Compile Skia image encoders

third_party/libwebp/mux/anim_encode.c

+// Copyright 2014 Google Inc. All Rights Reserved.
+//
+// Use of this source code is governed by a BSD-style license
+// that can be found in the COPYING file in the root of the source
+// tree. An additional intellectual property rights grant can be found
+// in the file PATENTS. All contributing project authors may
+// be found in the AUTHORS file in the root of the source tree.
+// -----------------------------------------------------------------------------
+//
+//  AnimEncoder implementation.
+//
+
+#include <assert.h>
+#include <limits.h>
+#include <math.h>    // for pow()
+#include <stdio.h>
+#include <stdlib.h>  // for abs()
+
+#include "../mux/animi.h"
+#include "../utils/utils.h"
+#include "../webp/decode.h"
+#include "../webp/encode.h"
+#include "../webp/format_constants.h"
+#include "../webp/mux.h"
+
+#if defined(_MSC_VER) && _MSC_VER < 1900
+#define snprintf _snprintf
+#endif
+
+#define ERROR_STR_MAX_LENGTH 100
+
+//------------------------------------------------------------------------------
+// Internal structs.
+
+// Stores frame rectangle dimensions.
+typedef struct {
+  int x_offset_, y_offset_, width_, height_;
+} FrameRect;
+
+// Used to store two candidates of encoded data for an animation frame. One of
+// the two will be chosen later.
+typedef struct {
+  WebPMuxFrameInfo sub_frame_;  // Encoded frame rectangle.
+  WebPMuxFrameInfo key_frame_;  // Encoded frame if it is a key-frame.
+  int is_key_frame_;            // True if 'key_frame' has been chosen.
+} EncodedFrame;
+
+struct WebPAnimEncoder {
+  const int canvas_width_;                  // Canvas width.
+  const int canvas_height_;                 // Canvas height.
+  const WebPAnimEncoderOptions options_;    // Global encoding options.
+
+  FrameRect prev_rect_;               // Previous WebP frame rectangle.
+  WebPConfig last_config_;            // Cached in case a re-encode is needed.
+  WebPConfig last_config_reversed_;   // If 'last_config_' uses lossless, then
+                                      // this config uses lossy and vice versa;
+                                      // only valid if 'options_.allow_mixed'
+                                      // is true.
+
+  WebPPicture* curr_canvas_;          // Only pointer; we don't own memory.
+
+  // Canvas buffers.
+  WebPPicture curr_canvas_copy_;      // Possibly modified current canvas.
+  int curr_canvas_copy_modified_;     // True if pixels in 'curr_canvas_copy_'
+                                      // differ from those in 'curr_canvas_'.
+
+  WebPPicture prev_canvas_;           // Previous canvas.
+  WebPPicture prev_canvas_disposed_;  // Previous canvas disposed to background.
+
+  // Encoded data.
+  EncodedFrame* encoded_frames_;      // Array of encoded frames.
+  size_t size_;             // Number of allocated frames.
+  size_t start_;            // Frame start index.
+  size_t count_;            // Number of valid frames.
+  size_t flush_count_;      // If >0, 'flush_count' frames starting from
+                            // 'start' are ready to be added to mux.
+
+  // key-frame related.
+  int64_t best_delta_;      // min(canvas size - frame size) over the frames.
+                            // Can be negative in certain cases due to
+                            // transparent pixels in a frame.
+  int keyframe_;            // Index of selected key-frame relative to 'start_'.
+  int count_since_key_frame_;     // Frames seen since the last key-frame.
+
+  int first_timestamp_;           // Timestamp of the first frame.
+  int prev_timestamp_;            // Timestamp of the last added frame.
+  int prev_candidate_undecided_;  // True if it's not yet decided if previous
+                                  // frame would be a sub-frame or a key-frame.
+
+  // Misc.
+  int is_first_frame_;  // True if first frame is yet to be added/being added.
+  int got_null_frame_;  // True if WebPAnimEncoderAdd() has already been called
+                        // with a NULL frame.
+
+  size_t in_frame_count_;   // Number of input frames processed so far.
+  size_t out_frame_count_;  // Number of frames added to mux so far. This may be
+                            // different from 'in_frame_count_' due to merging.
+
+  WebPMux* mux_;        // Muxer to assemble the WebP bitstream.
+  char error_str_[ERROR_STR_MAX_LENGTH];  // Error string. Empty if no error.
+};
+
+// -----------------------------------------------------------------------------
+// Life of WebPAnimEncoder object.
+
+#define DELTA_INFINITY      (1ULL << 32)
+#define KEYFRAME_NONE       (-1)
+
+// Reset the counters in the WebPAnimEncoder.
+static void ResetCounters(WebPAnimEncoder* const enc) {
+  enc->start_ = 0;
+  enc->count_ = 0;
+  enc->flush_count_ = 0;
+  enc->best_delta_ = DELTA_INFINITY;
+  enc->keyframe_ = KEYFRAME_NONE;
+}
+
+static void DisableKeyframes(WebPAnimEncoderOptions* const enc_options) {
+  enc_options->kmax = INT_MAX;
+  enc_options->kmin = enc_options->kmax - 1;
+}
+
+#define MAX_CACHED_FRAMES 30
+
+static void SanitizeEncoderOptions(WebPAnimEncoderOptions* const enc_options) {
+  int print_warning = enc_options->verbose;
+
+  if (enc_options->minimize_size) {
+    DisableKeyframes(enc_options);
+  }
+
+  if (enc_options->kmax == 1) {  // All frames will be key-frames.
+    enc_options->kmin = 0;
+    enc_options->kmax = 0;
+    return;
+  } else if (enc_options->kmax <= 0) {
+    DisableKeyframes(enc_options);
+    print_warning = 0;
+  }
+
+  if (enc_options->kmin >= enc_options->kmax) {
+    enc_options->kmin = enc_options->kmax - 1;
+    if (print_warning) {
+      fprintf(stderr, "WARNING: Setting kmin = %d, so that kmin < kmax.\n",
+              enc_options->kmin);
+    }
+  } else {
+    const int kmin_limit = enc_options->kmax / 2 + 1;
+    if (enc_options->kmin < kmin_limit && kmin_limit < enc_options->kmax) {
+      // This ensures that enc.keyframe + kmin >= kmax is always true. So, we
+      // can flush all the frames in the 'count_since_key_frame == kmax' case.
+      enc_options->kmin = kmin_limit;
+      if (print_warning) {
+        fprintf(stderr,
+                "WARNING: Setting kmin = %d, so that kmin >= kmax / 2 + 1.\n",
+                enc_options->kmin);
+      }
+    }
+  }
+  // Limit the max number of frames that are allocated.
+  if (enc_options->kmax - enc_options->kmin > MAX_CACHED_FRAMES) {
+    enc_options->kmin = enc_options->kmax - MAX_CACHED_FRAMES;
+    if (print_warning) {
+      fprintf(stderr,
+              "WARNING: Setting kmin = %d, so that kmax - kmin <= %d.\n",
+              enc_options->kmin, MAX_CACHED_FRAMES);
+    }
+  }
+  assert(enc_options->kmin < enc_options->kmax);
+}
+
+#undef MAX_CACHED_FRAMES
+
+static void DefaultEncoderOptions(WebPAnimEncoderOptions* const enc_options) {
+  enc_options->anim_params.loop_count = 0;
+  enc_options->anim_params.bgcolor = 0xffffffff;  // White.
+  enc_options->minimize_size = 0;
+  DisableKeyframes(enc_options);
+  enc_options->allow_mixed = 0;
+  enc_options->verbose = 0;
+}
+
+int WebPAnimEncoderOptionsInitInternal(WebPAnimEncoderOptions* enc_options,
+                                       int abi_version) {
+  if (enc_options == NULL ||
+      WEBP_ABI_IS_INCOMPATIBLE(abi_version, WEBP_MUX_ABI_VERSION)) {
+    return 0;
+  }
+  DefaultEncoderOptions(enc_options);
+  return 1;
+}
+
+// This starting value is more fit to WebPCleanupTransparentAreaLossless().
