gif2webp: Handle frames with odd offsets + disposal to background.

Snapping odd offsets in GIF to even offsets in WebP was causing extra row/column
being disposed in such cases.

Code is rewritten to maintain previous and current canvas (it used to maintain
previous canvas and current frame earlier). And we recompute change rectangles
as those from GIF may no longer apply.
Also, this renders methods like ReduceTransparency() and ConvertToKeyFrame()
redundant, as internally maintained current canvas is always independent of
previous canvases.

Disposal method choice: we pick the disposal method that results in the smallest
change rectangle.

(cherry picked from commit e4c829efe9)

Conflicts:
	examples/gif2webp_util.c

Change-Id: Ic31186d98fe1a2a790a89d1571b17e3abd127e79

examples/gif2webp_util.c

@@ -81,21 +81,118 @@ static void BlendPixels(const WebPPicture* const src,
   }
 }
 
-// Replace transparent pixels within 'dst_rect' of 'dst' by those in the 'src'.
+// Returns true if 'length' number of pixels in 'src' and 'dst' are identical,
-static void ReduceTransparency(const WebPPicture* const src,
+// assuming the given step sizes between pixels.
+static WEBP_INLINE int ComparePixels(const uint32_t* src, int src_step,
+                                     const uint32_t* dst, int dst_step,
+                                     int length) {
+  assert(length > 0);
+  while (length-- > 0) {
+    if (*src != *dst) {
+      return 0;
+    }
+    src += src_step;
+    dst += dst_step;
+  }
+  return 1;
+}
+
+// Assumes that an initial valid guess of change rectangle 'rect' is passed.
+static void MinimizeChangeRectangle(const WebPPicture* const src,
+                                    const WebPPicture* const dst,
+                                    WebPFrameRect* const rect) {
+  int i, j;
+  // Sanity checks.
+  assert(src->width == dst->width && src->height == dst->height);
+  assert(rect->x_offset + rect->width <= dst->width);
+  assert(rect->y_offset + rect->height <= dst->height);
+
+  // Left boundary.
+  for (i = rect->x_offset; i < rect->x_offset + rect->width; ++i) {
+    const uint32_t* const src_argb =
+        &src->argb[rect->y_offset * src->argb_stride + i];
+    const uint32_t* const dst_argb =
+        &dst->argb[rect->y_offset * dst->argb_stride + i];
+    if (ComparePixels(src_argb, src->argb_stride, dst_argb, dst->argb_stride,
+                      rect->height)) {
+      --rect->width;  // Redundant column.
+      ++rect->x_offset;
+    } else {
+      break;
+    }
+  }
+  if (rect->width == 0) goto End;
+
+  // Right boundary.
+  for (i = rect->x_offset + rect->width - 1; i >= rect->x_offset; --i) {
+    const uint32_t* const src_argb =
+        &src->argb[rect->y_offset * src->argb_stride + i];
+    const uint32_t* const dst_argb =
+        &dst->argb[rect->y_offset * dst->argb_stride + i];
+    if (ComparePixels(src_argb, src->argb_stride, dst_argb, dst->argb_stride,
+                      rect->height)) {
+      --rect->width;  // Redundant column.
+    } else {
+      break;
+    }
+  }
+  if (rect->width == 0) goto End;
+
+  // Top boundary.
+  for (j = rect->y_offset; j < rect->y_offset + rect->height; ++j) {
+    const uint32_t* const src_argb =
+        &src->argb[j * src->argb_stride + rect->x_offset];
+    const uint32_t* const dst_argb =
+        &dst->argb[j * dst->argb_stride + rect->x_offset];
+    if (ComparePixels(src_argb, 1, dst_argb, 1, rect->width)) {
+      --rect->height;  // Redundant row.
+      ++rect->y_offset;
+    } else {
+      break;
+    }
+  }
+  if (rect->height == 0) goto End;
+
+  // Bottom boundary.
+  for (j = rect->y_offset + rect->height - 1; j >= rect->y_offset; --j) {
+    const uint32_t* const src_argb =
+        &src->argb[j * src->argb_stride + rect->x_offset];
+    const uint32_t* const dst_argb =
+        &dst->argb[j * dst->argb_stride + rect->x_offset];
+    if (ComparePixels(src_argb, 1, dst_argb, 1, rect->width)) {
+      --rect->height;  // Redundant row.
+    } else {
+      break;
+    }
+  }
+  if (rect->height == 0) goto End;
+
+  if (rect->width == 0 || rect->height == 0) {
+ End:
+    // TODO(later): This rare case can happen for a bad GIF. In such a case, the
+    // frame should not be encoded at all and the duration of prev frame should
+    // be increased instead. For now, we just create a 1x1 frame at zero offset.
+    rect->x_offset = 0;
+    rect->y_offset = 0;
+    rect->width = 1;
+    rect->height = 1;
+  }
+}
+
+// For pixels in 'rect', replace those pixels in 'dst' that are same as 'src' by
+// transparent pixels.
+static void IncreaseTransparency(const WebPPicture* const src,
                                  const WebPFrameRect* const rect,
                                  WebPPicture* const dst) {
   int i, j;
   assert(src != NULL && dst != NULL && rect != NULL);
   assert(src->width == dst->width && src->height == dst->height);
   for (j = rect->y_offset; j < rect->y_offset + rect->height; ++j) {
+    const uint32_t* const psrc = src->argb + j * src->argb_stride;
+    uint32_t* const pdst = dst->argb + j * dst->argb_stride;
     for (i = rect->x_offset; i < rect->x_offset + rect->width; ++i) {
-      const uint32_t src_pixel = src->argb[j * src->argb_stride + i];
+      if (psrc[i] == pdst[i]) {
-      const int src_alpha = src_pixel >> 24;
+        pdst[i] = WEBP_UTIL_TRANSPARENT_COLOR;
-      const uint32_t dst_pixel = dst->argb[j * dst->argb_stride + i];
-      const int dst_alpha = dst_pixel >> 24;
-      if (dst_alpha == 0 && src_alpha == 0xff) {
-        dst->argb[j * dst->argb_stride + i] = src_pixel;
       }
     }
   }
@@ -153,84 +250,6 @@ static void FlattenSimilarBlocks(const WebPPicture* const src,
   }
 }
 
