@@ -86,32 +86,19 @@ torch::Tensor decode_gif(const torch::Tensor& encoded_data) {
8686 // This check should already done within DGifSlurp(), just to be safe
8787 TORCH_CHECK (num_images > 0 , " GIF file should contain at least one image!"
8888
89- // Note:
90- // The GIF format has this notion of "canvas" and "canvas size", where each
91- // image could be displayed on the canvas at different offsets, forming a
92- // mosaic/picture wall like so:
93- //
94- // <--- canvas W --->
95- // ------------------------ ^
96- // | | | |
97- // | img1 | img3 | |
98- // | |------------| canvas H
99- // |---------- | |
100- // | img2 | img4 | |
101- // | | | |
102- // ------------------------ v
103- // The GifLib docs indicate that this is mostly vestigial
104- // (https://giflib.sourceforge.net/whatsinagif/bits_and_bytes.html), and
105- // modern viewers ignore the canvas size as well as image offsets. Hence,
106- // we're ignoring that too:
107- // - We're ignoring the canvas width and height and assume that the shape of
108- // the canvas and of all images is the shape of the first image.
109- // - We're enforcing that all images have the same shape.
110- // - Left and Top offsets of each image are ignored as well and assumed to be
111- // 0.
112-
113- auto out_h = gifFile->SavedImages [0 ].ImageDesc .Height ;
114- auto out_w = gifFile->SavedImages [0 ].ImageDesc .Width ;
89+ GifColorType bg = {0 , 0 , 0 };
90+ if (gifFile->SColorMap ) {
91+ bg = gifFile->SColorMap ->Colors [gifFile->SBackGroundColor ];
92+ }
93+
94+ // The GIFLIB docs say that the canvas's height and width are potentially
95+ // ignored by modern viewers, so to be on the safe side we set the output
96+ // height to max(canvas_heigh, first_image_height). Same for width.
97+ // https://giflib.sourceforge.net/whatsinagif/bits_and_bytes.html
98+ auto out_h =
99+ std::max (gifFile->SHeight , gifFile->SavedImages [0 ].ImageDesc .Height );
100+ auto out_w =
101+ std::max (gifFile->SWidth , gifFile->SavedImages [0 ].ImageDesc .Width );
115102
116103 // We output a channels-last tensor for consistency with other image decoders.
117104 // Torchvision's resize tends to be is faster on uint8 channels-last tensors.
@@ -121,30 +108,59 @@ torch::Tensor decode_gif(const torch::Tensor& encoded_data) {
121108 auto out = torch::empty (
122109 {int64_t (num_images), 3 , int64_t (out_h), int64_t (out_w)}, options);
123110 auto out_a = out.accessor <uint8_t , 4 >();
124-
125111 for (int i = 0 ; i < num_images; i++) {
126112 const SavedImage& img = gifFile->SavedImages [i];
127- const GifImageDesc& desc = img.ImageDesc ;
128- TORCH_CHECK (
129- desc.Width == out_w && desc.Height == out_h,
130- " All images in the gif should have the same dimensions."
131113
114+ GraphicsControlBlock gcb;
115+ DGifSavedExtensionToGCB (gifFile, i, &gcb);
116+
117+ const GifImageDesc& desc = img.ImageDesc ;
132118 const ColorMapObject* cmap =
133119 desc.ColorMap ? desc.ColorMap : gifFile->SColorMap ;
134120 TORCH_CHECK (
135121 cmap != nullptr ,
136122 " Global and local color maps are missing. This should never happen!"
137123
124+ // When going from one image to another, there is a "disposal method" which
125+ // specifies how to handle the transition. E.g. DISPOSE_DO_NOT means that
126+ // the current image should essentially be drawn on top of the previous
127+ // canvas. The pixels of that previous canvas will appear on the new one if
128+ // either:
129+ // - a pixel is transparent in the current image
130+ // - the current image is smaller than the canvas, hence exposing its pixels
131+ // The "background" disposal method means that the current canvas should be
132+ // set to the background color.
133+ // We only support these 2 modes and default to "background" when the
134+ // disposal method is unspecified, or when it's set to "DISPOSE_PREVIOUS"
135+ // which according to GIFLIB is not widely supported.
136+ // (https://giflib.sourceforge.net/whatsinagif/animation_and_transparency.html).
137+ if (i > 0 && gcb.DisposalMode == DISPOSE_DO_NOT) {
138+ out[i] = out[i - 1 ];
139+ } else {
140+ // Background. If bg wasn't defined, it will be (0, 0, 0)
141+ for (int h = 0 ; h < gifFile->SHeight ; h++) {
142+ for (int w = 0 ; w < gifFile->SWidth ; w++) {
143+ out_a[i][0 ][h][w] = bg.Red ;
144+ out_a[i][1 ][h][w] = bg.Green ;
145+ out_a[i][2 ][h][w] = bg.Blue ;
146+ }
147+ }
148+ }
149+
138150 for (int h = 0 ; h < desc.Height ; h++) {
139151 for (int w = 0 ; w < desc.Width ; w++) {
140152 auto c = img.RasterBits [h * desc.Width + w];
153+ if (c == gcb.TransparentColor ) {
154+ continue ;
155+ }
141156 GifColorType rgb = cmap->Colors [c];
142- out_a[i][0 ][h][w] = rgb.Red ;
143- out_a[i][1 ][h][w] = rgb.Green ;
144- out_a[i][2 ][h][w] = rgb.Blue ;
157+ out_a[i][0 ][h + desc. Top ][w + desc. Left ] = rgb.Red ;
158+ out_a[i][1 ][h + desc. Top ][w + desc. Left ] = rgb.Green ;
159+ out_a[i][2 ][h + desc. Top ][w + desc. Left ] = rgb.Blue ;
145160 }
146161 }
147162 }
163+
148164 out = out.squeeze (0 ); // remove batch dim if there's only one image
149165
150166 DGifCloseFile (gifFile, &error);
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