+#define TRANSPARENT_COLOR   0x00000000
+
+static void ClearRectangle(WebPPicture* const picture,
+                           int left, int top, int width, int height) {
+  int j;
+  for (j = top; j < top + height; ++j) {
+    uint32_t* const dst = picture->argb + j * picture->argb_stride;
+    int i;
+    for (i = left; i < left + width; ++i) {
+      dst[i] = TRANSPARENT_COLOR;
+    }
+  }
+}
+
+static void WebPUtilClearPic(WebPPicture* const picture,
+                             const FrameRect* const rect) {
+  if (rect != NULL) {
+    ClearRectangle(picture, rect->x_offset_, rect->y_offset_,
+                   rect->width_, rect->height_);
+  } else {
+    ClearRectangle(picture, 0, 0, picture->width, picture->height);
+  }
+}
+
+static void MarkNoError(WebPAnimEncoder* const enc) {
+  enc->error_str_[0] = '\0';  // Empty string.
+}
+
+static void MarkError(WebPAnimEncoder* const enc, const char* str) {
+  if (snprintf(enc->error_str_, ERROR_STR_MAX_LENGTH, "%s.", str) < 0) {
+    assert(0);  // FIX ME!
+  }
+}
+
+static void MarkError2(WebPAnimEncoder* const enc,
+                       const char* str, int error_code) {
+  if (snprintf(enc->error_str_, ERROR_STR_MAX_LENGTH, "%s: %d.", str,
+               error_code) < 0) {
+    assert(0);  // FIX ME!
+  }
+}
+
+WebPAnimEncoder* WebPAnimEncoderNewInternal(
+    int width, int height, const WebPAnimEncoderOptions* enc_options,
+    int abi_version) {
+  WebPAnimEncoder* enc;
+
+  if (WEBP_ABI_IS_INCOMPATIBLE(abi_version, WEBP_MUX_ABI_VERSION)) {
+    return NULL;
+  }
+  if (width <= 0 || height <= 0 ||
+      (width * (uint64_t)height) >= MAX_IMAGE_AREA) {
+    return NULL;
+  }
+
+  enc = (WebPAnimEncoder*)WebPSafeCalloc(1, sizeof(*enc));
+  if (enc == NULL) return NULL;
+  // sanity inits, so we can call WebPAnimEncoderDelete():
+  enc->encoded_frames_ = NULL;
+  enc->mux_ = NULL;
+  MarkNoError(enc);
+
+  // Dimensions and options.
+  *(int*)&enc->canvas_width_ = width;
+  *(int*)&enc->canvas_height_ = height;
+  if (enc_options != NULL) {
+    *(WebPAnimEncoderOptions*)&enc->options_ = *enc_options;
+    SanitizeEncoderOptions((WebPAnimEncoderOptions*)&enc->options_);
+  } else {
+    DefaultEncoderOptions((WebPAnimEncoderOptions*)&enc->options_);
+  }
+
+  // Canvas buffers.
+  if (!WebPPictureInit(&enc->curr_canvas_copy_) ||
+      !WebPPictureInit(&enc->prev_canvas_) ||
+      !WebPPictureInit(&enc->prev_canvas_disposed_)) {
+    goto Err;
+  }
+  enc->curr_canvas_copy_.width = width;
+  enc->curr_canvas_copy_.height = height;
+  enc->curr_canvas_copy_.use_argb = 1;
+  if (!WebPPictureAlloc(&enc->curr_canvas_copy_) ||
+      !WebPPictureCopy(&enc->curr_canvas_copy_, &enc->prev_canvas_) ||
+      !WebPPictureCopy(&enc->curr_canvas_copy_, &enc->prev_canvas_disposed_)) {
+    goto Err;
+  }
+  WebPUtilClearPic(&enc->prev_canvas_, NULL);
+  enc->curr_canvas_copy_modified_ = 1;
+
+  // Encoded frames.
+  ResetCounters(enc);
+  // Note: one extra storage is for the previous frame.
+  enc->size_ = enc->options_.kmax - enc->options_.kmin + 1;
+  // We need space for at least 2 frames. But when kmin, kmax are both zero,
+  // enc->size_ will be 1. So we handle that special case below.
+  if (enc->size_ < 2) enc->size_ = 2;
+  enc->encoded_frames_ =
+      (EncodedFrame*)WebPSafeCalloc(enc->size_, sizeof(*enc->encoded_frames_));
+  if (enc->encoded_frames_ == NULL) goto Err;
+
+  enc->mux_ = WebPMuxNew();
+  if (enc->mux_ == NULL) goto Err;
+
+  enc->count_since_key_frame_ = 0;
+  enc->first_timestamp_ = 0;
+  enc->prev_timestamp_ = 0;
+  enc->prev_candidate_undecided_ = 0;
+  enc->is_first_frame_ = 1;
+  enc->got_null_frame_ = 0;
+
+  return enc;  // All OK.
+
+ Err:
+  WebPAnimEncoderDelete(enc);
+  return NULL;
+}
+
+// Release the data contained by 'encoded_frame'.
+static void FrameRelease(EncodedFrame* const encoded_frame) {
+  if (encoded_frame != NULL) {
+    WebPDataClear(&encoded_frame->sub_frame_.bitstream);
+    WebPDataClear(&encoded_frame->key_frame_.bitstream);
+    memset(encoded_frame, 0, sizeof(*encoded_frame));
+  }
+}
+
+void WebPAnimEncoderDelete(WebPAnimEncoder* enc) {
+  if (enc != NULL) {
+    WebPPictureFree(&enc->curr_canvas_copy_);
+    WebPPictureFree(&enc->prev_canvas_);
+    WebPPictureFree(&enc->prev_canvas_disposed_);
+    if (enc->encoded_frames_ != NULL) {
+      size_t i;
+      for (i = 0; i < enc->size_; ++i) {
+        FrameRelease(&enc->encoded_frames_[i]);
+      }
+      WebPSafeFree(enc->encoded_frames_);
+    }
+    WebPMuxDelete(enc->mux_);
+    WebPSafeFree(enc);
+  }
+}
+
+// -----------------------------------------------------------------------------
+// Frame addition.
+
+// Returns cached frame at the given 'position'.
+static EncodedFrame* GetFrame(const WebPAnimEncoder* const enc,
+                              size_t position) {
+  assert(enc->start_ + position < enc->size_);
+  return &enc->encoded_frames_[enc->start_ + position];
+}
+
+typedef int (*ComparePixelsFunc)(const uint32_t*, int, const uint32_t*, int,
+                                 int, int);
+
+// Returns true if 'length' number of pixels in 'src' and 'dst' are equal,
+// assuming the given step sizes between pixels.
+// 'max_allowed_diff' is unused and only there to allow function pointer use.
+static WEBP_INLINE int ComparePixelsLossless(const uint32_t* src, int src_step,
+                                             const uint32_t* dst, int dst_step,
+                                             int length, int max_allowed_diff) {
+  (void)max_allowed_diff;
+  assert(length > 0);
+  while (length-- > 0) {
+    if (*src != *dst) {
+      return 0;
+    }
+    src += src_step;
+    dst += dst_step;
+  }
+  return 1;
+}
+
+// Helper to check if each channel in 'src' and 'dst' is at most off by
+// 'max_allowed_diff'.
