Always encode an alpha AV1 payload if the first frame has an alpha plane, unless it is known to be a single image
diff --git a/src/write.c b/src/write.c
index ce1cf9a..fce0b13 100644
--- a/src/write.c
+++ b/src/write.c
@@ -365,8 +365,20 @@
}
encoder->data->primaryItemID = encoder->data->colorItem->id;
- avifBool imageIsOpaque = avifImageIsOpaque(image);
- if (!imageIsOpaque) {
+ avifBool needsAlpha = (image->alphaPlane != NULL);
+ if (addImageFlags & AVIF_ADD_IMAGE_FLAG_SINGLE) {
+ // If encoding a single image in which the alpha plane exists but is entirely opaque,
+ // simply skip writing an alpha AV1 payload entirely, as it'll be interpreted as opaque
+ // and is less bytes.
+ //
+ // However, if encoding an image sequence, the first frame's alpha plane being entirely
+ // opaque could be a false positive for removing the alpha AV1 payload, as it might simply
+ // be a fade out later in the sequence. This is why avifImageIsOpaque() is only called
+ // when encoding a single image.
+
+ needsAlpha = !avifImageIsOpaque(image);
+ }
+ if (needsAlpha) {
encoder->data->alphaItem = avifEncoderDataCreateItem(encoder->data, "av01", "Alpha", 6);
encoder->data->alphaItem->codec = avifCodecCreate(encoder->codecChoice, AVIF_CODEC_FLAG_CAN_ENCODE);
if (!encoder->data->alphaItem->codec) {
@@ -435,6 +447,12 @@
}
} else {
// Another frame in an image sequence
+
+ if (encoder->data->alphaItem && !image->alphaPlane) {
+ // If the first image in the sequence had an alpha plane (even if fully opaque), all
+ // subsequence images must have alpha as well.
+ return AVIF_RESULT_ENCODE_ALPHA_FAILED;
+ }
}
// -----------------------------------------------------------------------
