[FFmpeg-devel] [PATCH v2 2/3] avcodec/pngdec: support decoding sRGB chunks

[Leo Izen]

If an sRGB chunk is present in the PNG file, this commit will cause the
png decoder to ignore the cHRM and gAMA chunks and tag the resulting AVFrames
with BT.709 primaries, and ISO/IEC 61966-2-1 transfer. If these tags are
present in the AVFrame, pngenc.c already writes this chunk, so no change was
needed on the encode-side.

The PNG spec does not define what happens if sRGB and iCCP are present at
the same time, it just recommends that this not happen. As of this patch,
the decoder will have the ICC profile take precedence, and it will not tag
the pixel data as sRGB.

Signed-off-by: Leo Izen <leo.izen at gmail.com>
---
 libavcodec/pngdec.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/libavcodec/pngdec.c b/libavcodec/pngdec.c
index 32e7773bcb..1f9ed894de 100644
--- a/libavcodec/pngdec.c
+++ b/libavcodec/pngdec.c
@@ -75,6 +75,7 @@ typedef struct PNGDecContext {
     int have_chrm;
     uint32_t white_point[2];
     uint32_t display_primaries[3][2];
+    int have_srgb;
 
     enum PNGHeaderState hdr_state;
     enum PNGImageState pic_state;
@@ -1313,6 +1314,11 @@ static int decode_frame_common(AVCodecContext *avctx, PNGDecContext *s,
             }
             break;
         }
+        case MKTAG('s', 'R', 'G', 'B'):
+            /* skip rendering intent byte */
+            bytestream2_skip(&gb_chunk, 1);
+            s->have_srgb = 1;
+            break;
         case MKTAG('i', 'C', 'C', 'P'): {
             if ((ret = decode_iccp_chunk(s, &gb_chunk, p)) < 0)
                 goto fail;
@@ -1468,6 +1474,7 @@ static void clear_frame_metadata(PNGDecContext *s)
     s->stereo_mode = -1;
 
     s->have_chrm = 0;
+    s->have_srgb = 0;
 
     av_dict_free(&s->frame_metadata);
 }
@@ -1486,6 +1493,9 @@ static int output_frame(PNGDecContext *s, AVFrame *f)
         memcpy(sd->data, s->iccp_data, s->iccp_data_len);
 
         av_dict_set(&sd->metadata, "name", s->iccp_name, 0);
+    } else if (s->have_srgb) {
+        avctx->color_primaries = f->color_primaries = AVCOL_PRI_BT709;
+        avctx->color_trc = f->color_trc = AVCOL_TRC_IEC61966_2_1;
     } else if (s->have_chrm) {
         AVColorPrimariesDesc desc;
         enum AVColorPrimaries prim;
@@ -1504,8 +1514,8 @@ static int output_frame(PNGDecContext *s, AVFrame *f)
             av_log(avctx, AV_LOG_WARNING, "unknown cHRM primaries\n");
     }
 
-    /* this chunk overrides gAMA */
-    if (s->iccp_data)
+    /* these chunks overrides gAMA */
+    if (s->iccp_data || s->have_srgb)
         av_dict_set(&s->frame_metadata, "gamma", NULL, 0);
 
     avctx->colorspace = f->colorspace = AVCOL_SPC_RGB;
-- 
2.39.0