+static WEBP_INLINE int PixelsAreSimilar(uint32_t src, uint32_t dst,
+                                        int max_allowed_diff) {
+  const int src_a = (src >> 24) & 0xff;
+  const int src_r = (src >> 16) & 0xff;
+  const int src_g = (src >> 8) & 0xff;
+  const int src_b = (src >> 0) & 0xff;
+  const int dst_a = (dst >> 24) & 0xff;
+  const int dst_r = (dst >> 16) & 0xff;
+  const int dst_g = (dst >> 8) & 0xff;
+  const int dst_b = (dst >> 0) & 0xff;
+
+  return (src_a == dst_a) &&
+         (abs(src_r - dst_r) * dst_a <= (max_allowed_diff * 255)) &&
+         (abs(src_g - dst_g) * dst_a <= (max_allowed_diff * 255)) &&
+         (abs(src_b - dst_b) * dst_a <= (max_allowed_diff * 255));
+}
+
+// Returns true if 'length' number of pixels in 'src' and 'dst' are within an
+// error bound, assuming the given step sizes between pixels.
+static WEBP_INLINE int ComparePixelsLossy(const uint32_t* src, int src_step,
+                                          const uint32_t* dst, int dst_step,
+                                          int length, int max_allowed_diff) {
+  assert(length > 0);
+  while (length-- > 0) {
+    if (!PixelsAreSimilar(*src, *dst, max_allowed_diff)) {
+      return 0;
+    }
+    src += src_step;
+    dst += dst_step;
+  }
+  return 1;
+}
+
+static int IsEmptyRect(const FrameRect* const rect) {
+  return (rect->width_ == 0) || (rect->height_ == 0);
+}
+
+static int QualityToMaxDiff(float quality) {
+  const double val = pow(quality / 100., 0.5);
+  const double max_diff = 31 * (1 - val) + 1 * val;
+  return (int)(max_diff + 0.5);
+}
+
+// Assumes that an initial valid guess of change rectangle 'rect' is passed.
+static void MinimizeChangeRectangle(const WebPPicture* const src,
+                                    const WebPPicture* const dst,
+                                    FrameRect* const rect,
+                                    int is_lossless, float quality) {
+  int i, j;
+  const ComparePixelsFunc compare_pixels =
+      is_lossless ? ComparePixelsLossless : ComparePixelsLossy;
+  const int max_allowed_diff_lossy = QualityToMaxDiff(quality);
+  const int max_allowed_diff = is_lossless ? 0 : max_allowed_diff_lossy;
+
+  // 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 (compare_pixels(src_argb, src->argb_stride, dst_argb, dst->argb_stride,
+                       rect->height_, max_allowed_diff)) {
+      --rect->width_;  // Redundant column.
+      ++rect->x_offset_;
+    } else {
+      break;
+    }
+  }
+  if (rect->width_ == 0) goto NoChange;
+
+  // 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 (compare_pixels(src_argb, src->argb_stride, dst_argb, dst->argb_stride,
+                       rect->height_, max_allowed_diff)) {
+      --rect->width_;  // Redundant column.
+    } else {
+      break;
+    }
+  }
+  if (rect->width_ == 0) goto NoChange;
+
+  // 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 (compare_pixels(src_argb, 1, dst_argb, 1, rect->width_,
+                       max_allowed_diff)) {
+      --rect->height_;  // Redundant row.
+      ++rect->y_offset_;
+    } else {
+      break;
+    }
+  }
+  if (rect->height_ == 0) goto NoChange;
+
+  // 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 (compare_pixels(src_argb, 1, dst_argb, 1, rect->width_,
+                       max_allowed_diff)) {
+      --rect->height_;  // Redundant row.
+    } else {
+      break;
+    }
+  }
+  if (rect->height_ == 0) goto NoChange;
+
+  if (IsEmptyRect(rect)) {
+ NoChange:
+    rect->x_offset_ = 0;
+    rect->y_offset_ = 0;
+    rect->width_ = 0;
+    rect->height_ = 0;
+  }
+}
+
+// Snap rectangle to even offsets (and adjust dimensions if needed).
+static WEBP_INLINE void SnapToEvenOffsets(FrameRect* const rect) {
+  rect->width_ += (rect->x_offset_ & 1);
+  rect->height_ += (rect->y_offset_ & 1);
+  rect->x_offset_ &= ~1;
+  rect->y_offset_ &= ~1;
+}
+
+typedef struct {
+  int should_try_;               // Should try this set of parameters.
+  int empty_rect_allowed_;       // Frame with empty rectangle can be skipped.
+  FrameRect rect_ll_;            // Frame rectangle for lossless compression.
+  WebPPicture sub_frame_ll_;     // Sub-frame pic for lossless compression.
+  FrameRect rect_lossy_;         // Frame rectangle for lossy compression.
+                                 // Could be smaller than rect_ll_ as pixels
+                                 // with small diffs can be ignored.
+  WebPPicture sub_frame_lossy_;  // Sub-frame pic for lossless compression.
+} SubFrameParams;
+
+static int SubFrameParamsInit(SubFrameParams* const params,
+                              int should_try, int empty_rect_allowed) {
+  params->should_try_ = should_try;
+  params->empty_rect_allowed_ = empty_rect_allowed;
+  if (!WebPPictureInit(&params->sub_frame_ll_) ||
+      !WebPPictureInit(&params->sub_frame_lossy_)) {
+    return 0;
+  }
+  return 1;
+}
+
+static void SubFrameParamsFree(SubFrameParams* const params) {
+  WebPPictureFree(&params->sub_frame_ll_);
+  WebPPictureFree(&params->sub_frame_lossy_);
+}
+
+// Given previous and current canvas, picks the optimal rectangle for the
+// current frame based on 'is_lossless' and other parameters. Assumes that the
+// initial guess 'rect' is valid.
+static int GetSubRect(const WebPPicture* const prev_canvas,
+                      const WebPPicture* const curr_canvas, int is_key_frame,
+                      int is_first_frame, int empty_rect_allowed,
+                      int is_lossless, float quality, FrameRect* const rect,
+                      WebPPicture* const sub_frame) {
+  if (!is_key_frame || is_first_frame) {  // Optimize frame rectangle.
+    // Note: This behaves as expected for first frame, as 'prev_canvas' is
+    // initialized to a fully transparent canvas in the beginning.
+    MinimizeChangeRectangle(prev_canvas, curr_canvas, rect,
+                            is_lossless, quality);
+  }
+
+  if (IsEmptyRect(rect)) {
+    if (empty_rect_allowed) {  // No need to get 'sub_frame'.
+      return 1;
+    } else {                   // Force a 1x1 rectangle.
+      rect->width_ = 1;
+      rect->height_ = 1;
+      assert(rect->x_offset_ == 0);
+      assert(rect->y_offset_ == 0);
+    }
+  }
+
+  SnapToEvenOffsets(rect);
+  return WebPPictureView(curr_canvas, rect->x_offset_, rect->y_offset_,
+                         rect->width_, rect->height_, sub_frame);
+}
+
+// Picks optimal frame rectangle for both lossless and lossy compression. The
+// initial guess for frame rectangles will be the full canvas.
+static int GetSubRects(const WebPPicture* const prev_canvas,
+                       const WebPPicture* const curr_canvas, int is_key_frame,
+                       int is_first_frame, float quality,
+                       SubFrameParams* const params) {
+  // Lossless frame rectangle.
+  params->rect_ll_.x_offset_ = 0;
+  params->rect_ll_.y_offset_ = 0;
+  params->rect_ll_.width_ = curr_canvas->width;
+  params->rect_ll_.height_ = curr_canvas->height;
+  if (!GetSubRect(prev_canvas, curr_canvas, is_key_frame, is_first_frame,
+                  params->empty_rect_allowed_, 1, quality,
+                  &params->rect_ll_, &params->sub_frame_ll_)) {
+    return 0;
+  }
+  // Lossy frame rectangle.
+  params->rect_lossy_ = params->rect_ll_;  // seed with lossless rect.
