int WebPAnimDecoderGetNext()

in Extended/libwebp/src/demux/anim_decode.c [309:424]
92 lines of code
19 McCabe index (conditional complexity)

int WebPAnimDecoderGetNext(WebPAnimDecoder* dec,
                           uint8_t** buf_ptr, int* timestamp_ptr) {
  WebPIterator iter;
  uint32_t width;
  uint32_t height;
  int is_key_frame;
  int timestamp;
  BlendRowFunc blend_row;

  if (dec == NULL || buf_ptr == NULL || timestamp_ptr == NULL) return 0;
  if (!WebPAnimDecoderHasMoreFrames(dec)) return 0;

  width = dec->info_.canvas_width;
  height = dec->info_.canvas_height;
  blend_row = dec->blend_func_;

  // Get compressed frame.
  if (!WebPDemuxGetFrame(dec->demux_, dec->next_frame_, &iter)) {
    return 0;
  }
  timestamp = dec->prev_frame_timestamp_ + iter.duration;

  // Initialize.
  is_key_frame = IsKeyFrame(&iter, &dec->prev_iter_,
                            dec->prev_frame_was_keyframe_, width, height);
  if (is_key_frame) {
    if (!ZeroFillCanvas(dec->curr_frame_, width, height)) {
      goto Error;
    }
  } else {
    if (!CopyCanvas(dec->prev_frame_disposed_, dec->curr_frame_,
                    width, height)) {
      goto Error;
    }
  }

  // Decode.
  {
    const uint8_t* in = iter.fragment.bytes;
    const size_t in_size = iter.fragment.size;
    const size_t out_offset =
        (iter.y_offset * width + iter.x_offset) * NUM_CHANNELS;
    WebPDecoderConfig* const config = &dec->config_;
    WebPRGBABuffer* const buf = &config->output.u.RGBA;
    buf->stride = NUM_CHANNELS * width;
    buf->size = buf->stride * iter.height;
    buf->rgba = dec->curr_frame_ + out_offset;

    if (WebPDecode(in, in_size, config) != VP8_STATUS_OK) {
      goto Error;
    }
  }

  // During the decoding of current frame, we may have set some pixels to be
  // transparent (i.e. alpha < 255). However, the value of each of these
  // pixels should have been determined by blending it against the value of
  // that pixel in the previous frame if blending method of is WEBP_MUX_BLEND.
  if (iter.frame_num > 1 && iter.blend_method == WEBP_MUX_BLEND &&
      !is_key_frame) {
    if (dec->prev_iter_.dispose_method == WEBP_MUX_DISPOSE_NONE) {
      int y;
      // Blend transparent pixels with pixels in previous canvas.
      for (y = 0; y < iter.height; ++y) {
        const size_t offset =
            (iter.y_offset + y) * width + iter.x_offset;
        blend_row((uint32_t*)dec->curr_frame_ + offset,
                  (uint32_t*)dec->prev_frame_disposed_ + offset, iter.width);
      }
    } else {
      int y;
      assert(dec->prev_iter_.dispose_method == WEBP_MUX_DISPOSE_BACKGROUND);
      // We need to blend a transparent pixel with its value just after
      // initialization. That is, blend it with:
      // * Fully transparent pixel if it belongs to prevRect <-- No-op.
      // * The pixel in the previous canvas otherwise <-- Need alpha-blending.
      for (y = 0; y < iter.height; ++y) {
        const int canvas_y = iter.y_offset + y;
        int left1, width1, left2, width2;
        FindBlendRangeAtRow(&iter, &dec->prev_iter_, canvas_y, &left1, &width1,
                            &left2, &width2);
        if (width1 > 0) {
          const size_t offset1 = canvas_y * width + left1;
          blend_row((uint32_t*)dec->curr_frame_ + offset1,
                    (uint32_t*)dec->prev_frame_disposed_ + offset1, width1);
        }
        if (width2 > 0) {
          const size_t offset2 = canvas_y * width + left2;
          blend_row((uint32_t*)dec->curr_frame_ + offset2,
                    (uint32_t*)dec->prev_frame_disposed_ + offset2, width2);
        }
      }
    }
  }

  // Update info of the previous frame and dispose it for the next iteration.
  dec->prev_frame_timestamp_ = timestamp;
  WebPDemuxReleaseIterator(&dec->prev_iter_);
  dec->prev_iter_ = iter;
  dec->prev_frame_was_keyframe_ = is_key_frame;
  CopyCanvas(dec->curr_frame_, dec->prev_frame_disposed_, width, height);
  if (dec->prev_iter_.dispose_method == WEBP_MUX_DISPOSE_BACKGROUND) {
    ZeroFillFrameRect(dec->prev_frame_disposed_, width * NUM_CHANNELS,
                      dec->prev_iter_.x_offset, dec->prev_iter_.y_offset,
                      dec->prev_iter_.width, dec->prev_iter_.height);
  }
  ++dec->next_frame_;

  // All OK, fill in the values.
  *buf_ptr = dec->curr_frame_;
  *timestamp_ptr = timestamp;
  return 1;

 Error:
  WebPDemuxReleaseIterator(&iter);
  return 0;
}