-//------------------------------------------------------------------------------
-// Key frame related utilities.
-
-// Returns true if 'curr' frame with frame rectangle 'curr_rect' is a key frame,
-// that is, it can be decoded independently of 'prev' canvas.
-static int IsKeyFrame(const WebPPicture* const curr,
-                      const WebPFrameRect* const curr_rect,
-                      const WebPPicture* const prev) {
-  int i, j;
-  int is_key_frame = 1;
-
-  // If previous canvas (with previous frame disposed) is all transparent,
-  // current frame is a key frame.
-  for (j = 0; j < prev->height; ++j) {
-    const uint32_t* const row = &prev->argb[j * prev->argb_stride];
-    for (i = 0; i < prev->width; ++i) {
-      if (row[i] & 0xff000000u) {   // has alpha?
-        is_key_frame = 0;
-        break;
-      }
-    }
-    if (!is_key_frame) break;
-  }
-  if (is_key_frame) return 1;
-
-  // If current frame covers the whole canvas and does not contain any
-  // transparent pixels that depend on previous canvas, then current frame is
-  // a key frame.
-  if (curr_rect->width == curr->width && curr_rect->height == curr->height) {
-    assert(curr_rect->x_offset == 0 && curr_rect->y_offset == 0);
-    is_key_frame = 1;
-    for (j = 0; j < prev->height; ++j) {
-      for (i = 0; i < prev->width; ++i) {
-        const uint32_t prev_alpha =
-            (prev->argb[j * prev->argb_stride + i]) >> 24;
-        const uint32_t curr_alpha =
-            (curr->argb[j * curr->argb_stride + i]) >> 24;
-        if (curr_alpha != 0xff && prev_alpha != 0) {
-          is_key_frame = 0;
-          break;
-        }
-      }
-      if (!is_key_frame) break;
-    }
-    if (is_key_frame) return 1;
-  }
-
-  return 0;
-}
-
-// Given 'prev' frame and current frame rectangle 'rect', convert 'curr' frame
-// to a key frame.
-static void ConvertToKeyFrame(const WebPPicture* const prev,
-                              WebPFrameRect* const rect,
-                              WebPPicture* const curr) {
-  int j;
-  assert(curr->width == prev->width && curr->height == prev->height);
-
-  // Replace transparent pixels of current canvas with those from previous
-  // canvas (with previous frame disposed).
-  for (j = 0; j < curr->height; ++j) {
-    int i;
-    for (i = 0; i < curr->width; ++i) {
-      uint32_t* const curr_pixel = curr->argb + j * curr->argb_stride + i;
-      const int curr_alpha = *curr_pixel >> 24;
-      if (curr_alpha == 0) {
-        *curr_pixel = prev->argb[j * prev->argb_stride + i];
-      }
-    }
-  }
-
-  // Frame rectangle now covers the whole canvas.
-  rect->x_offset = 0;
-  rect->y_offset = 0;
-  rect->width = curr->width;
-  rect->height = curr->height;
-}
-
 //------------------------------------------------------------------------------
 // Encoded frame.
 
@@ -239,6 +258,7 @@ static void ConvertToKeyFrame(const WebPPicture* const prev,
 typedef struct {
   WebPMuxFrameInfo sub_frame;  // Encoded frame rectangle.
   WebPMuxFrameInfo key_frame;  // Encoded frame if it was converted to keyframe.
+  int is_key_frame;            // True if 'key_frame' has been chosen.
 } EncodedFrame;
 
 // Release the data contained by 'encoded_frame'.
@@ -270,8 +290,17 @@ struct WebPFrameCache {
   size_t kmax;                   // Max distance between key frames.
   size_t count_since_key_frame;  // Frames seen since the last key frame.
   int allow_mixed;           // If true, each frame can be lossy or lossless.
-  WebPPicture prev_canvas;   // Previous canvas (properly disposed).
+
-  WebPPicture curr_canvas;   // Current canvas (temporary buffer).
+  WebPFrameRect prev_orig_rect;  // Previous input (e.g. GIF) frame rectangle.
+  WebPFrameRect prev_webp_rect;  // Previous WebP frame rectangle.
+  WebPMuxAnimDispose prev_orig_dispose;  // Previous input dispose method.
+  int prev_candidate_undecided;  // True if sub-frame vs keyframe decision
+                                 // hasn't been made for the previous frame yet.
+
+  WebPPicture prev_canvas;       // Previous canvas (NOT disposed).
+  WebPPicture curr_canvas;       // Current canvas (NOT disposed).
+  WebPPicture prev_canvas_disposed;  // Previous canvas disposed to background.
+  WebPPicture curr_canvas_tmp;   // Temporary storage for current canvas.
   int is_first_frame;        // True if no frames have been added to the cache
                              // since WebPFrameCacheNew().
 };
@@ -294,18 +323,23 @@ WebPFrameCache* WebPFrameCacheNew(int width, int height,
   // sanity init, so we can call WebPFrameCacheDelete():
   cache->encoded_frames = NULL;
 