+  return GetSubRect(prev_canvas, curr_canvas, is_key_frame, is_first_frame,
+                    params->empty_rect_allowed_, 0, quality,
+                    &params->rect_lossy_, &params->sub_frame_lossy_);
+}
+
+static WEBP_INLINE int clip(int v, int min_v, int max_v) {
+  return (v < min_v) ? min_v : (v > max_v) ? max_v : v;
+}
+
+int WebPAnimEncoderRefineRect(
+    const WebPPicture* const prev_canvas, const WebPPicture* const curr_canvas,
+    int is_lossless, float quality, int* const x_offset, int* const y_offset,
+    int* const width, int* const height) {
+  FrameRect rect;
+  const int right = clip(*x_offset + *width, 0, curr_canvas->width);
+  const int left = clip(*x_offset, 0, curr_canvas->width - 1);
+  const int bottom = clip(*y_offset + *height, 0, curr_canvas->height);
+  const int top = clip(*y_offset, 0, curr_canvas->height - 1);
+  if (prev_canvas == NULL || curr_canvas == NULL ||
+      prev_canvas->width != curr_canvas->width ||
+      prev_canvas->height != curr_canvas->height ||
+      !prev_canvas->use_argb || !curr_canvas->use_argb) {
+    return 0;
+  }
+  rect.x_offset_ = left;
+  rect.y_offset_ = top;
+  rect.width_ = clip(right - left, 0, curr_canvas->width - rect.x_offset_);
+  rect.height_ = clip(bottom - top, 0, curr_canvas->height - rect.y_offset_);
+  MinimizeChangeRectangle(prev_canvas, curr_canvas, &rect, is_lossless,
+                          quality);
+  SnapToEvenOffsets(&rect);
+  *x_offset = rect.x_offset_;
+  *y_offset = rect.y_offset_;
+  *width = rect.width_;
+  *height = rect.height_;
+  return 1;
+}
+
+static void DisposeFrameRectangle(int dispose_method,
+                                  const FrameRect* const rect,
+                                  WebPPicture* const curr_canvas) {
+  assert(rect != NULL);
+  if (dispose_method == WEBP_MUX_DISPOSE_BACKGROUND) {
+    WebPUtilClearPic(curr_canvas, rect);
+  }
+}
+
+static uint32_t RectArea(const FrameRect* const rect) {
+  return (uint32_t)rect->width_ * rect->height_;
+}
+
+static int IsLosslessBlendingPossible(const WebPPicture* const src,
+                                      const WebPPicture* const dst,
+                                      const FrameRect* 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 IsLossyBlendingPossible(const WebPPicture* const src,
+                                   const WebPPicture* const dst,
+                                   const FrameRect* const rect,
+                                   float quality) {
+  const int max_allowed_diff_lossy = QualityToMaxDiff(quality);
+  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 &&
+          !PixelsAreSimilar(src_pixel, dst_pixel, max_allowed_diff_lossy)) {
+        // 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;
+}
+
+// For pixels in 'rect', replace those pixels in 'dst' that are same as 'src' by
+// transparent pixels.
+// Returns true if at least one pixel gets modified.
+static int IncreaseTransparency(const WebPPicture* const src,
+                                const FrameRect* const rect,
+                                WebPPicture* const dst) {
+  int i, j;
+  int modified = 0;
+  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) {
+      if (psrc[i] == pdst[i] && pdst[i] != TRANSPARENT_COLOR) {
+        pdst[i] = TRANSPARENT_COLOR;
+        modified = 1;
+      }
+    }
+  }
+  return modified;
+}
+
+#undef TRANSPARENT_COLOR
+
+// Replace similar blocks of pixels by a 'see-through' transparent block
+// with uniform average color.
+// Assumes lossy compression is being used.
+// Returns true if at least one pixel gets modified.
+static int FlattenSimilarBlocks(const WebPPicture* const src,
+                                const FrameRect* const rect,
+                                WebPPicture* const dst, float quality) {
+  const int max_allowed_diff_lossy = QualityToMaxDiff(quality);
+  int i, j;
+  int modified = 0;
+  const int block_size = 8;
+  const int y_start = (rect->y_offset_ + block_size) & ~(block_size - 1);
+  const int y_end = (rect->y_offset_ + rect->height_) & ~(block_size - 1);
+  const int x_start = (rect->x_offset_ + block_size) & ~(block_size - 1);
+  const int x_end = (rect->x_offset_ + rect->width_) & ~(block_size - 1);
+  assert(src != NULL && dst != NULL && rect != NULL);
+  assert(src->width == dst->width && src->height == dst->height);
+  assert((block_size & (block_size - 1)) == 0);  // must be a power of 2
+  // Iterate over each block and count similar pixels.
+  for (j = y_start; j < y_end; j += block_size) {
+    for (i = x_start; i < x_end; i += block_size) {
+      int cnt = 0;
+      int avg_r = 0, avg_g = 0, avg_b = 0;
+      int x, y;
+      const uint32_t* const psrc = src->argb + j * src->argb_stride + i;
+      uint32_t* const pdst = dst->argb + j * dst->argb_stride + i;
+      for (y = 0; y < block_size; ++y) {
+        for (x = 0; x < block_size; ++x) {
+          const uint32_t src_pixel = psrc[x + y * src->argb_stride];
+          const int alpha = src_pixel >> 24;
+          if (alpha == 0xff &&
+              PixelsAreSimilar(src_pixel, pdst[x + y * dst->argb_stride],
+                               max_allowed_diff_lossy)) {
+            ++cnt;
+            avg_r += (src_pixel >> 16) & 0xff;
+            avg_g += (src_pixel >> 8) & 0xff;
+            avg_b += (src_pixel >> 0) & 0xff;
+          }
+        }
+      }
+      // If we have a fully similar block, we replace it with an
+      // average transparent block. This compresses better in lossy mode.
+      if (cnt == block_size * block_size) {
+        const uint32_t color = (0x00          << 24) |
+                               ((avg_r / cnt) << 16) |
+                               ((avg_g / cnt) <<  8) |
+                               ((avg_b / cnt) <<  0);
+        for (y = 0; y < block_size; ++y) {
+          for (x = 0; x < block_size; ++x) {
+            pdst[x + y * dst->argb_stride] = color;
+          }
+        }
+        modified = 1;
+      }
+    }
+  }
+  return modified;
+}
+
+static int EncodeFrame(const WebPConfig* const config, WebPPicture* const pic,
+                       WebPMemoryWriter* const memory) {
+  pic->use_argb = 1;
+  pic->writer = WebPMemoryWrite;
+  pic->custom_ptr = memory;
+  if (!WebPEncode(config, pic)) {
+    return 0;
+  }
+  return 1;
+}
+
+// Struct representing a candidate encoded frame including its metadata.
+typedef struct {
+  WebPMemoryWriter  mem_;
+  WebPMuxFrameInfo  info_;
+  FrameRect         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 FrameRect* const rect,
+                                         const WebPConfig* const encoder_config,
+                                         int use_blending,
+                                         Candidate* const candidate) {
+  WebPConfig config = *encoder_config;
+  WebPEncodingError error_code = VP8_ENC_OK;
+  assert(candidate != NULL);
+  memset(candidate, 0, sizeof(*candidate));
+
+  // Set frame rect and info.
+  candidate->rect_ = *rect;
+  candidate->info_.id = WEBP_CHUNK_ANMF;
+  candidate->info_.x_offset = rect->x_offset_;
+  candidate->info_.y_offset = rect->y_offset_;
+  candidate->info_.dispose_method = WEBP_MUX_DISPOSE_NONE;  // Set later.
+  candidate->info_.blend_method =
+      use_blending ? WEBP_MUX_BLEND : WEBP_MUX_NO_BLEND;
+  candidate->info_.duration = 0;  // Set in next call to WebPAnimEncoderAdd().
+
+  // Encode picture.
+  WebPMemoryWriterInit(&candidate->mem_);
+
+  if (!config.lossless && use_blending) {
+    // Disable filtering to avoid blockiness in reconstructed frames at the
+    // time of decoding.
