[FFmpeg-devel] [PATCH 1/2] avcodec/pngdec: use 8-bit sBIT cap for indexed PNGs per spec

[FFmpeg-devel] [PATCH 1/2] avcodec/pngdec: use 8-bit sBIT cap for indexed PNGs per spec

[Leo Izen]

The PNG specification[1] says that sBIT entries must be at most the bit
depth specified in IHDR, unless the PNG is indexed-color, in which case
sBIT must be between 1 and 8. We should not reject valid sBITs on PNGs
with indexed color.

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

Regression since 84b454935fae2633a8a5dd075e22393f3e8f932f.

Signed-off-by: Leo Izen <leo.izen@gmail.com>
Reported-by: Ramiro Polla <ramiro.polla@gmail.com>
---
 libavcodec/pngdec.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libavcodec/pngdec.c b/libavcodec/pngdec.c
index cb861e5f60..c5b32c166d 100644
--- a/libavcodec/pngdec.c
+++ b/libavcodec/pngdec.c
@@ -1097,7 +1097,7 @@ static int decode_sbit_chunk(AVCodecContext *avctx, PNGDecContext *s,
         bits = FFMAX(b, bits);
     }
 
-    if (bits < 0 || bits > s->bit_depth) {
+    if (bits <= 0 || bits > (s->color_type & PNG_COLOR_MASK_PALETTE ? 8 : s->bit_depth)) {
         av_log(avctx, AV_LOG_ERROR, "Invalid significant bits: %d\n", bits);
         return AVERROR_INVALIDDATA;
     }
-- 
2.45.2

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[FFmpeg-devel] [PATCH 2/2] avcodec/pngenc: fix sBIT writing for indexed-color PNGs

[Leo Izen]

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@gmail.com>
Reported-by: Ramiro Polla: <ramiro.polla@gmail.com>
---
 libavcodec/pngenc.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/libavcodec/pngenc.c b/libavcodec/pngenc.c
index 7ae16fa2c5..cb79c04e11 100644
--- a/libavcodec/pngenc.c
+++ b/libavcodec/pngenc.c
@@ -469,8 +469,9 @@ 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);
     }
-- 
2.45.2

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Re: [FFmpeg-devel] [PATCH 1/2] avcodec/pngdec: use 8-bit sBIT cap for indexed PNGs per spec

[Leo Izen]

On 7/19/24 12:04 PM, Leo Izen wrote:
> The PNG specification[1] says that sBIT entries must be at most the bit
> depth specified in IHDR, unless the PNG is indexed-color, in which case
> sBIT must be between 1 and 8. We should not reject valid sBITs on PNGs
> with indexed color.
> 
> [1]: https://www.w3.org/TR/png-3/#11sBIT
> 
> Regression since 84b454935fae2633a8a5dd075e22393f3e8f932f.
> 
> Signed-off-by: Leo Izen <leo.izen@gmail.com>
> Reported-by: Ramiro Polla <ramiro.polla@gmail.com>
> ---

Will merge soon.

- Leo Izen (Traneptora)

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