+  cache->prev_candidate_undecided = 0;
   cache->is_first_frame = 1;
 
   // Picture buffers.
   if (!WebPPictureInit(&cache->prev_canvas) ||
-      !WebPPictureInit(&cache->curr_canvas)) {
+      !WebPPictureInit(&cache->curr_canvas) ||
+      !WebPPictureInit(&cache->prev_canvas_disposed) ||
+      !WebPPictureInit(&cache->curr_canvas_tmp)) {
     return NULL;
   }
   cache->prev_canvas.width = width;
   cache->prev_canvas.height = height;
   cache->prev_canvas.use_argb = 1;
   if (!WebPPictureAlloc(&cache->prev_canvas) ||
-      !WebPPictureCopy(&cache->prev_canvas, &cache->curr_canvas)) {
+      !WebPPictureCopy(&cache->prev_canvas, &cache->curr_canvas) ||
+      !WebPPictureCopy(&cache->prev_canvas, &cache->prev_canvas_disposed) ||
+      !WebPPictureCopy(&cache->prev_canvas, &cache->curr_canvas_tmp)) {
     goto Err;
   }
   WebPUtilClearPic(&cache->prev_canvas, NULL);
@@ -316,7 +350,7 @@ WebPFrameCache* WebPFrameCacheNew(int width, int height,
   cache->kmax = kmax;
   cache->count_since_key_frame = 0;
   assert(kmax > kmin);
-  cache->size = kmax - kmin;
+  cache->size = kmax - kmin + 1;  // One extra storage for previous frame.
   cache->encoded_frames =
       (EncodedFrame*)calloc(cache->size, sizeof(*cache->encoded_frames));
   if (cache->encoded_frames == NULL) goto Err;
@@ -339,6 +373,8 @@ void WebPFrameCacheDelete(WebPFrameCache* const cache) {
     }
     WebPPictureFree(&cache->prev_canvas);
     WebPPictureFree(&cache->curr_canvas);
+    WebPPictureFree(&cache->prev_canvas_disposed);
+    WebPPictureFree(&cache->curr_canvas_tmp);
     free(cache);
   }
 }
@@ -415,101 +451,128 @@ static int GetColorCount(const WebPPicture* const pic) {
 #undef HASH_SIZE
 #undef HASH_RIGHT_SHIFT
 
-static WebPEncodingError SetFrame(const WebPConfig* const config,
+static void DisposeFullFrame(WebPMuxAnimDispose dispose_method,
-                                  int allow_mixed, int is_key_frame,
+                             WebPPicture* const frame) {
-                                  const WebPPicture* const prev_canvas,
+  if (dispose_method == WEBP_MUX_DISPOSE_BACKGROUND) {
-                                  WebPPicture* const frame,
+    WebPUtilClearPic(frame, NULL);
+  }
+}
+
+static void DisposeFrameRectangle(WebPMuxAnimDispose dispose_method,
+                                  const WebPFrameRect* const gif_rect,
+                                  WebPPicture* const frame) {
+  if (dispose_method == WEBP_MUX_DISPOSE_BACKGROUND) {
+    WebPUtilClearPic(frame, gif_rect);
+  }
+}
+
+// Snap rectangle to even offsets (and adjust dimensions if needed).
+static WEBP_INLINE void SnapToEvenOffsets(WebPFrameRect* const rect) {
+  rect->width += (rect->x_offset & 1);
+  rect->height += (rect->y_offset & 1);
+  rect->x_offset &= ~1;
+  rect->y_offset &= ~1;
+}
+
+// Given previous and current canvas, picks the optimal rectangle for the
+// current frame.
+// The initial guess for 'rect' will be 'orig_rect' if is non-NULL, otherwise
+// the initial guess will be the full canvas.
+static int GetSubRect(const WebPPicture* const prev_canvas,
+                      const WebPPicture* const curr_canvas,
+                      const WebPFrameRect* const orig_rect, int is_key_frame,
+                      WebPFrameRect* const rect, WebPPicture* const sub_frame) {
+  if (orig_rect != NULL) {
+    *rect = *orig_rect;
+  } else {
+    rect->x_offset = 0;
+    rect->y_offset = 0;
+    rect->width = curr_canvas->width;
+    rect->height = curr_canvas->height;
+  }
+  if (!is_key_frame) {  // Optimize frame rectangle.
+    MinimizeChangeRectangle(prev_canvas, curr_canvas, rect);
+  }
+  SnapToEvenOffsets(rect);
+
+  return WebPPictureView(curr_canvas, rect->x_offset, rect->y_offset,
+                         rect->width, rect->height, sub_frame);
+}
+
+static int IsBlendingPossible(const WebPPicture* const src,
+                              const WebPPicture* const dst,
+                              const WebPFrameRect* const rect) {
+  int i, j;
+  assert(src->width == dst->width && src->height == dst->height);
+  assert(rect->x_offset + rect->width <= dst->width);
+  assert(rect->y_offset + rect->height <= dst->height);
+  for (j = rect->y_offset; j < rect->y_offset + rect->height; ++j) {
+    for (i = rect->x_offset; i < rect->x_offset + rect->width; ++i) {
+      const uint32_t src_pixel = src->argb[j * src->argb_stride + i];
+      const uint32_t dst_pixel = dst->argb[j * dst->argb_stride + i];
+      const uint32_t dst_alpha = dst_pixel >> 24;
+      if (dst_alpha != 0xff && src_pixel != dst_pixel) {
+        // In this case, if we use blending, we can't attain the desired
+        // 'dst_pixel' value for this pixel. So, blending is not possible.
+        return 0;
+      }
+    }
+  }
+  return 1;
+}
+
+static int RectArea(const WebPFrameRect* const rect) {
+  return rect->width * rect->height;
+}
+
+// Struct representing a candidate encoded frame including its metadata.
+typedef struct {
+  WebPMemoryWriter  mem;
+  WebPMuxFrameInfo  info;
+  WebPFrameRect     rect;
+  int               evaluate;  // True if this candidate should be evaluated.
+} Candidate;
+
+// Generates a candidate encoded frame given a picture and metadata.
+static WebPEncodingError EncodeCandidate(WebPPicture* const sub_frame,
                                          const WebPFrameRect* const rect,
                                          const WebPMuxFrameInfo* const info,
-                                  WebPPicture* const sub_frame,
+                                         const WebPConfig* const config,
-                                  EncodedFrame* encoded_frame) {
+                                         int use_blending,
+                                         Candidate* const candidate) {
   WebPEncodingError error_code = VP8_ENC_OK;
-  int try_lossless;
+  assert(candidate != NULL);
-  int try_lossy;
+  memset(candidate, 0, sizeof(*candidate));
-  int try_both;
-  WebPMemoryWriter mem1, mem2;
-  WebPData* encoded_data;
-  WebPMuxFrameInfo* const dst =
-      is_key_frame ? &encoded_frame->key_frame : &encoded_frame->sub_frame;
-  *dst = *info;
-  encoded_data = &dst->bitstream;
-  WebPMemoryWriterInit(&mem1);
-  WebPMemoryWriterInit(&mem2);
 
