Do not use single decoder for non-grid images with alpha
In this case, we will steal the planes from the decoder. So
invoking the same decoder for the alpha plane after having used
it for the color planes will invalidate the color planes.
This bug was introduced in 39682be.
diff --git a/src/read.c b/src/read.c
index 5bf628f..36a9058 100644
--- a/src/read.c
+++ b/src/read.c
@@ -3368,6 +3368,11 @@
static avifBool avifTilesCanBeDecodedWithSameAV1Decoder(avifDecoderData * data)
{
+ if (data->color.tileCount == 1 && data->alpha.tileCount == 1) {
+ // Single tile image with alpha plane. In this case each tile needs its own decoder since the planes will be "stolen".
+ // Stealing either the color or the alpha plane will invalidate the other one when decode is called the second time.
+ return AVIF_FALSE;
+ }
const uint8_t firstTileOperatingPoint = data->tiles.tile[0].operatingPoint;
const avifBool firstTileAllLayers = data->tiles.tile[0].input->allLayers;
for (unsigned int i = 1; i < data->tiles.count; ++i) {
@@ -3402,6 +3407,8 @@
// and do not allow it to be changed later).
// - All the tiles must have the same value for allLayers (because AV1 decoders take allLayers once at initialization
// and do not allow it to be changed later).
+ // - If the image has a single tile, it must not have an alpha plane (in this case we will steal the planes from the
+ // decoder, so we cannot use the same decoder for both the color and the alpha planes).
// Otherwise, we will use |tiles.count| decoder instances (one instance for each tile).
avifBool canUseSingleDecoder = (data->tiles.count == 1) ||
(decoder->imageCount == 1 && avifTilesCanBeDecodedWithSameAV1Decoder(data));
