On Mon, Aug 30, 2021 at 5:11 AM Maryla <maryla-at-google....@ffmpeg.org> wrote:
>
> When a color indexing transform with 16 or fewer colors is used,
> WebP uses "pixel packing", i.e. storing several pixels in one byte,
> which virtually reduces the width of the image (see WebPContext's
> reduced_width field). This reduced_width should always be used when
> reading and applying subsequent transforms.
>
> Updated patch with added fate test.
> The source image dual_transform.webp can be downloaded by cloning
> https://chromium.googlesource.com/webm/libwebp-test-data/
>
> Fixes: 9368
> ---
> libavcodec/webp.c | 34 ++++++++++---------
> tests/fate/image.mak | 3 ++
> .../fate/webp-rgb-lossless-palette-predictor | 6 ++++
> 3 files changed, 27 insertions(+), 16 deletions(-)
> create mode 100644 tests/ref/fate/webp-rgb-lossless-palette-predictor
>
This works locally and matches the output from libwebp. I sent a
request to samples-request@ to add the file. This should have a micro
version bump for libavcodec/version.h; I've added that locally.
> diff --git a/libavcodec/webp.c b/libavcodec/webp.c
> index 3efd4438d9..e4c67adc3a 100644
> --- a/libavcodec/webp.c
> +++ b/libavcodec/webp.c
> @@ -181,7 +181,10 @@ typedef struct ImageContext {
> uint32_t *color_cache; /* color cache data */
> int nb_huffman_groups; /* number of huffman groups */
> HuffReader *huffman_groups; /* reader for each huffman group */
> - int size_reduction; /* relative size compared to primary
> image, log2 */
> + /* relative size compared to primary image, log2.
> + * for IMAGE_ROLE_COLOR_INDEXING with <= 16 colors, this is log2 of the
> + * number of pixels per byte in the primary image (pixel packing) */
> + int size_reduction;
> int is_alpha_primary;
> } ImageContext;
>
> @@ -205,7 +208,9 @@ typedef struct WebPContext {
>
> int nb_transforms; /* number of transforms */
> enum TransformType transforms[4]; /* transformations used in the
> image, in order */
> - int reduced_width; /* reduced width for index image, if
> applicable */
> + /* reduced width when using a color indexing transform with <= 16 colors
> (pixel packing)
> + * before pixels are unpacked, or same as width otherwise. */
> + int reduced_width;
> int nb_huffman_groups; /* number of huffman groups in the
> primary image */
> ImageContext image[IMAGE_ROLE_NB]; /* image context for each role */
> } WebPContext;
> @@ -425,13 +430,9 @@ static int decode_entropy_coded_image(WebPContext *s,
> enum ImageRole role,
> static int decode_entropy_image(WebPContext *s)
> {
> ImageContext *img;
> - int ret, block_bits, width, blocks_w, blocks_h, x, y, max;
> + int ret, block_bits, blocks_w, blocks_h, x, y, max;
>
> - width = s->width;
> - if (s->reduced_width > 0)
> - width = s->reduced_width;
> -
> - PARSE_BLOCK_SIZE(width, s->height);
> + PARSE_BLOCK_SIZE(s->reduced_width, s->height);
>
> ret = decode_entropy_coded_image(s, IMAGE_ROLE_ENTROPY, blocks_w,
> blocks_h);
> if (ret < 0)
> @@ -460,7 +461,7 @@ static int parse_transform_predictor(WebPContext *s)
> {
> int block_bits, blocks_w, blocks_h, ret;
>
> - PARSE_BLOCK_SIZE(s->width, s->height);
> + PARSE_BLOCK_SIZE(s->reduced_width, s->height);
>
> ret = decode_entropy_coded_image(s, IMAGE_ROLE_PREDICTOR, blocks_w,
> blocks_h);
> @@ -476,7 +477,7 @@ static int parse_transform_color(WebPContext *s)
> {
> int block_bits, blocks_w, blocks_h, ret;
>
> - PARSE_BLOCK_SIZE(s->width, s->height);
> + PARSE_BLOCK_SIZE(s->reduced_width, s->height);
>