+    config.autofilter = 0;
+    config.filter_strength = 0;
+  }
+  if (!EncodeFrame(&config, sub_frame, &candidate->mem_)) {
+    error_code = sub_frame->error_code;
+    goto Err;
+  }
+
+  candidate->evaluate_ = 1;
+  return error_code;
+
+ Err:
+  WebPMemoryWriterClear(&candidate->mem_);
+  return error_code;
+}
+
+static void CopyCurrentCanvas(WebPAnimEncoder* const enc) {
+  if (enc->curr_canvas_copy_modified_) {
+    WebPCopyPixels(enc->curr_canvas_, &enc->curr_canvas_copy_);
+    enc->curr_canvas_copy_.progress_hook = enc->curr_canvas_->progress_hook;
+    enc->curr_canvas_copy_.user_data = enc->curr_canvas_->user_data;
+    enc->curr_canvas_copy_modified_ = 0;
+  }
+}
+
+enum {
+  LL_DISP_NONE = 0,
+  LL_DISP_BG,
+  LOSSY_DISP_NONE,
+  LOSSY_DISP_BG,
+  CANDIDATE_COUNT
+};
+
+#define MIN_COLORS_LOSSY     31  // Don't try lossy below this threshold.
+#define MAX_COLORS_LOSSLESS 194  // Don't try lossless above this threshold.
+
+// Generates candidates for a given dispose method given pre-filled sub-frame
+// 'params'.
+static WebPEncodingError GenerateCandidates(
+    WebPAnimEncoder* const enc, Candidate candidates[CANDIDATE_COUNT],
+    WebPMuxAnimDispose dispose_method, int is_lossless, int is_key_frame,
+    SubFrameParams* const params,
+    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];
+  WebPPicture* const curr_canvas = &enc->curr_canvas_copy_;
+  const WebPPicture* const prev_canvas =
+      is_dispose_none ? &enc->prev_canvas_ : &enc->prev_canvas_disposed_;
+  int use_blending_ll, use_blending_lossy;
+  int evaluate_ll, evaluate_lossy;
+
+  CopyCurrentCanvas(enc);
+  use_blending_ll =
+      !is_key_frame &&
+      IsLosslessBlendingPossible(prev_canvas, curr_canvas, &params->rect_ll_);
+  use_blending_lossy =
+      !is_key_frame &&
+      IsLossyBlendingPossible(prev_canvas, curr_canvas, &params->rect_lossy_,
+                              config_lossy->quality);
+
+  // Pick candidates to be tried.
+  if (!enc->options_.allow_mixed) {
+    evaluate_ll = is_lossless;
+    evaluate_lossy = !is_lossless;
+  } else if (enc->options_.minimize_size) {
+    evaluate_ll = 1;
+    evaluate_lossy = 1;
+  } else {  // Use a heuristic for trying lossless and/or lossy compression.
+    const int num_colors = WebPGetColorPalette(&params->sub_frame_ll_, NULL);
+    evaluate_ll = (num_colors < MAX_COLORS_LOSSLESS);
+    evaluate_lossy = (num_colors >= MIN_COLORS_LOSSY);
+  }
+
+  // Generate candidates.
+  if (evaluate_ll) {
+    CopyCurrentCanvas(enc);
+    if (use_blending_ll) {
+      enc->curr_canvas_copy_modified_ =
+          IncreaseTransparency(prev_canvas, &params->rect_ll_, curr_canvas);
+    }
+    error_code = EncodeCandidate(&params->sub_frame_ll_, &params->rect_ll_,
+                                 config_ll, use_blending_ll, candidate_ll);
+    if (error_code != VP8_ENC_OK) return error_code;
+  }
+  if (evaluate_lossy) {
+    CopyCurrentCanvas(enc);
+    if (use_blending_lossy) {
+      enc->curr_canvas_copy_modified_ =
+          FlattenSimilarBlocks(prev_canvas, &params->rect_lossy_, curr_canvas,
+                               config_lossy->quality);
+    }
+    error_code =
+        EncodeCandidate(&params->sub_frame_lossy_, &params->rect_lossy_,
+                        config_lossy, use_blending_lossy, candidate_lossy);
+    if (error_code != VP8_ENC_OK) return error_code;
+    enc->curr_canvas_copy_modified_ = 1;
+  }
+  return error_code;
+}
+
+#undef MIN_COLORS_LOSSY
+#undef MAX_COLORS_LOSSLESS
+
+static void GetEncodedData(const WebPMemoryWriter* const memory,
+                           WebPData* const encoded_data) {
+  encoded_data->bytes = memory->mem;
+  encoded_data->size  = memory->size;
+}
+
+// Sets dispose method of the previous frame to be 'dispose_method'.
+static void SetPreviousDisposeMethod(WebPAnimEncoder* const enc,
+                                     WebPMuxAnimDispose dispose_method) {
+  const size_t position = enc->count_ - 2;
+  EncodedFrame* const prev_enc_frame = GetFrame(enc, position);
+  assert(enc->count_ >= 2);  // As current and previous frames are in enc.
+
+  if (enc->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;
+  }
+}
+
+static int IncreasePreviousDuration(WebPAnimEncoder* const enc, int duration) {
+  const size_t position = enc->count_ - 1;
+  EncodedFrame* const prev_enc_frame = GetFrame(enc, position);
+  int new_duration;
+
+  assert(enc->count_ >= 1);
+  assert(prev_enc_frame->sub_frame_.duration ==
+         prev_enc_frame->key_frame_.duration);
+  assert(prev_enc_frame->sub_frame_.duration ==
+         (prev_enc_frame->sub_frame_.duration & (MAX_DURATION - 1)));
+  assert(duration == (duration & (MAX_DURATION - 1)));
+
+  new_duration = prev_enc_frame->sub_frame_.duration + duration;
+  if (new_duration >= MAX_DURATION) {  // Special case.
+    // Separate out previous frame from earlier merged frames to avoid overflow.
+    // We add a 1x1 transparent frame for the previous frame, with blending on.
+    const FrameRect rect = { 0, 0, 1, 1 };
+    const uint8_t lossless_1x1_bytes[] = {
+      0x52, 0x49, 0x46, 0x46, 0x14, 0x00, 0x00, 0x00, 0x57, 0x45, 0x42, 0x50,
+      0x56, 0x50, 0x38, 0x4c, 0x08, 0x00, 0x00, 0x00, 0x2f, 0x00, 0x00, 0x00,
+      0x10, 0x88, 0x88, 0x08
+    };
+    const WebPData lossless_1x1 = {
+        lossless_1x1_bytes, sizeof(lossless_1x1_bytes)
+    };
+    const uint8_t lossy_1x1_bytes[] = {
+      0x52, 0x49, 0x46, 0x46, 0x40, 0x00, 0x00, 0x00, 0x57, 0x45, 0x42, 0x50,
+      0x56, 0x50, 0x38, 0x58, 0x0a, 0x00, 0x00, 0x00, 0x10, 0x00, 0x00, 0x00,
+      0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x41, 0x4c, 0x50, 0x48, 0x02, 0x00,
+      0x00, 0x00, 0x00, 0x00, 0x56, 0x50, 0x38, 0x20, 0x18, 0x00, 0x00, 0x00,
+      0x30, 0x01, 0x00, 0x9d, 0x01, 0x2a, 0x01, 0x00, 0x01, 0x00, 0x02, 0x00,
+      0x34, 0x25, 0xa4, 0x00, 0x03, 0x70, 0x00, 0xfe, 0xfb, 0xfd, 0x50, 0x00
+    };
+    const WebPData lossy_1x1 = { lossy_1x1_bytes, sizeof(lossy_1x1_bytes) };
+    const int can_use_lossless =
+        (enc->last_config_.lossless || enc->options_.allow_mixed);
+    EncodedFrame* const curr_enc_frame = GetFrame(enc, enc->count_);
+    curr_enc_frame->is_key_frame_ = 0;
+    curr_enc_frame->sub_frame_.id = WEBP_CHUNK_ANMF;
+    curr_enc_frame->sub_frame_.x_offset = 0;
+    curr_enc_frame->sub_frame_.y_offset = 0;
+    curr_enc_frame->sub_frame_.dispose_method = WEBP_MUX_DISPOSE_NONE;
+    curr_enc_frame->sub_frame_.blend_method = WEBP_MUX_BLEND;
+    curr_enc_frame->sub_frame_.duration = duration;
+    if (!WebPDataCopy(can_use_lossless ? &lossless_1x1 : &lossy_1x1,
+                      &curr_enc_frame->sub_frame_.bitstream)) {
+      return 0;
+    }
+    ++enc->count_;
+    ++enc->count_since_key_frame_;
+    enc->flush_count_ = enc->count_ - 1;
+    enc->prev_candidate_undecided_ = 0;
+    enc->prev_rect_ = rect;
+  } else {                           // Regular case.
