h264: Add support for alternative transfer characterics SEI

The use of this SEI is for backward compatibility in HLG HDR systems:
older devices that cannot interpret the "arib-std-b67" transfer will
get the compatible transfer (usually bt709 or bt2020) from the VUI,
while newer devices that can interpret HDR will read the SEI and use
its value instead.
Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>

libavcodec/h264_sei.c

@@ -346,6 +346,14 @@ static int decode_display_orientation(H264SEIDisplayOrientation *h,
     return 0;
 }
 
+static int decode_alternative_transfer(H264SEIAlternativeTransfer *h,
+                                       GetBitContext *gb)
+{
+    h->present = 1;
+    h->preferred_transfer_characteristics = get_bits(gb, 8);
+    return 0;
+}
+
 int ff_h264_sei_decode(H264SEIContext *h, GetBitContext *gb,
                        const H264ParamSets *ps, void *logctx)
 {
@@ -396,6 +404,9 @@ int ff_h264_sei_decode(H264SEIContext *h, GetBitContext *gb,
         case H264_SEI_TYPE_DISPLAY_ORIENTATION:
             ret = decode_display_orientation(&h->display_orientation, gb);
             break;
+        case H264_SEI_TYPE_ALTERNATIVE_TRANSFER:
+            ret = decode_alternative_transfer(&h->alternative_transfer, gb);
+            break;
         default:
             av_log(logctx, AV_LOG_DEBUG, "unknown SEI type %d\n", type);
             skip_bits(gb, 8 * size);

libavcodec/h264_sei.h

@@ -33,6 +33,7 @@ typedef enum {
     H264_SEI_TYPE_RECOVERY_POINT         = 6,   ///< recovery point (frame # to decoder sync)
     H264_SEI_TYPE_FRAME_PACKING          = 45,  ///< frame packing arrangement
     H264_SEI_TYPE_DISPLAY_ORIENTATION    = 47,  ///< display orientation
+    H264_SEI_TYPE_ALTERNATIVE_TRANSFER   = 147, ///< alternative transfer
 } H264_SEI_Type;
 
 /**
@@ -115,6 +116,11 @@ typedef struct H264SEIDisplayOrientation {
     int hflip, vflip;
 } H264SEIDisplayOrientation;
 
+typedef struct H264SEIAlternativeTransfer {
+    int present;
+    int preferred_transfer_characteristics;
+} H264SEIAlternativeTransfer;
+
 typedef struct H264SEIContext {
     H264SEIPictureTiming picture_timing;
     H264SEIAFD afd;
@@ -124,6 +130,7 @@ typedef struct H264SEIContext {
     H264SEIBufferingPeriod buffering_period;
     H264SEIFramePacking frame_packing;
     H264SEIDisplayOrientation display_orientation;
+    H264SEIAlternativeTransfer alternative_transfer;
 } H264SEIContext;
 
 struct H264ParamSets;

libavcodec/h264_slice.c

@@ -1154,6 +1154,12 @@ static int h264_export_frame_props(H264Context *h)
         a53->a53_caption_size = 0;
     }
 
+    if (h->sei.alternative_transfer.present &&
+        av_color_transfer_name(h->sei.alternative_transfer.preferred_transfer_characteristics) &&
+        h->sei.alternative_transfer.preferred_transfer_characteristics != AVCOL_TRC_UNSPECIFIED) {
+        h->avctx->color_trc = cur->f->color_trc = h->sei.alternative_transfer.preferred_transfer_characteristics;
+    }
+
     return 0;
 }