webp: fix transforms after a palette with pixel packing.

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
Signed-off-by: James Zern <jzern@google.com>

2 files changed, 19 insertions, 17 deletions

diff --git a/libavcodec/version.h b/libavcodec/version.h
index e36c9fbac3..70c4e4d0fe 100644
--- a/libavcodec/version.h
+++ b/libavcodec/version.h
@@ -29,7 +29,7 @@
 
 #define LIBAVCODEC_VERSION_MAJOR  59
 #define LIBAVCODEC_VERSION_MINOR   9
-#define LIBAVCODEC_VERSION_MICRO 100
+#define LIBAVCODEC_VERSION_MICRO 101
 
 #define LIBAVCODEC_VERSION_INT  AV_VERSION_INT(LIBAVCODEC_VERSION_MAJOR, \
                                                LIBAVCODEC_VERSION_MINOR, \
diff --git a/libavcodec/webp.c b/libavcodec/webp.c
index 8b19ba4e64..d5a81fd527 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);