+    // Increase duration of the previous frame by 'duration'.
+    prev_enc_frame->sub_frame_.duration = new_duration;
+    prev_enc_frame->key_frame_.duration = new_duration;
+  }
+  return 1;
+}
+
+// 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(WebPAnimEncoder* const enc,
+                              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(enc, prev_dispose_method);
+        }
+        enc->prev_rect_ = candidates[i].rect_;  // save for next frame.
+      } else {
+        WebPMemoryWriterClear(&candidates[i].mem_);
+        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'.
+// 'frame_skipped' will be set to true if this frame should actually be skipped.
+static WebPEncodingError SetFrame(WebPAnimEncoder* const enc,
+                                  const WebPConfig* const config,
+                                  int is_key_frame,
+                                  EncodedFrame* const encoded_frame,
+                                  int* const frame_skipped) {
+  int i;
+  WebPEncodingError error_code = VP8_ENC_OK;
+  const WebPPicture* const curr_canvas = &enc->curr_canvas_copy_;
+  const WebPPicture* const prev_canvas = &enc->prev_canvas_;
+  Candidate candidates[CANDIDATE_COUNT];
+  const int is_lossless = config->lossless;
+  const int consider_lossless = is_lossless || enc->options_.allow_mixed;
+  const int consider_lossy = !is_lossless || enc->options_.allow_mixed;
+  const int is_first_frame = enc->is_first_frame_;
+
+  // First frame cannot be skipped as there is no 'previous frame' to merge it
+  // to. So, empty rectangle is not allowed for the first frame.
+  const int empty_rect_allowed_none = !is_first_frame;
+
+  // Even if there is exact pixel match between 'disposed previous canvas' and
+  // 'current canvas', we can't skip current frame, as there may not be exact
+  // pixel match between 'previous canvas' and 'current canvas'. So, we don't
+  // allow empty rectangle in this case.
+  const int empty_rect_allowed_bg = 0;
+
+  // 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 key-frame insertion is on, and previous frame could be picked as
+  // either a sub-frame or a key-frame, 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 && !enc->prev_candidate_undecided_;
+
+  SubFrameParams dispose_none_params;
+  SubFrameParams dispose_bg_params;
+
+  WebPConfig config_ll = *config;
+  WebPConfig config_lossy = *config;
+  config_ll.lossless = 1;
+  config_lossy.lossless = 0;
+  enc->last_config_ = *config;
+  enc->last_config_reversed_ = config->lossless ? config_lossy : config_ll;
+  *frame_skipped = 0;
+
+  if (!SubFrameParamsInit(&dispose_none_params, 1, empty_rect_allowed_none) ||
+      !SubFrameParamsInit(&dispose_bg_params, 0, empty_rect_allowed_bg)) {
+    return VP8_ENC_ERROR_INVALID_CONFIGURATION;
+  }
+
+  memset(candidates, 0, sizeof(candidates));
+
+  // Change-rectangle assuming previous frame was DISPOSE_NONE.
+  if (!GetSubRects(prev_canvas, curr_canvas, is_key_frame, is_first_frame,
+                   config_lossy.quality, &dispose_none_params)) {
+    error_code = VP8_ENC_ERROR_INVALID_CONFIGURATION;
+    goto Err;
+  }
+
+  if ((consider_lossless && IsEmptyRect(&dispose_none_params.rect_ll_)) ||
+      (consider_lossy && IsEmptyRect(&dispose_none_params.rect_lossy_))) {
+    // Don't encode the frame at all. Instead, the duration of the previous
+    // frame will be increased later.
+    assert(empty_rect_allowed_none);
+    *frame_skipped = 1;
+    goto End;
+  }
+
+  if (dispose_bg_possible) {
+    // Change-rectangle assuming previous frame was DISPOSE_BACKGROUND.
+    WebPPicture* const prev_canvas_disposed = &enc->prev_canvas_disposed_;
+    WebPCopyPixels(prev_canvas, prev_canvas_disposed);
+    DisposeFrameRectangle(WEBP_MUX_DISPOSE_BACKGROUND, &enc->prev_rect_,
+                          prev_canvas_disposed);
+
+    if (!GetSubRects(prev_canvas_disposed, curr_canvas, is_key_frame,
+                     is_first_frame, config_lossy.quality,
+                     &dispose_bg_params)) {
+      error_code = VP8_ENC_ERROR_INVALID_CONFIGURATION;
+      goto Err;
+    }
+    assert(!IsEmptyRect(&dispose_bg_params.rect_ll_));
+    assert(!IsEmptyRect(&dispose_bg_params.rect_lossy_));
+
+    if (enc->options_.minimize_size) {  // Try both dispose methods.
+      dispose_bg_params.should_try_ = 1;
+      dispose_none_params.should_try_ = 1;
+    } else if ((is_lossless &&
+                RectArea(&dispose_bg_params.rect_ll_) <
+                    RectArea(&dispose_none_params.rect_ll_)) ||
+               (!is_lossless &&
+                RectArea(&dispose_bg_params.rect_lossy_) <
+                    RectArea(&dispose_none_params.rect_lossy_))) {
+      dispose_bg_params.should_try_ = 1;  // Pick DISPOSE_BACKGROUND.
+      dispose_none_params.should_try_ = 0;
+    }
+  }
+
+  if (dispose_none_params.should_try_) {
+    error_code = GenerateCandidates(
+        enc, candidates, WEBP_MUX_DISPOSE_NONE, is_lossless, is_key_frame,
+        &dispose_none_params, &config_ll, &config_lossy);
+    if (error_code != VP8_ENC_OK) goto Err;
+  }
+
+  if (dispose_bg_params.should_try_) {
+    assert(!enc->is_first_frame_);
+    assert(dispose_bg_possible);
+    error_code = GenerateCandidates(
+        enc, candidates, WEBP_MUX_DISPOSE_BACKGROUND, is_lossless, is_key_frame,
+        &dispose_bg_params, &config_ll, &config_lossy);
+    if (error_code != VP8_ENC_OK) goto Err;
+  }
+
+  PickBestCandidate(enc, candidates, is_key_frame, encoded_frame);
+
+  goto End;
+
+ Err:
+  for (i = 0; i < CANDIDATE_COUNT; ++i) {
+    if (candidates[i].evaluate_) {
+      WebPMemoryWriterClear(&candidates[i].mem_);
+    }
+  }
+
+ End:
+  SubFrameParamsFree(&dispose_none_params);
+  SubFrameParamsFree(&dispose_bg_params);
+  return error_code;
+}
+
+// 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) {
+  return ((int64_t)encoded_frame->key_frame_.bitstream.size -
+          encoded_frame->sub_frame_.bitstream.size);
+}
+
+static int CacheFrame(WebPAnimEncoder* const enc,
+                      const WebPConfig* const config) {
+  int ok = 0;
+  int frame_skipped = 0;
+  WebPEncodingError error_code = VP8_ENC_OK;
+  const size_t position = enc->count_;
+  EncodedFrame* const encoded_frame = GetFrame(enc, position);
+
+  ++enc->count_;
+
+  if (enc->is_first_frame_) {  // Add this as a key-frame.