-  if (!allow_mixed) {
+  // Set frame rect and info.
-    try_lossless = config->lossless;
+  candidate->rect = *rect;
-    try_lossy = !try_lossless;
+  candidate->info = *info;
-  } else {  // Use a heuristic for trying lossless and/or lossy compression.
+  candidate->info.x_offset = rect->x_offset;
-    const int num_colors = GetColorCount(sub_frame);
+  candidate->info.y_offset = rect->y_offset;
-    try_lossless = (num_colors < MAX_COLORS_LOSSLESS);
+  candidate->info.blend_method =
-    try_lossy = (num_colors >= MIN_COLORS_LOSSY);
+      use_blending ? WEBP_MUX_BLEND : WEBP_MUX_NO_BLEND;
-  }
-  try_both = try_lossless && try_lossy;
 
-  if (try_lossless) {
+  // Encode picture.
-    WebPConfig config_ll = *config;
+  WebPMemoryWriterInit(&candidate->mem);
-    config_ll.lossless = 1;
+
-    if (!EncodeFrame(&config_ll, sub_frame, &mem1)) {
+  if (!EncodeFrame(config, sub_frame, &candidate->mem)) {
     error_code = sub_frame->error_code;
     goto Err;
   }
-  }
 
-  if (try_lossy) {
+  candidate->evaluate = 1;
-    WebPConfig config_lossy = *config;
-    config_lossy.lossless = 0;
-    if (!is_key_frame) {
-      // For lossy compression of a frame, it's better to replace transparent
-      // pixels of 'curr' with actual RGB values, whenever possible.
-      ReduceTransparency(prev_canvas, rect, frame);
-      // TODO(later): Investigate if this helps lossless compression as well.
-      FlattenSimilarBlocks(prev_canvas, rect, frame);
-    }
-    if (!EncodeFrame(&config_lossy, sub_frame, &mem2)) {
-      error_code = sub_frame->error_code;
-      goto Err;
-    }
-  }
-
-  if (try_both) {  // Pick the encoding with smallest size.
-    // TODO(later): Perhaps a rough SSIM/PSNR produced by the encoder should
-    // also be a criteria, in addition to sizes.
-    if (mem1.size <= mem2.size) {
-#if WEBP_ENCODER_ABI_VERSION > 0x0203
-      WebPMemoryWriterClear(&mem2);
-#else
-      free(mem2.mem);
-      memset(&mem2, 0, sizeof(mem2));
-#endif
-      GetEncodedData(&mem1, encoded_data);
-    } else {
-#if WEBP_ENCODER_ABI_VERSION > 0x0203
-      WebPMemoryWriterClear(&mem1);
-#else
-      free(mem1.mem);
-      memset(&mem1, 0, sizeof(mem1));
-#endif
-      GetEncodedData(&mem2, encoded_data);
-    }
-  } else {
-    GetEncodedData(try_lossless ? &mem1 : &mem2, encoded_data);
-  }
   return error_code;
 
  Err:
 #if WEBP_ENCODER_ABI_VERSION > 0x0203
-  WebPMemoryWriterClear(&mem1);
+  WebPMemoryWriterClear(&candidate->mem);
-  WebPMemoryWriterClear(&mem2);
 #else
-  free(mem1.mem);
+  free(candidate->mem.mem);
-  free(mem2.mem);
+  memset(&candidate->mem, 0, sizeof(candidate->mem));
 #endif
   return error_code;
 }
 
-#undef MIN_COLORS_LOSSY
-#undef MAX_COLORS_LOSSLESS
-
 // Returns cached frame at given 'position' index.
 static EncodedFrame* CacheGetFrame(const WebPFrameCache* const cache,
                                    size_t position) {
@@ -517,6 +580,259 @@ static EncodedFrame* CacheGetFrame(const WebPFrameCache* const cache,
   return &cache->encoded_frames[cache->start + position];
 }
 
