX_MAX; i++) {
+            if (!mBoxes.get(i).mUseViewSize) return true;
+        }
+        return false;
+    }
+
+    private boolean hasPrevImages() {
+        for (int i = -1; i >= -BOX_MAX; i--) {
+            if (!mBoxes.get(i).mUseViewSize) return true;
         }
+        return false;
     }
 
-    private boolean viewHigherThanScaledImage(float scale) {
-        return FloatMath.floor(mImageH * scale) <= mViewH;
+    private boolean viewTallerThanScaledImage(float scale) {
+        return mViewH >= heightOf(mBoxes.get(0), scale);
     }
 
     private boolean viewWiderThanScaledImage(float scale) {
-        return FloatMath.floor(mImageW * scale) <= mViewW;
+        return mViewW >= widthOf(mBoxes.get(0), scale);
     }
 
-    private boolean useCurrentValueAsTarget() {
-        return mAnimationStartTime == NO_ANIMATION ||
-                mAnimationKind == ANIM_KIND_SNAPBACK ||
-                mAnimationKind == ANIM_KIND_FLING;
+    private float getTargetScale(Box b) {
+        return useCurrentValueAsTarget(b) ? b.mCurrentScale : b.mToScale;
     }
 
-    private float getTargetScale() {
-        return useCurrentValueAsTarget() ? mCurrentScale : mToScale;
+    private int getTargetX(Platform p) {
+        return useCurrentValueAsTarget(p) ? p.mCurrentX : p.mToX;
     }
 
-    private int getTargetX() {
-        return useCurrentValueAsTarget() ? mCurrentX : mToX;
+    private int getTargetY(Box b) {
+        return useCurrentValueAsTarget(b) ? b.mCurrentY : b.mToY;
     }
 
-    private int getTargetY() {
-        return useCurrentValueAsTarget() ? mCurrentY : mToY;
+    private boolean useCurrentValueAsTarget(Animatable a) {
+        return a.mAnimationStartTime == NO_ANIMATION ||
+                a.mAnimationKind == ANIM_KIND_SNAPBACK ||
+                a.mAnimationKind == ANIM_KIND_FLING;
     }
 
-    public RectF getImageBounds() {
-        float points[] = mTempPoints;
+    // Returns the index of the anchor box.
+    private int anchorIndex(int i) {
+        if (i > 0) return i - 1;
+        if (i < 0) return i + 1;
+        throw new IllegalArgumentException();
+    }
+
+    ////////////////////////////////////////////////////////////////////////////
+    //  Animatable: an thing which can do animation.
+    ////////////////////////////////////////////////////////////////////////////
+    private abstract static class Animatable {
+        public long mAnimationStartTime;
+        public int mAnimationKind;
+        public int mAnimationDuration;
+
+        // This should be overidden in subclass to change the animation values
+        // give the progress value in [0, 1].
+        protected abstract boolean interpolate(float progress);
+        public abstract boolean startSnapback();
+
+        // Returns true if the animation values changes, so things need to be
+        // redrawn.
+        public boolean advanceAnimation() {
+            if (mAnimationStartTime == NO_ANIMATION) {
+                return false;
+            }
+            if (mAnimationStartTime == LAST_ANIMATION) {
+                mAnimationStartTime = NO_ANIMATION;
+                return startSnapback();
+            }
+
+            float progress;
+            if (mAnimationDuration == 0) {
+                progress = 1;
+            } else {
+                long now = AnimationTime.get();
+                progress =
+                    (float) (now - mAnimationStartTime) / mAnimationDuration;
+            }
+
+            if (progress >= 1) {
+                progress = 1;
+            } else {
+                progress = applyInterpolationCurve(mAnimationKind, progress);
+            }
 
-        /*
-         * (p0,p1)----------(p2,p3)
-         *   |                  |
-         *   |                  |
-         * (p4,p5)----------(p6,p7)
-         */
-        points[0] = points[4] = -mCurrentX;
-        points[1] = points[3] = -mCurrentY;
-        points[2] = points[6] = mImageW - mCurrentX;
-        points[5] = points[7] = mImageH - mCurrentY;
+            boolean done = interpolate(progress);
 
-        RectF rect = mTempRect;
-        rect.set(Float.POSITIVE_INFINITY, Float.POSITIVE_INFINITY,
-                Float.NEGATIVE_INFINITY, Float.NEGATIVE_INFINITY);
+            if (done) {
+                mAnimationStartTime = LAST_ANIMATION;
+            }
 