+    error_code = SetFrame(enc, config, 1, encoded_frame, &frame_skipped);
+    if (error_code != VP8_ENC_OK) goto End;
+    assert(frame_skipped == 0);  // First frame can't be skipped, even if empty.
+    assert(position == 0 && enc->count_ == 1);
+    encoded_frame->is_key_frame_ = 1;
+    enc->flush_count_ = 0;
+    enc->count_since_key_frame_ = 0;
+    enc->prev_candidate_undecided_ = 0;
+  } else {
+    ++enc->count_since_key_frame_;
+    if (enc->count_since_key_frame_ <= enc->options_.kmin) {
+      // Add this as a frame rectangle.
+      error_code = SetFrame(enc, config, 0, encoded_frame, &frame_skipped);
+      if (error_code != VP8_ENC_OK) goto End;
+      if (frame_skipped) goto Skip;
+      encoded_frame->is_key_frame_ = 0;
+      enc->flush_count_ = enc->count_ - 1;
+      enc->prev_candidate_undecided_ = 0;
+    } else {
+      int64_t curr_delta;
+      FrameRect prev_rect_key, prev_rect_sub;
+
+      // Add this as a frame rectangle to enc.
+      error_code = SetFrame(enc, config, 0, encoded_frame, &frame_skipped);
+      if (error_code != VP8_ENC_OK) goto End;
+      if (frame_skipped) goto Skip;
+      prev_rect_sub = enc->prev_rect_;
+
+
+      // Add this as a key-frame to enc, too.
+      error_code = SetFrame(enc, config, 1, encoded_frame, &frame_skipped);
+      if (error_code != VP8_ENC_OK) goto End;
+      assert(frame_skipped == 0);  // Key-frame cannot be an empty rectangle.
+      prev_rect_key = enc->prev_rect_;
+
+      // Analyze size difference of the two variants.
+      curr_delta = KeyFramePenalty(encoded_frame);
+      if (curr_delta <= enc->best_delta_) {  // Pick this as the key-frame.
+        if (enc->keyframe_ != KEYFRAME_NONE) {
+          EncodedFrame* const old_keyframe = GetFrame(enc, enc->keyframe_);
+          assert(old_keyframe->is_key_frame_);
+          old_keyframe->is_key_frame_ = 0;
+        }
+        encoded_frame->is_key_frame_ = 1;
+        enc->prev_candidate_undecided_ = 1;
+        enc->keyframe_ = (int)position;
+        enc->best_delta_ = curr_delta;
+        enc->flush_count_ = enc->count_ - 1;  // We can flush previous frames.
+      } else {
+        encoded_frame->is_key_frame_ = 0;
+        enc->prev_candidate_undecided_ = 0;
+      }
+      // Note: We need '>=' below because when kmin and kmax are both zero,
+      // count_since_key_frame will always be > kmax.
+      if (enc->count_since_key_frame_ >= enc->options_.kmax) {
+        enc->flush_count_ = enc->count_ - 1;
+        enc->count_since_key_frame_ = 0;
+        enc->keyframe_ = KEYFRAME_NONE;
+        enc->best_delta_ = DELTA_INFINITY;
+      }
+      if (!enc->prev_candidate_undecided_) {
+        enc->prev_rect_ =
+            encoded_frame->is_key_frame_ ? prev_rect_key : prev_rect_sub;
+      }
+    }
+  }
+
+  // Update previous to previous and previous canvases for next call.
+  WebPCopyPixels(enc->curr_canvas_, &enc->prev_canvas_);
+  enc->is_first_frame_ = 0;
+
+ Skip:
+  ok = 1;
+  ++enc->in_frame_count_;
+
+ End:
+  if (!ok || frame_skipped) {
+    FrameRelease(encoded_frame);
+    // We reset some counters, as the frame addition failed/was skipped.
+    --enc->count_;
+    if (!enc->is_first_frame_) --enc->count_since_key_frame_;
+    if (!ok) {
+      MarkError2(enc, "ERROR adding frame. WebPEncodingError", error_code);
+    }
+  }
+  enc->curr_canvas_->error_code = error_code;   // report error_code
+  assert(ok || error_code != VP8_ENC_OK);
+  return ok;
+}
+
+static int FlushFrames(WebPAnimEncoder* const enc) {
+  while (enc->flush_count_ > 0) {
+    WebPMuxError err;
+    EncodedFrame* const curr = GetFrame(enc, 0);
+    const WebPMuxFrameInfo* const info =
+        curr->is_key_frame_ ? &curr->key_frame_ : &curr->sub_frame_;
+    assert(enc->mux_ != NULL);
+    err = WebPMuxPushFrame(enc->mux_, info, 1);
+    if (err != WEBP_MUX_OK) {
+      MarkError2(enc, "ERROR adding frame. WebPMuxError", err);
+      return 0;
+    }
+    if (enc->options_.verbose) {
+      fprintf(stderr, "INFO: Added frame. offset:%d,%d dispose:%d blend:%d\n",
+              info->x_offset, info->y_offset, info->dispose_method,
+              info->blend_method);
+    }
+    ++enc->out_frame_count_;
+    FrameRelease(curr);
+    ++enc->start_;
+    --enc->flush_count_;
+    --enc->count_;
+    if (enc->keyframe_ != KEYFRAME_NONE) --enc->keyframe_;
+  }
+
+  if (enc->count_ == 1 && enc->start_ != 0) {
+    // Move enc->start to index 0.
+    const int enc_start_tmp = (int)enc->start_;
+    EncodedFrame temp = enc->encoded_frames_[0];
+    enc->encoded_frames_[0] = enc->encoded_frames_[enc_start_tmp];
+    enc->encoded_frames_[enc_start_tmp] = temp;
+    FrameRelease(&enc->encoded_frames_[enc_start_tmp]);
+    enc->start_ = 0;
+  }
+  return 1;
+}
+
+#undef DELTA_INFINITY
+#undef KEYFRAME_NONE
+
+int WebPAnimEncoderAdd(WebPAnimEncoder* enc, WebPPicture* frame, int timestamp,
+                       const WebPConfig* encoder_config) {
+  WebPConfig config;
+  int ok;
+
+  if (enc == NULL) {
+    return 0;
+  }
+  MarkNoError(enc);
+
+  if (!enc->is_first_frame_) {
+    // Make sure timestamps are non-decreasing (integer wrap-around is OK).
+    const uint32_t prev_frame_duration =
+        (uint32_t)timestamp - enc->prev_timestamp_;
+    if (prev_frame_duration >= MAX_DURATION) {
+      if (frame != NULL) {
+        frame->error_code = VP8_ENC_ERROR_INVALID_CONFIGURATION;
+      }
+      MarkError(enc, "ERROR adding frame: timestamps must be non-decreasing");
+      return 0;
+    }
+    if (!IncreasePreviousDuration(enc, (int)prev_frame_duration)) {
+      return 0;
+    }
+  } else {
+    enc->first_timestamp_ = timestamp;
+  }
+
+  if (frame == NULL) {  // Special: last call.
+    enc->got_null_frame_ = 1;
+    enc->prev_timestamp_ = timestamp;
+    return 1;
+  }
+
+  if (frame->width != enc->canvas_width_ ||
+      frame->height != enc->canvas_height_) {
+    frame->error_code = VP8_ENC_ERROR_INVALID_CONFIGURATION;
+    MarkError(enc, "ERROR adding frame: Invalid frame dimensions");
+    return 0;
+  }
+
+  if (!frame->use_argb) {  // Convert frame from YUV(A) to ARGB.
