Add support for reading sBIT chunks, which mark the significant
bit depth of the PNG file. This passes the metadata using the field
bits_per_raw_sample of AVCodecContext.
Signed-off-by: Leo Izen <leo.i...@gmail.com>
---
libavcodec/pngdec.c | 57 +++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 57 insertions(+)
diff --git a/libavcodec/pngdec.c b/libavcodec/pngdec.c
index 0d969decf2..48313db671 100644
--- a/libavcodec/pngdec.c
+++ b/libavcodec/pngdec.c
@@ -97,6 +97,7 @@ typedef struct PNGDecContext {
int bpp;
int has_trns;
uint8_t transparent_color_be[6];
+ int significant_bits;
uint32_t palette[256];
uint8_t *crow_buf;
@@ -916,6 +917,55 @@ fail:
return ret;
}
+static int decode_sbit_chunk(AVCodecContext *avctx, PNGDecContext *s,
+ GetByteContext *gb)
+{
+ int bits = 0;
+
+ if (!(s->hdr_state & PNG_IHDR)) {
+ av_log(avctx, AV_LOG_ERROR, "sBIT before IHDR\n");
+ return AVERROR_INVALIDDATA;
+ }
+
+ switch (s->color_type) {
+ case 0:
+ bits = bytestream2_get_byteu(gb);
+ break;
+ case 2:
+ case 3: {
+ int r = bytestream2_get_byteu(gb);
+ int g = bytestream2_get_byteu(gb);
+ int b = bytestream2_get_byteu(gb);
+ bits = FFMAX3(r, g, b);
+ break;
+ }
+ case 4: {
+ int y = bytestream2_get_byteu(gb);
+ int a = bytestream2_get_byteu(gb);
+ bits = FFMAX(y, a);
+ break;
+ }
+ case 6: {
+ int r = bytestream2_get_byteu(gb);
+ int g = bytestream2_get_byteu(gb);
+ int b = bytestream2_get_byteu(gb);
+ int a = bytestream2_get_byteu(gb);
+ bits = FFMAX(FFMAX3(r, g, b), a);
+ break;
+ }
+ default:
+ return AVERROR_INVALIDDATA;
+ }
+
+ if (bits < 0 || bits > s->bit_depth) {
+ av_log(avctx, AV_LOG_ERROR, "Invalid significant bits: %d\n", bits);
+ return AVERROR_INVALIDDATA;
+ }
+ s->significant_bits = bits;
+
+ return 0;
+}
+
static void handle_small_bpp(PNGDecContext *s, AVFrame *p)
{
if (s->bits_per_pixel == 1 && s->color_type == PNG_COLOR_TYPE_PALETTE) {
@@ -1355,6 +1405,9 @@ static int decode_frame_common(AVCodecContext *avctx,
PNGDecContext *s,
break;
}
+ case MKTAG('s', 'B', 'I', 'T'):
+ decode_sbit_chunk(avctx, s, &gb_chunk);
+ break;
case MKTAG('g', 'A', 'M', 'A'): {
AVBPrint bp;
char *gamma_str;
@@ -1560,6 +1613,10 @@ static int output_frame(PNGDecContext *s, AVFrame *f)
stereo3d->flags = s->stereo_mode ? 0 : AV_STEREO3D_FLAG_INVERT;
}
+ /* tRNS sets alpha depth to full, so we ignore sBIT if set */
+ if (!s->has_trns && s->significant_bits > 0)
+ avctx->bits_per_raw_sample = s->significant_bits;
+
FFSWAP(AVDictionary*, f->metadata, s->frame_metadata);
return 0;
--
2.39.1
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