+// Sets dispose method of the previous frame to be 'dispose_method'.
+static void SetPreviousDisposeMethod(WebPFrameCache* const cache,
+                                     WebPMuxAnimDispose dispose_method) {
+  const size_t position = cache->count - 2;
+  EncodedFrame* const prev_enc_frame = CacheGetFrame(cache, position);
+  assert(cache->count >= 2);  // As current and previous frames are in cache.
+
+  if (cache->prev_candidate_undecided) {
+    assert(dispose_method == WEBP_MUX_DISPOSE_NONE);
+    prev_enc_frame->sub_frame.dispose_method = dispose_method;
+    prev_enc_frame->key_frame.dispose_method = dispose_method;
+  } else {
+    WebPMuxFrameInfo* const prev_info = prev_enc_frame->is_key_frame
+                                        ? &prev_enc_frame->key_frame
+                                        : &prev_enc_frame->sub_frame;
+    prev_info->dispose_method = dispose_method;
+  }
+}
+
+enum {
+  LL_DISP_NONE = 0,
+  LL_DISP_BG,
+  LOSSY_DISP_NONE,
+  LOSSY_DISP_BG,
+  CANDIDATE_COUNT
+};
+
+// Generates candidates for a given dispose method given pre-filled 'rect'
+// and 'sub_frame'.
+static WebPEncodingError GenerateCandidates(
+    WebPFrameCache* const cache, Candidate candidates[CANDIDATE_COUNT],
+    WebPMuxAnimDispose dispose_method, int is_lossless, int is_key_frame,
+    const WebPFrameRect* const rect, WebPPicture* sub_frame,
+    const WebPMuxFrameInfo* const info,
+    const WebPConfig* const config_ll, const WebPConfig* const config_lossy) {
+  WebPEncodingError error_code = VP8_ENC_OK;
+  const int is_dispose_none = (dispose_method == WEBP_MUX_DISPOSE_NONE);
+  Candidate* const candidate_ll =
+      is_dispose_none ? &candidates[LL_DISP_NONE] : &candidates[LL_DISP_BG];
+  Candidate* const candidate_lossy = is_dispose_none
+                                     ? &candidates[LOSSY_DISP_NONE]
+                                     : &candidates[LOSSY_DISP_BG];
+  const WebPPicture* const prev_canvas =
+      is_dispose_none ? &cache->prev_canvas : &cache->prev_canvas_disposed;
+  WebPPicture* const curr_canvas = &cache->curr_canvas;
+  WebPPicture* const curr_canvas_tmp = &cache->curr_canvas_tmp;
+  const int use_blending =
+      !is_key_frame &&
+      IsBlendingPossible(prev_canvas, curr_canvas, rect);
+  int curr_canvas_saved = 0;  // If 'curr_canvas' is saved in 'curr_canvas_tmp'.
+
+  // Pick candidates to be tried.
+  if (!cache->allow_mixed) {
+    candidate_ll->evaluate = is_lossless;
+    candidate_lossy->evaluate = !is_lossless;
+  } else {  // Use a heuristic for trying lossless and/or lossy compression.
+    const int num_colors = GetColorCount(sub_frame);
+    candidate_ll->evaluate = (num_colors < MAX_COLORS_LOSSLESS);
+    candidate_lossy->evaluate = (num_colors >= MIN_COLORS_LOSSY);
+  }
+
+  // Generate candidates.
+  if (candidate_ll->evaluate) {
+    if (use_blending) {
+      CopyPixels(curr_canvas, curr_canvas_tmp);  // save
+      curr_canvas_saved = 1;
+      IncreaseTransparency(prev_canvas, rect, curr_canvas);
+    }
+    error_code = EncodeCandidate(sub_frame, rect, info, config_ll, use_blending,
+                                 candidate_ll);
+    if (error_code != VP8_ENC_OK) return error_code;
+    if (use_blending) {
+      CopyPixels(curr_canvas_tmp, curr_canvas);  // restore
+    }
+  }
+  if (candidate_lossy->evaluate) {
+    if (!is_key_frame) {
+      // For lossy compression of a frame, it's better to:
+      // * Replace transparent pixels of 'curr' with actual RGB values,
+      //   whenever possible, and
+      // * Replace similar blocks of pixels by a transparent block.
+      if (!curr_canvas_saved) {  // save if not already done so.
+        CopyPixels(curr_canvas, curr_canvas_tmp);
+      }
+      FlattenSimilarBlocks(prev_canvas, rect, curr_canvas);
+    }
+    error_code = EncodeCandidate(sub_frame, rect, info, config_lossy,
+                                 use_blending, candidate_lossy);
+    if (error_code != VP8_ENC_OK) return error_code;
+    if (!is_key_frame) {
+      CopyPixels(curr_canvas_tmp, curr_canvas);  // restore
+    }
+  }
+  return error_code;
+}
+
+// Pick the candidate encoded frame with smallest size and release other
+// candidates.
+// TODO(later): Perhaps a rough SSIM/PSNR produced by the encoder should
+// also be a criteria, in addition to sizes.
+static void PickBestCandidate(WebPFrameCache* const cache,
+                              Candidate* const candidates, int is_key_frame,
+                              EncodedFrame* const encoded_frame) {
+  int i;
+  int best_idx = -1;
+  size_t best_size = ~0;
+  for (i = 0; i < CANDIDATE_COUNT; ++i) {
+    if (candidates[i].evaluate) {
+      const size_t candidate_size = candidates[i].mem.size;
+      if (candidate_size < best_size) {
+        best_idx = i;
+        best_size = candidate_size;
+      }
+    }
+  }
+  assert(best_idx != -1);
+  for (i = 0; i < CANDIDATE_COUNT; ++i) {
+    if (candidates[i].evaluate) {
+      if (i == best_idx) {
+        WebPMuxFrameInfo* const dst = is_key_frame
+                                      ? &encoded_frame->key_frame
+                                      : &encoded_frame->sub_frame;
+        *dst = candidates[i].info;
+        GetEncodedData(&candidates[i].mem, &dst->bitstream);
+        if (!is_key_frame) {
+          // Note: Previous dispose method only matters for non-keyframes.
+          // Also, we don't want to modify previous dispose method that was
+          // selected when a non key-frame was assumed.
+          const WebPMuxAnimDispose prev_dispose_method =
+              (best_idx == LL_DISP_NONE || best_idx == LOSSY_DISP_NONE)
+                  ? WEBP_MUX_DISPOSE_NONE
+                  : WEBP_MUX_DISPOSE_BACKGROUND;
+          SetPreviousDisposeMethod(cache, prev_dispose_method);
+        }
+        cache->prev_webp_rect = candidates[i].rect;  // save for next frame.
+      } else {
+#if WEBP_ENCODER_ABI_VERSION > 0x0203
+        WebPMemoryWriterClear(&candidates[i].mem);
+#else
+        free(candidates[i].mem.mem);
+        memset(&candidates[i].mem, 0, sizeof(candidates[i].mem));
+#endif
+        candidates[i].evaluate = 0;
+      }
+    }
+  }
+}
+
+// Depending on the configuration, tries different compressions
+// (lossy/lossless), dispose methods, blending methods etc to encode the current
+// frame and outputs the best one in 'encoded_frame'.
+static WebPEncodingError SetFrame(WebPFrameCache* const cache,
+                                  const WebPConfig* const config,
+                                  const WebPMuxFrameInfo* const info,
+                                  const WebPFrameRect* const orig_rect,
+                                  int is_key_frame,
+                                  EncodedFrame* const encoded_frame) {
+  int i;
+  WebPEncodingError error_code = VP8_ENC_OK;
+  const WebPPicture* const prev_canvas = &cache->prev_canvas;
+  WebPPicture* const prev_canvas_disposed = &cache->prev_canvas_disposed;
+  WebPPicture* const curr_canvas = &cache->curr_canvas;
+  Candidate candidates[CANDIDATE_COUNT];
+  const int is_lossless = config->lossless;
+
+  int try_dispose_none = 1;  // Default.
+  WebPFrameRect rect_none;
+  WebPPicture sub_frame_none;
+
+  // If current frame is a key-frame, dispose method of previous frame doesn't
+  // matter, so we don't try dispose to background.
+  // Also, if keyframe insertion is on, and previous frame could be picked as
+  // either a sub-frame or a keyframe, then we can't be sure about what frame
+  // rectangle would be disposed. In that case too, we don't try dispose to
+  // background.
+  const int dispose_bg_possible =
+      !is_key_frame && !cache->prev_candidate_undecided;
+  int try_dispose_bg = 0;  // Default.
+  WebPFrameRect rect_bg;
+  WebPPicture sub_frame_bg;
+
+  WebPConfig config_ll = *config;
+  WebPConfig config_lossy = *config;
+  config_ll.lossless = 1;
+  config_lossy.lossless = 0;
+
+  if (!WebPPictureInit(&sub_frame_none) || !WebPPictureInit(&sub_frame_bg)) {
+    return VP8_ENC_ERROR_INVALID_CONFIGURATION;
+  }
+
+  for (i = 0; i < CANDIDATE_COUNT; ++i) {
+    candidates[i].evaluate = 0;
+  }
+
+  // Change-rectangle assuming previous frame was DISPOSE_NONE.
+  GetSubRect(prev_canvas, curr_canvas, orig_rect, is_key_frame,
+             &rect_none, &sub_frame_none);
+
+  if (dispose_bg_possible) {
+    // Change-rectangle assuming previous frame was DISPOSE_BACKGROUND.
+    CopyPixels(prev_canvas, prev_canvas_disposed);
+    DisposeFrameRectangle(WEBP_MUX_DISPOSE_BACKGROUND, &cache->prev_webp_rect,
+                          prev_canvas_disposed);
+    GetSubRect(prev_canvas_disposed, curr_canvas, orig_rect, is_key_frame,
+               &rect_bg, &sub_frame_bg);
+
+    if (RectArea(&rect_bg) < RectArea(&rect_none)) {
+      try_dispose_bg = 1;  // Pick DISPOSE_BACKGROUND.
+      try_dispose_none = 0;
+    }
+  }
+
+  if (try_dispose_none) {
+    error_code = GenerateCandidates(
+        cache, candidates, WEBP_MUX_DISPOSE_NONE, is_lossless, is_key_frame,
+        &rect_none, &sub_frame_none, info, &config_ll, &config_lossy);
+    if (error_code != VP8_ENC_OK) goto Err;
+  }
+
+  if (try_dispose_bg) {
+    assert(!cache->is_first_frame);
+    assert(dispose_bg_possible);
+    error_code = GenerateCandidates(
+        cache, candidates, WEBP_MUX_DISPOSE_BACKGROUND, is_lossless,
+        is_key_frame, &rect_bg, &sub_frame_bg, info, &config_ll, &config_lossy);
+    if (error_code != VP8_ENC_OK) goto Err;
+  }
+
+  PickBestCandidate(cache, candidates, is_key_frame, encoded_frame);
+
+  goto End;
+
+ Err:
+  for (i = 0; i < CANDIDATE_COUNT; ++i) {
+    if (candidates[i].evaluate) {
+#if WEBP_ENCODER_ABI_VERSION > 0x0203
+      WebPMemoryWriterClear(&candidates[i].mem);
+#else
+      free(candidates[i].mem.mem);
+      memset(&candidates[i].mem, 0, sizeof(candidates[i].mem));
+#endif
+    }
+  }
+
+ End:
+  WebPPictureFree(&sub_frame_none);
+  WebPPictureFree(&sub_frame_bg);
+  return error_code;
+}
+
+#undef MIN_COLORS_LOSSY
+#undef MAX_COLORS_LOSSLESS
+
 // Calculate the penalty incurred if we encode given frame as a key frame
 // instead of a sub-frame.
 static int64_t KeyFramePenalty(const EncodedFrame* const encoded_frame) {
@@ -524,27 +840,17 @@ static int64_t KeyFramePenalty(const EncodedFrame* const encoded_frame) {
           encoded_frame->sub_frame.bitstream.size);
 }
 