> ret = decode_entropy_coded_image(s, IMAGE_ROLE_COLOR_TRANSFORM, blocks_w,
> blocks_h);
> @@ -620,7 +621,7 @@ static int decode_entropy_coded_image(WebPContext *s,
> enum ImageRole role,
> }
>
> width = img->frame->width;
> - if (role == IMAGE_ROLE_ARGB && s->reduced_width > 0)
> + if (role == IMAGE_ROLE_ARGB)
> width = s->reduced_width;
>
> x = 0; y = 0;
> @@ -925,7 +926,7 @@ static int apply_predictor_transform(WebPContext *s)
> int x, y;
>
> for (y = 0; y < img->frame->height; y++) {
> - for (x = 0; x < img->frame->width; x++) {
> + for (x = 0; x < s->reduced_width; x++) {
> int tx = x >> pimg->size_reduction;
> int ty = y >> pimg->size_reduction;
> enum PredictionMode m = GET_PIXEL_COMP(pimg->frame, tx, ty, 2);
> @@ -965,7 +966,7 @@ static int apply_color_transform(WebPContext *s)
> cimg = &s->image[IMAGE_ROLE_COLOR_TRANSFORM];
>
> for (y = 0; y < img->frame->height; y++) {
> - for (x = 0; x < img->frame->width; x++) {
> + for (x = 0; x < s->reduced_width; x++) {
> cx = x >> cimg->size_reduction;
> cy = y >> cimg->size_reduction;
> cp = GET_PIXEL(cimg->frame, cx, cy);
> @@ -985,7 +986,7 @@ static int apply_subtract_green_transform(WebPContext *s)
> ImageContext *img = &s->image[IMAGE_ROLE_ARGB];
>
> for (y = 0; y < img->frame->height; y++) {
> - for (x = 0; x < img->frame->width; x++) {
> + for (x = 0; x < s->reduced_width; x++) {
> uint8_t *p = GET_PIXEL(img->frame, x, y);
> p[1] += p[2];
> p[3] += p[2];
> @@ -1004,7 +1005,7 @@ static int apply_color_indexing_transform(WebPContext
> *s)
> img = &s->image[IMAGE_ROLE_ARGB];
> pal = &s->image[IMAGE_ROLE_COLOR_INDEXING];
>
> - if (pal->size_reduction > 0) {
> + if (pal->size_reduction > 0) { // undo pixel packing
> GetBitContext gb_g;
> uint8_t *line;
> int pixel_bits = 8 >> pal->size_reduction;
> @@ -1030,6 +1031,7 @@ static int apply_color_indexing_transform(WebPContext
> *s)
> }
> }
> av_free(line);
> + s->reduced_width = s->width; // we are back to full size
> }
>
> // switch to local palette if it's worth initializing it
> @@ -1126,7 +1128,7 @@ static int vp8_lossless_decode_frame(AVCodecContext
> *avctx, AVFrame *p,
>
> /* parse transformations */
> s->nb_transforms = 0;
> - s->reduced_width = 0;
> + s->reduced_width = s->width;
> used = 0;
> while (get_bits1(&s->gb)) {
> enum TransformType transform = get_bits(&s->gb, 2);
> diff --git a/tests/fate/image.mak b/tests/fate/image.mak
> index 3b58972a53..ea9c801990 100644
> --- a/tests/fate/image.mak
> +++ b/tests/fate/image.mak
> @@ -517,6 +517,9 @@ fate-webp-rgb-lena-lossless-rgb24: CMD = framecrc -i
> $(TARGET_SAMPLES)/webp/rgb_
> FATE_WEBP += fate-webp-rgba-lossless
> fate-webp-rgba-lossless: CMD = framecrc -i
> $(TARGET_SAMPLES)/webp/rgba_lossless.webp
>
> +FATE_WEBP += fate-webp-rgb-lossless-palette-predictor
> +fate-webp-rgb-lossless-palette-predictor: CMD = framecrc -i
> $(TARGET_SAMPLES)/webp/dual_transform.webp
> +
> FATE_WEBP += fate-webp-rgb-lossy-q80
> fate-webp-rgb-lossy-q80: CMD = framecrc -i
> $(TARGET_SAMPLES)/webp/rgb_q80.webp
>
> diff --git a/tests/ref/fate/webp-rgb-lossless-palette-predictor
> b/tests/ref/fate/webp-rgb-lossless-palette-predictor
> new file mode 100644
> index 0000000000..92a4ad9810
> --- /dev/null
> +++ b/tests/ref/fate/webp-rgb-lossless-palette-predictor
> @@ -0,0 +1,6 @@
> +#tb 0: 1/25
> +#media_type 0: video
> +#codec_id 0: rawvideo
> +#dimensions 0: 100x30
> +#sar 0: 0/1
> +0, 0, 0, 1, 12000, 0xb200d843
> --
> 2.33.0.259.gc128427fd7-goog
>
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