-        float scale = mCurrentScale;
-        float offsetX = mViewW / 2;
-        float offsetY = mViewH / 2;
-        for (int i = 0; i < 4; ++i) {
-            float x = points[i + i] * scale + offsetX;
-            float y = points[i + i + 1] * scale + offsetY;
-            if (x < rect.left) rect.left = x;
-            if (x > rect.right) rect.right = x;
-            if (y < rect.top) rect.top = y;
-            if (y > rect.bottom) rect.bottom = y;
+            return true;
         }
-        return rect;
-    }
 
-    public int getImageWidth() {
-        return mImageW;
+        private static float applyInterpolationCurve(int kind, float progress) {
+            float f = 1 - progress;
+            switch (kind) {
+                case ANIM_KIND_SCROLL:
+                case ANIM_KIND_FLING:
+                    progress = 1 - f;  // linear
+                    break;
+                case ANIM_KIND_SCALE:
+                    progress = 1 - f * f;  // quadratic
+                    break;
+                case ANIM_KIND_SNAPBACK:
+                case ANIM_KIND_ZOOM:
+                case ANIM_KIND_SLIDE:
+                case ANIM_KIND_OPENING:
+                    progress = 1 - f * f * f * f * f; // x^5
+                    break;
+            }
+            return progress;
+        }
     }
 
-    public int getImageHeight() {
-        return mImageH;
+    ////////////////////////////////////////////////////////////////////////////
+    //  Platform: captures the global X movement.
+    ////////////////////////////////////////////////////////////////////////////
+    private class Platform extends Animatable {
+        public int mCurrentX, mFromX, mToX;
+        public int mFlingOffset;
+
+        @Override
+        public boolean startSnapback() {
+            if (mAnimationStartTime != NO_ANIMATION) return false;
+            if (mAnimationKind == ANIM_KIND_SCROLL
+                    && mListener.isDown()) return false;
+
+            Box b = mBoxes.get(0);
+            float scaleMin = mExtraScalingRange ?
+                b.mScaleMin * SCALE_MIN_EXTRA : b.mScaleMin;
+            float scaleMax = mExtraScalingRange ?
+                b.mScaleMax * SCALE_MAX_EXTRA : b.mScaleMax;
+            float scale = Utils.clamp(b.mCurrentScale, scaleMin, scaleMax);
+            int x = mCurrentX;
+            if (mFilmMode) {
+                if (!hasNextImages()) x = Math.max(x, mViewW / 2);
+                if (!hasPrevImages()) x = Math.min(x, mViewW / 2);
+            } else {
+                calculateStableBound(scale, HORIZONTAL_SLACK);
+                x = Utils.clamp(x, mBoundLeft, mBoundRight);
+            }
+            if (mCurrentX != x) {
+                return doAnimation(x, ANIM_KIND_SNAPBACK);
+            }
+            return false;
+        }
+
+        // Starts an animation for the platform.
+        public boolean doAnimation(int targetX, int kind) {
+            if (mCurrentX == targetX) return false;
+            mAnimationKind = kind;
+            mFromX = mCurrentX;
+            mToX = targetX;
+            mAnimationStartTime = AnimationTime.startTime();
+            mAnimationDuration = ANIM_TIME[kind];
+            mFlingOffset = 0;
+            advanceAnimation();
+            return true;
+        }
+
+        @Override
+        protected boolean interpolate(float progress) {
+            if (mAnimationKind == ANIM_KIND_FLING) {
+                return mFilmMode
+                        ? interpolateFlingFilm(progress)
+                        : interpolateFlingPage(progress);
+            } else {
+                return interpolateLinear(progress);
+            }
+        }
+
+        private boolean interpolateFlingFilm(float progress) {
+            mFilmScroller.computeScrollOffset();
+            mCurrentX = mFilmScroller.getCurrX() + mFlingOffset;
+
+            int dir = EdgeView.INVALID_DIRECTION;
+            if (mCurrentX < mViewW / 2) {
+                if (!hasNextImages()) {
+                    dir = EdgeView.RIGHT;
+                }
+            } else if (mCurrentX > mViewW / 2) {
+                if (!hasPrevImages()) {
+                    dir = EdgeView.LEFT;
+                }
+            }
+            if (dir != EdgeView.INVALID_DIRECTION) {
+                int v = (int) (mFilmScroller.getCurrVelocity() + 0.5f);
+                mListener.onAbsorb(v, dir);
+                mFilmScroller.forceFinished(true);
+                mCurrentX = mViewW / 2;
+            }
+            return mFilmScroller.isFinished();
+        }
+
+        private boolean interpolateFlingPage(float progress) {
+            mPageScroller.computeScrollOffset(progress);
+            Box b = mBoxes.get(0);
+            calculateStableBound(b.mCurrentScale);
+
+            int oldX = mCurrentX;
+            mCurrentX = mPageScroller.getCurrX();
+
+            // Check if we hit the edges; show edge effects if we do.
+            if (oldX > mBoundLeft && mCurrentX == mBoundLeft) {
+                int v = (int) (-mPageScroller.getCurrVelocityX() + 0.5f);
+                mListener.onAbsorb(v, EdgeView.RIGHT);
+            } else if (oldX < mBoundRight && mCurrentX == mBoundRight) {
+                int v = (int) (mPageScroller.getCurrVelocityX() + 0.5f);
+                mListener.onAbsorb(v, EdgeView.LEFT);
+            }
+
+            return progress >= 1;
+        }
+
+        private boolean interpolateLinear(float progress) {
+            // Other animations
+            if (progress >= 1) {
+                mCurrentX = mToX;
+                return true;
+            } else {
+                mCurrentX = (int) (mFromX + progress * (mToX - mFromX));
+                return (mCurrentX == mToX);
+            }
+        }
     }
 