+    if (enc->options_.verbose) {
+      fprintf(stderr, "WARNING: Converting frame from YUV(A) to ARGB format; "
+              "this incurs a small loss.\n");
+    }
+    if (!WebPPictureYUVAToARGB(frame)) {
+      MarkError(enc, "ERROR converting frame from YUV(A) to ARGB");
+      return 0;
+    }
+  }
+
+  if (encoder_config != NULL) {
+    if (!WebPValidateConfig(encoder_config)) {
+      MarkError(enc, "ERROR adding frame: Invalid WebPConfig");
+      return 0;
+    }
+    config = *encoder_config;
+  } else {
+    WebPConfigInit(&config);
+    config.lossless = 1;
+  }
+  assert(enc->curr_canvas_ == NULL);
+  enc->curr_canvas_ = frame;  // Store reference.
+  assert(enc->curr_canvas_copy_modified_ == 1);
+  CopyCurrentCanvas(enc);
+
+  ok = CacheFrame(enc, &config) && FlushFrames(enc);
+
+  enc->curr_canvas_ = NULL;
+  enc->curr_canvas_copy_modified_ = 1;
+  if (ok) {
+    enc->prev_timestamp_ = timestamp;
+  }
+  return ok;
+}
+
+// -----------------------------------------------------------------------------
+// Bitstream assembly.
+
+static int DecodeFrameOntoCanvas(const WebPMuxFrameInfo* const frame,
+                                 WebPPicture* const canvas) {
+  const WebPData* const image = &frame->bitstream;
+  WebPPicture sub_image;
+  WebPDecoderConfig config;
+  WebPInitDecoderConfig(&config);
+  WebPUtilClearPic(canvas, NULL);
+  if (WebPGetFeatures(image->bytes, image->size, &config.input) !=
+      VP8_STATUS_OK) {
+    return 0;
+  }
+  if (!WebPPictureView(canvas, frame->x_offset, frame->y_offset,
+                       config.input.width, config.input.height, &sub_image)) {
+    return 0;
+  }
+  config.output.is_external_memory = 1;
+  config.output.colorspace = MODE_BGRA;
+  config.output.u.RGBA.rgba = (uint8_t*)sub_image.argb;
+  config.output.u.RGBA.stride = sub_image.argb_stride * 4;
+  config.output.u.RGBA.size = config.output.u.RGBA.stride * sub_image.height;
+
+  if (WebPDecode(image->bytes, image->size, &config) != VP8_STATUS_OK) {
+    return 0;
+  }
+  return 1;
+}
+
+static int FrameToFullCanvas(WebPAnimEncoder* const enc,
+                             const WebPMuxFrameInfo* const frame,
+                             WebPData* const full_image) {
+  WebPPicture* const canvas_buf = &enc->curr_canvas_copy_;
+  WebPMemoryWriter mem1, mem2;
+  WebPMemoryWriterInit(&mem1);
+  WebPMemoryWriterInit(&mem2);
+
+  if (!DecodeFrameOntoCanvas(frame, canvas_buf)) goto Err;
+  if (!EncodeFrame(&enc->last_config_, canvas_buf, &mem1)) goto Err;
+  GetEncodedData(&mem1, full_image);
+
+  if (enc->options_.allow_mixed) {
+    if (!EncodeFrame(&enc->last_config_reversed_, canvas_buf, &mem2)) goto Err;
+    if (mem2.size < mem1.size) {
+      GetEncodedData(&mem2, full_image);
+      WebPMemoryWriterClear(&mem1);
+    } else {
+      WebPMemoryWriterClear(&mem2);
+    }
+  }
+  return 1;
+
+ Err:
+  WebPMemoryWriterClear(&mem1);
+  WebPMemoryWriterClear(&mem2);
+  return 0;
+}
+
+// Convert a single-frame animation to a non-animated image if appropriate.
+// TODO(urvang): Can we pick one of the two heuristically (based on frame
+// rectangle and/or presence of alpha)?
+static WebPMuxError OptimizeSingleFrame(WebPAnimEncoder* const enc,
+                                        WebPData* const webp_data) {
+  WebPMuxError err = WEBP_MUX_OK;
+  int canvas_width, canvas_height;
+  WebPMuxFrameInfo frame;
+  WebPData full_image;
+  WebPData webp_data2;
+  WebPMux* const mux = WebPMuxCreate(webp_data, 0);
+  if (mux == NULL) return WEBP_MUX_BAD_DATA;
+  assert(enc->out_frame_count_ == 1);
+  WebPDataInit(&frame.bitstream);
+  WebPDataInit(&full_image);
+  WebPDataInit(&webp_data2);
+
+  err = WebPMuxGetFrame(mux, 1, &frame);
+  if (err != WEBP_MUX_OK) goto End;
+  if (frame.id != WEBP_CHUNK_ANMF) goto End;  // Non-animation: nothing to do.
+  err = WebPMuxGetCanvasSize(mux, &canvas_width, &canvas_height);
+  if (err != WEBP_MUX_OK) goto End;
+  if (!FrameToFullCanvas(enc, &frame, &full_image)) {
+    err = WEBP_MUX_BAD_DATA;
+    goto End;
+  }
+  err = WebPMuxSetImage(mux, &full_image, 1);
+  if (err != WEBP_MUX_OK) goto End;
+  err = WebPMuxAssemble(mux, &webp_data2);
+  if (err != WEBP_MUX_OK) goto End;
+
+  if (webp_data2.size < webp_data->size) {  // Pick 'webp_data2' if smaller.
+    WebPDataClear(webp_data);
+    *webp_data = webp_data2;
+    WebPDataInit(&webp_data2);
+  }
+
+ End:
+  WebPDataClear(&frame.bitstream);
+  WebPDataClear(&full_image);
+  WebPMuxDelete(mux);
+  WebPDataClear(&webp_data2);
+  return err;
+}
+
+int WebPAnimEncoderAssemble(WebPAnimEncoder* enc, WebPData* webp_data) {
+  WebPMux* mux;
+  WebPMuxError err;
+
+  if (enc == NULL) {
+    return 0;
+  }
+  MarkNoError(enc);
+
+  if (webp_data == NULL) {
+    MarkError(enc, "ERROR assembling: NULL input");
+    return 0;
+  }
+
+  if (enc->in_frame_count_ == 0) {
+    MarkError(enc, "ERROR: No frames to assemble");
+    return 0;
+  }
+
+  if (!enc->got_null_frame_ && enc->in_frame_count_ > 1 && enc->count_ > 0) {
+    // set duration of the last frame to be avg of durations of previous frames.
+    const double delta_time = enc->prev_timestamp_ - enc->first_timestamp_;
+    const int average_duration = (int)(delta_time / (enc->in_frame_count_ - 1));
+    if (!IncreasePreviousDuration(enc, average_duration)) {
+      return 0;
+    }
+  }
+
+  // Flush any remaining frames.
+  enc->flush_count_ = enc->count_;
+  if (!FlushFrames(enc)) {
+    return 0;
+  }
+
+  // Set definitive canvas size.
+  mux = enc->mux_;
+  err = WebPMuxSetCanvasSize(mux, enc->canvas_width_, enc->canvas_height_);
+  if (err != WEBP_MUX_OK) goto Err;
+
+  err = WebPMuxSetAnimationParams(mux, &enc->options_.anim_params);
+  if (err != WEBP_MUX_OK) goto Err;
+
+  // Assemble into a WebP bitstream.
+  err = WebPMuxAssemble(mux, webp_data);
+  if (err != WEBP_MUX_OK) goto Err;
+
+  if (enc->out_frame_count_ == 1) {
+    err = OptimizeSingleFrame(enc, webp_data);
+    if (err != WEBP_MUX_OK) goto Err;
+  }
+  return 1;
+
+ Err:
+  MarkError2(enc, "ERROR assembling WebP", err);
+  return 0;
+}
+
+const char* WebPAnimEncoderGetError(WebPAnimEncoder* enc) {
+  if (enc == NULL) return NULL;
+  return enc->error_str_;
+}
+
+// -----------------------------------------------------------------------------