[FFmpeg-devel] [PATCH 1/2] avcodec/pngdec: support sBIT chunks

[Leo Izen <leo.izen@gmail.com>]

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.izen@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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