-    public int getImageAtEdges() {
-        calculateStableBound(mCurrentScale);
-        int edges = 0;
-        if (mCurrentX <= mBoundLeft) {
-            edges |= IMAGE_AT_LEFT_EDGE;
+    ////////////////////////////////////////////////////////////////////////////
+    //  Box: represents a rectangular area which shows a picture.
+    ////////////////////////////////////////////////////////////////////////////
+    private class Box extends Animatable {
+        // Size of the bitmap
+        public int mImageW, mImageH;
+
+        // This is true if we assume the image size is the same as view size
+        // until we know the actual size of image. This is also used to
+        // determine if there is an image ready to show.
+        public boolean mUseViewSize;
+
+        // The minimum and maximum scale we allow for this box.
+        public float mScaleMin, mScaleMax;
+
+        // The X/Y value indicates where the center of the box is on the view
+        // coordinate. We always keep the mCurrent{X,Y,Scale} sync with the
+        // actual values used currently. Note that the X values are implicitly
+        // defined by Platform and Gaps.
+        public int mCurrentY, mFromY, mToY;
+        public float mCurrentScale, mFromScale, mToScale;
+
+        // The absolute X coordinate of the center of the box. This is only used
+        // during moveBox().
+        public int mAbsoluteX;
+
+        @Override
+        public boolean startSnapback() {
+            if (mAnimationStartTime != NO_ANIMATION) return false;
+            if (mAnimationKind == ANIM_KIND_SCROLL
+                    && mListener.isDown()) return false;
+            if (mInScale && this == mBoxes.get(0)) return false;
+
+            int y;
+            float scale;
+
+            if (this == mBoxes.get(0)) {
+                float scaleMin = mExtraScalingRange ?
+                    mScaleMin * SCALE_MIN_EXTRA : mScaleMin;
+                float scaleMax = mExtraScalingRange ?
+                    mScaleMax * SCALE_MAX_EXTRA : mScaleMax;
+                scale = Utils.clamp(mCurrentScale, scaleMin, scaleMax);
+                if (mFilmMode) {
+                    y = mViewH / 2;
+                } else {
+                    calculateStableBound(scale, HORIZONTAL_SLACK);
+                    y = Utils.clamp(mCurrentY, mBoundTop, mBoundBottom);
+                }
+            } else {
+                y = mViewH / 2;
+                scale = mScaleMin;
+            }
+
+            if (mCurrentY != y || mCurrentScale != scale) {
+                return doAnimation(y, scale, ANIM_KIND_SNAPBACK);
+            }
+            return false;
         }
-        if (mCurrentX >= mBoundRight) {
-            edges |= IMAGE_AT_RIGHT_EDGE;
+
+        private boolean doAnimation(int targetY, float targetScale, int kind) {
+            targetScale = Utils.clamp(targetScale,
+                    SCALE_MIN_EXTRA * mScaleMin,
+                    SCALE_MAX_EXTRA * mScaleMax);
+
+            // If the scaled height is smaller than the view height, force it to be
+            // in the center.  (We do this for height only, not width, because the
+            // user may want to scroll to the previous/next image.)
+            if (!mInScale && viewTallerThanScaledImage(targetScale)) {
+                targetY = mViewH / 2;
+            }
+
+            if (mCurrentY == targetY && mCurrentScale == targetScale) {
+                return false;
+            }
+
+            // Now starts an animation for the box.
+            mAnimationKind = kind;
+            mFromY = mCurrentY;
+            mFromScale = mCurrentScale;
+            mToY = targetY;
