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Refactor libwebp anim_encode curr prev rectangles

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Previously, WebPAnimEncoder::prev_rect was set in
SetFrame()>PickBestCandidate(), and SetFrame() may be called twice in
a row in CacheFrame(), making prev_rect value wrong in the second
SetFrame() call. Fortunately, the second call is always for a
keyframe, and in that case SetFrame() does not use the value of
prev_rect.

Since commit b1feaa4 that may have been a bigger issue as
PickBestCandidate() was moved from the end of SetFrame() to
SetFrame()>GenerateCandidates()>PickBestCandidate(), effectively
changing the value of prev_rect for non-keyframe candidates within the
same SetFrame() call. Fortunately, prev_rect is only used at the
beginning of SetFrame(), before any call to GenerateCandidates(),
hence the no-op change.

Change-Id: I693dbf9d264b8768d25a29d75b511e616a3db56b
This commit is contained in:
Yannis Guyon
2025-08-20 07:52:53 +00:00
parent f2061209d0
commit ddcdfa6a42
1 changed files with 23 additions and 14 deletions
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37
src/mux/anim_encode.c
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@@ -53,7 +53,6 @@ struct WebPAnimEncoder {
const int canvas_height; // Canvas height.
const WebPAnimEncoderOptions options; // Global encoding options.
FrameRectangle 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;
@@ -90,6 +89,9 @@ struct WebPAnimEncoder {
int prev_candidate_undecided; // True if it's not yet decided if previous
// frame would be a sub-frame or a key-frame.
FrameRectangle prev_rect; // Previous WebP frame rectangle. Only valid if
// prev_candidate_undecided is true.
// 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
@@ -894,7 +896,6 @@ static void PickBestCandidate(WebPAnimEncoder* const enc,
// selected when a non key-frame was assumed.
SetPreviousDisposeMethod(enc, dispose_method);
}
enc->prev_rect = candidate->rect; // save for next frame.
// Release the memory of the previous best candidate if any.
if (*best_candidate != NULL) {
@@ -1051,6 +1052,7 @@ static int IncreasePreviousDuration(WebPAnimEncoder* const enc, int duration) {
static WebPEncodingError SetFrame(WebPAnimEncoder* const enc,
const WebPConfig* const config,
int is_key_frame,
FrameRectangle* const best_candidate_rect,
EncodedFrame* const encoded_frame,
int* const frame_skipped) {
int i;
@@ -1165,6 +1167,7 @@ static WebPEncodingError SetFrame(WebPAnimEncoder* const enc,
}
assert(best_candidate != NULL);
*best_candidate_rect = best_candidate->rect;
goto End;
Err:
@@ -1194,11 +1197,13 @@ static int CacheFrame(WebPAnimEncoder* const enc,
WebPEncodingError error_code = VP8_ENC_OK;
const size_t position = enc->count;
EncodedFrame* const encoded_frame = GetFrame(enc, position);
FrameRectangle best_key_candidate_rect, best_sub_candidate_rect;
++enc->count;
if (enc->is_first_frame) { // Add this as a key-frame.
error_code = SetFrame(enc, config, 1, encoded_frame, &frame_skipped);
error_code = SetFrame(enc, config, 1, &best_key_candidate_rect,
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);
@@ -1210,7 +1215,8 @@ static int CacheFrame(WebPAnimEncoder* const enc,
++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);
error_code = SetFrame(enc, config, 0, &best_sub_candidate_rect,
encoded_frame, &frame_skipped);
if (error_code != VP8_ENC_OK) goto End;
if (frame_skipped) goto Skip;
encoded_frame->is_key_frame = 0;
@@ -1218,19 +1224,18 @@ static int CacheFrame(WebPAnimEncoder* const enc,
enc->prev_candidate_undecided = 0;
} else {
int64_t curr_delta;
FrameRectangle prev_rect_key, prev_rect_sub;
// Add this as a frame rectangle to enc.
error_code = SetFrame(enc, config, 0, encoded_frame, &frame_skipped);
error_code = SetFrame(enc, config, 0, &best_sub_candidate_rect,
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);
error_code = SetFrame(enc, config, 1, &best_key_candidate_rect,
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);
@@ -1263,15 +1268,19 @@ static int CacheFrame(WebPAnimEncoder* const enc,
// Flush all previous frames.
enc->flush_count = enc->count - 1;
}
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.
// Save the current frame environment as the previous frame environment for
// the next call to this function.
WebPCopyPixels(enc->curr_canvas, &enc->prev_canvas);
if (enc->prev_candidate_undecided) {
// The previous frame rectangle is not known for sure. Do not save it.
} else {
enc->prev_rect = encoded_frame->is_key_frame ? best_key_candidate_rect
: best_sub_candidate_rect;
}
enc->is_first_frame = 0;
Skip: