[FFmpeg-cvslog] avcodec/pngenc: fix sBIT writing for indexed-color PNGs

[Leo Izen]

ffmpeg | branch: release/6.1 | Leo Izen <leo.izen at gmail.com> | Fri Jul 19 12:04:20 2024 -0400| [0bd31a8f91bb65b370bbe9234e2636a1f359a4cd] | committer: Leo Izen

avcodec/pngenc: fix sBIT writing for indexed-color PNGs

We currently write invalid sBIT entries for indexed PNGs, which by PNG
specification[1] must be 3-bytes long. The values also are capped at 8
for indexed-color PNGs, not the palette depth. This patch fixes both of
these issues previously fixed in the decoder, but not the encoder.

[1]: https://www.w3.org/TR/png-3/#11sBIT

Regression since: c125860892e931d9b10f88ace73c91484815c3a8.

Signed-off-by: Leo Izen <leo.izen at gmail.com>
Reported-by: Ramiro Polla: <ramiro.polla at gmail.com>

> http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=0bd31a8f91bb65b370bbe9234e2636a1f359a4cd
---

 libavcodec/pngenc.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/libavcodec/pngenc.c b/libavcodec/pngenc.c
index f0650962d2..f570ed5d08 100644
--- a/libavcodec/pngenc.c
+++ b/libavcodec/pngenc.c
@@ -442,8 +442,9 @@ static int encode_headers(AVCodecContext *avctx, const AVFrame *pict)
     if (png_get_gama(pict->color_trc, s->buf))
         png_write_chunk(&s->bytestream, MKTAG('g', 'A', 'M', 'A'), s->buf, 4);
 
-    if (avctx->bits_per_raw_sample > 0 && avctx->bits_per_raw_sample < s->bit_depth) {
-        int len = ff_png_get_nb_channels(s->color_type);
+    if (avctx->bits_per_raw_sample > 0 &&
+            avctx->bits_per_raw_sample < (s->color_type & PNG_COLOR_MASK_PALETTE ? 8 : s->bit_depth)) {
+        int len = s->color_type & PNG_COLOR_MASK_PALETTE ? 3 : ff_png_get_nb_channels(s->color_type);
         memset(s->buf, avctx->bits_per_raw_sample, len);
         png_write_chunk(&s->bytestream, MKTAG('s', 'B', 'I', 'T'), s->buf, len);
     }