+            mToScale = targetScale;
+            mAnimationStartTime = AnimationTime.startTime();
+            mAnimationDuration = ANIM_TIME[kind];
+            advanceAnimation();
+            return true;
         }
-        if (mCurrentY <= mBoundTop) {
-            edges |= IMAGE_AT_TOP_EDGE;
+
+        @Override
+        protected boolean interpolate(float progress) {
+            if (mAnimationKind == ANIM_KIND_FLING) {
+                // Currently a Box can only be flung in page mode.
+                return interpolateFlingPage(progress);
+            } else {
+                return interpolateLinear(progress);
+            }
         }
-        if (mCurrentY >= mBoundBottom) {
-            edges |= IMAGE_AT_BOTTOM_EDGE;
+
+        private boolean interpolateFlingPage(float progress) {
+            mPageScroller.computeScrollOffset(progress);
+            calculateStableBound(mCurrentScale);
+
+            int oldY = mCurrentY;
+            mCurrentY = mPageScroller.getCurrY();
+
+            // Check if we hit the edges; show edge effects if we do.
+            if (oldY > mBoundTop && mCurrentY == mBoundTop) {
+                int v = (int) (-mPageScroller.getCurrVelocityY() + 0.5f);
+                mListener.onAbsorb(v, EdgeView.BOTTOM);
+            } else if (oldY < mBoundBottom && mCurrentY == mBoundBottom) {
+                int v = (int) (mPageScroller.getCurrVelocityY() + 0.5f);
+                mListener.onAbsorb(v, EdgeView.TOP);
+            }
+
+            return progress >= 1;
+        }
+
+        private boolean interpolateLinear(float progress) {
+            if (progress >= 1) {
+                mCurrentY = mToY;
+                mCurrentScale = mToScale;
+                return true;
+            } else {
+                mCurrentY = (int) (mFromY + progress * (mToY - mFromY));
+                mCurrentScale = mFromScale + progress * (mToScale - mFromScale);
+                return (mCurrentY == mToY && mCurrentScale == mToScale);
+            }
+        }
+    }
+
+    ////////////////////////////////////////////////////////////////////////////
+    //  Gap: represents a rectangular area which is between two boxes.
+    ////////////////////////////////////////////////////////////////////////////
+    private class Gap extends Animatable {
+        // The default gap size between two boxes. The value may vary for
+        // different image size of the boxes and for different modes (page or
+        // film).
+        public int mDefaultSize;
+
+        // The gap size between the two boxes.
+        public int mCurrentGap, mFromGap, mToGap;
+
+        @Override
+        public boolean startSnapback() {
+            if (mAnimationStartTime != NO_ANIMATION) return false;
+            return doAnimation(mDefaultSize);
+        }
+
+        // Starts an animation for a gap.
+        public boolean doAnimation(int targetSize) {
+            if (mCurrentGap == targetSize) return false;
+            mAnimationKind = ANIM_KIND_SNAPBACK;
+            mFromGap = mCurrentGap;
+            mToGap = targetSize;
+            mAnimationStartTime = AnimationTime.startTime();
+            mAnimationDuration = ANIM_TIME[mAnimationKind];
+            advanceAnimation();
+            return true;
+        }
+
+        @Override
+        protected boolean interpolate(float progress) {
+            if (progress >= 1) {
+                mCurrentGap = mToGap;
+                return true;
+            } else {
+                mCurrentGap = (int) (mFromGap + progress * (mToGap - mFromGap));
+                return (mCurrentGap == mToGap);
+            }
         }
-        return edges;
     }
 }