-static void DisposeFrame(WebPMuxAnimDispose dispose_method,
-                         const WebPFrameRect* const gif_rect,
-                         WebPPicture* const frame, WebPPicture* const canvas) {
-  if (dispose_method == WEBP_MUX_DISPOSE_BACKGROUND) {
-    WebPUtilClearPic(frame, NULL);
-    WebPUtilClearPic(canvas, gif_rect);
-  }
-}
-
 int WebPFrameCacheAddFrame(WebPFrameCache* const cache,
                            const WebPConfig* const config,
                            const WebPFrameRect* const orig_rect_ptr,
                            WebPPicture* const frame,
                            WebPMuxFrameInfo* const info) {
+  // Initialize.
   int ok = 0;
   WebPEncodingError error_code = VP8_ENC_OK;
-  WebPFrameRect rect;
-  WebPPicture sub_image;  // View extracted from 'frame' with rectangle 'rect'.
   WebPPicture* const prev_canvas = &cache->prev_canvas;
+  WebPPicture* const curr_canvas = &cache->curr_canvas;
   const size_t position = cache->count;
-  const int allow_mixed = cache->allow_mixed;
   EncodedFrame* const encoded_frame = CacheGetFrame(cache, position);
   WebPFrameRect orig_rect;
   assert(position < cache->size);
@@ -553,6 +859,12 @@ int WebPFrameCacheAddFrame(WebPFrameCache* const cache,
     return 0;
   }
 
