commit 65d0af9c0d0e37e4e343db6c785b210ec0e132a2
author Vignesh Venkatasubramanian <vigneshv@google.com> Tue Sep 20 09:57:58 2022 -0700
committer Vignesh Venkatasubramanian <vigneshvg@users.noreply.github.com> Tue Sep 20 15:34:13 2022 -0700
tree fdb28b01ba751593ba642aab563eed793873cf3e
parent 98faa17f139b1c280fbea40563078b1fcb438aa5

Write 'auxv' as handler_type for alpha channel track

For animated AVIF sequences, alpha channel track has to have the
handler_type set to 'auxv'.

This is mentioned in Section 7.5.3.1 of the HEIF specification.

libavif's parser ignores the 'hdlr' box that occurs inside the
'mdia' box. So this change is backward compatible (i.e.) files
produced after this change will still be parse'able by earlier
libavif versions.

src/write.c

diff --git a/src/write.c b/src/write.c
index e25e112..bca23d3 100644
--- a/src/write.c
+++ b/src/write.c
@@ -1480,10 +1480,10 @@
             avifRWStreamFinishBox(&s, mdhd);
 
             avifBoxMarker hdlrTrak = avifRWStreamWriteFullBox(&s, "hdlr", AVIF_BOX_SIZE_TBD, 0, 0);
-            avifRWStreamWriteU32(&s, 0);              // unsigned int(32) pre_defined = 0;
-            avifRWStreamWriteChars(&s, "pict", 4);    // unsigned int(32) handler_type;
-            avifRWStreamWriteZeros(&s, 12);           // const unsigned int(32)[3] reserved = 0;
-            avifRWStreamWriteChars(&s, "libavif", 8); // string name; (writing null terminator)
+            avifRWStreamWriteU32(&s, 0);                                  // unsigned int(32) pre_defined = 0;
+            avifRWStreamWriteChars(&s, item->alpha ? "auxv" : "pict", 4); // unsigned int(32) handler_type;
+            avifRWStreamWriteZeros(&s, 12);                               // const unsigned int(32)[3] reserved = 0;
+            avifRWStreamWriteChars(&s, "libavif", 8);                     // string name; (writing null terminator)
             avifRWStreamFinishBox(&s, hdlrTrak);
 
             avifBoxMarker minf = avifRWStreamWriteBox(&s, "minf", AVIF_BOX_SIZE_TBD);