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gif2webp: Handle frames with missing graphic control extension

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According to the GIF spec (http://www.w3.org/Graphics/GIF/spec-gif89a.txt),
this block is optional, and its scope is only the first graphic rendering block
that follows.

The spec doesn't mention what default values of frame dimensions, offsets,
duration and transparent index to use in such a case, though. So, we use the
defaults used by GIF reader in Chromium:
https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit/Source/platform/image-decoders/gif/GIFImageReader.h&l=186

(cherry picked from commit d51f3e4069)

Change-Id: Iecc6967847192483770e85ac15fe2835cd01ce7b
This commit is contained in:
Urvang Joshi 2014-09-11 14:35:21 -07:00 committed by James Zern
parent bf0eb74829
commit b7d209a448
1 changed files with 18 additions and 5 deletions
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23
examples/gif2webp.c
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@ -46,7 +46,18 @@
//------------------------------------------------------------------------------
static int transparent_index = -1; // Index of transparent color in the map.
static int transparent_index; // Index of transparent color in the map.
static void ResetFrameInfo(WebPMuxFrameInfo* const info) {
WebPDataInit(&info->bitstream);
info->x_offset = 0;
info->y_offset = 0;
info->duration = 0;
info->id = WEBP_CHUNK_ANMF;
info->dispose_method = WEBP_MUX_DISPOSE_NONE;
info->blend_method = WEBP_MUX_BLEND;
transparent_index = -1; // Opaque frame by default.
}
static void SanitizeKeyFrameIntervals(size_t* const kmin_ptr,
size_t* const kmax_ptr) {
@ -278,10 +289,7 @@ int main(int argc, const char *argv[]) {
size_t kmax = 0;
int allow_mixed = 0; // If true, each frame can be lossy or lossless.
memset(&info, 0, sizeof(info));
info.id = WEBP_CHUNK_ANMF;
info.dispose_method = WEBP_MUX_DISPOSE_BACKGROUND;
info.blend_method = WEBP_MUX_BLEND;
ResetFrameInfo(&info);
if (!WebPConfigInit(&config) || !WebPPictureInit(&frame)) {
fprintf(stderr, "Error! Version mismatch!\n");
@ -492,6 +500,11 @@ int main(int argc, const char *argv[]) {
goto End;
}
is_first_frame = 0;
// In GIF, graphic control extensions are optional for a frame, so we
// may not get one before reading the next frame. To handle this case,
// we reset frame properties to reasonable defaults for the next frame.
ResetFrameInfo(&info);
break;
}
case EXTENSION_RECORD_TYPE: {