+  // As we are encoding (part of) 'curr_canvas', and not 'frame' directly, make
+  // sure the progress is still reported back.
+  curr_canvas->progress_hook = frame->progress_hook;
+  curr_canvas->user_data = frame->user_data;
+  curr_canvas->stats = frame->stats;
+
   if (orig_rect_ptr == NULL) {
     orig_rect.width = frame->width;
     orig_rect.height = frame->height;
@@ -562,99 +874,93 @@ int WebPFrameCacheAddFrame(WebPFrameCache* const cache,
     orig_rect = *orig_rect_ptr;
   }
 
-  // Snap to even offsets (and adjust dimensions if needed).
+  // Main frame addition.
-  rect = orig_rect;
-  rect.width += (rect.x_offset & 1);
-  rect.height += (rect.y_offset & 1);
-  rect.x_offset &= ~1;
-  rect.y_offset &= ~1;
-
-  if (!WebPPictureView(frame, rect.x_offset, rect.y_offset,
-                       rect.width, rect.height, &sub_image)) {
-    return 0;
-  }
-  info->x_offset = rect.x_offset;
-  info->y_offset = rect.y_offset;
-
   ++cache->count;
 
-  if (cache->is_first_frame || IsKeyFrame(frame, &rect, prev_canvas)) {
+  if (cache->is_first_frame) {
+    // 'curr_canvas' is same as 'frame'.
+    CopyPixels(frame, curr_canvas);
     // Add this as a key frame.
-    error_code = SetFrame(config, allow_mixed, 1, NULL, NULL, NULL,
+    // Note: we use original rectangle as-is for the first frame.
-                          info, &sub_image, encoded_frame);
+    error_code = SetFrame(cache, config, info, &orig_rect, 1, encoded_frame);
     if (error_code != VP8_ENC_OK) {
       goto End;
     }
-    cache->keyframe = position;
+    assert(position == 0 && cache->count == 1);
-    cache->flush_count = cache->count;
+    encoded_frame->is_key_frame = 1;
+    cache->flush_count = 0;
     cache->count_since_key_frame = 0;
-    // Update prev_canvas by simply copying from 'curr'.
+    cache->prev_candidate_undecided = 0;
-    CopyPixels(frame, prev_canvas);
   } else {
+    // Store previous canvas.
+    CopyPixels(curr_canvas, prev_canvas);
+    // Create curr_canvas:
+    // * Start with disposed previous canvas.
+    // * Then blend 'frame' onto it.
+    DisposeFrameRectangle(cache->prev_orig_dispose, &cache->prev_orig_rect,
+                          curr_canvas);
+    BlendPixels(frame, &orig_rect, curr_canvas);
+
     ++cache->count_since_key_frame;
     if (cache->count_since_key_frame <= cache->kmin) {
       // Add this as a frame rectangle.
-      error_code = SetFrame(config, allow_mixed, 0, prev_canvas, frame,
+      error_code = SetFrame(cache, config, info, NULL, 0, encoded_frame);
-                            &rect, info, &sub_image, encoded_frame);
       if (error_code != VP8_ENC_OK) {
         goto End;
       }
-      cache->flush_count = cache->count;
+      encoded_frame->is_key_frame = 0;
-      // Update prev_canvas by blending 'curr' into it.
+      cache->flush_count = cache->count - 1;
-      BlendPixels(frame, &orig_rect, prev_canvas);
+      cache->prev_candidate_undecided = 0;
     } else {
-      WebPPicture full_image;
       WebPMuxFrameInfo full_image_info;
       int64_t curr_delta;
 
       // Add frame rectangle to cache.
-      error_code = SetFrame(config, allow_mixed, 0, prev_canvas, frame, &rect,
+      error_code = SetFrame(cache, config, info, NULL, 0, encoded_frame);
-                            info, &sub_image, encoded_frame);
       if (error_code != VP8_ENC_OK) {
         goto End;
       }
 
-      // Convert to a key frame.
-      CopyPixels(frame, &cache->curr_canvas);
-      ConvertToKeyFrame(prev_canvas, &rect, &cache->curr_canvas);
-      if (!WebPPictureView(&cache->curr_canvas, rect.x_offset, rect.y_offset,
-                           rect.width, rect.height, &full_image)) {
-        goto End;
-      }
-      full_image_info = *info;
-      full_image_info.x_offset = rect.x_offset;
-      full_image_info.y_offset = rect.y_offset;
-
       // Add key frame to cache, too.
-      error_code = SetFrame(config, allow_mixed, 1, NULL, NULL, NULL,
+      full_image_info = *info;
-                            &full_image_info, &full_image, encoded_frame);
+      full_image_info.x_offset = 0;
-      WebPPictureFree(&full_image);
+      full_image_info.y_offset = 0;
+      error_code =
+          SetFrame(cache, config, &full_image_info, NULL, 1, encoded_frame);
       if (error_code != VP8_ENC_OK) goto End;
 
       // Analyze size difference of the two variants.
       curr_delta = KeyFramePenalty(encoded_frame);
       if (curr_delta <= cache->best_delta) {  // Pick this as keyframe.
+        if (cache->keyframe != KEYFRAME_NONE) {
+          EncodedFrame* const old_keyframe =
+              CacheGetFrame(cache, cache->keyframe);
+          assert(old_keyframe->is_key_frame);
+          old_keyframe->is_key_frame = 0;
+        }
+        encoded_frame->is_key_frame = 1;
         cache->keyframe = position;
         cache->best_delta = curr_delta;
         cache->flush_count = cache->count - 1;  // We can flush previous frames.
+      } else {
+        encoded_frame->is_key_frame = 0;
       }
       if (cache->count_since_key_frame == cache->kmax) {
-        cache->flush_count = cache->count;
+        cache->flush_count = cache->count - 1;
         cache->count_since_key_frame = 0;
       }
-
+      cache->prev_candidate_undecided = 1;
-      // Update prev_canvas by simply copying from 'curr_canvas'.
-      CopyPixels(&cache->curr_canvas, prev_canvas);
     }
   }
 
-  DisposeFrame(info->dispose_method, &orig_rect, frame, prev_canvas);
+  DisposeFullFrame(info->dispose_method, frame);
 
   cache->is_first_frame = 0;
+  cache->prev_orig_dispose = info->dispose_method;
+  cache->prev_orig_rect = orig_rect;
   ok = 1;
 
  End:
-  WebPPictureFree(&sub_image);
   if (!ok) {
     FrameRelease(encoded_frame);
     --cache->count;  // We reset the count, as the frame addition failed.
@@ -671,14 +977,14 @@ WebPMuxError WebPFrameCacheFlush(WebPFrameCache* const cache, int verbose,
     WebPMuxError err;
     EncodedFrame* const curr = CacheGetFrame(cache, 0);
     // Pick frame or full canvas.
-    if (cache->keyframe == 0) {
+    if (curr->is_key_frame) {
       info = &curr->key_frame;
-      info->blend_method = WEBP_MUX_NO_BLEND;
+      if (cache->keyframe == 0) {
         cache->keyframe = KEYFRAME_NONE;
         cache->best_delta = DELTA_INFINITY;
+      }
     } else {
       info = &curr->sub_frame;
-      info->blend_method = WEBP_MUX_BLEND;
     }
     // Add to mux.
     err = WebPMuxPushFrame(mux, info, 1);
@@ -695,7 +1001,15 @@ WebPMuxError WebPFrameCacheFlush(WebPFrameCache* const cache, int verbose,
     if (cache->keyframe != KEYFRAME_NONE) --cache->keyframe;
   }
 
-  if (cache->count == 0) CacheReset(cache);
+  if (cache->count == 1 && cache->start != 0) {
+    // Move cache->start to index 0.
+    const int cache_start_tmp = (int)cache->start;
+    EncodedFrame temp = cache->encoded_frames[0];
+    cache->encoded_frames[0] = cache->encoded_frames[cache_start_tmp];
+    cache->encoded_frames[cache_start_tmp] = temp;
+    FrameRelease(&cache->encoded_frames[cache_start_tmp]);
+    cache->start = 0;
+  }
   return WEBP_MUX_OK;
 }