hevc: 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/hevc_sei.c

@@ -265,6 +265,13 @@ static int decode_nal_sei_active_parameter_sets(HEVCSEIContext *s, GetBitContext
     return 0;
 }
 
+static int decode_nal_sei_alternative_transfer(HEVCSEIAlternativeTransfer *s, GetBitContext *gb)
+{
+    s->present = 1;
+    s->preferred_transfer_characteristics = get_bits(gb, 8);
+    return 0;
+}
+
 static int decode_nal_sei_prefix(GetBitContext *gb, HEVCSEIContext *s, const HEVCParamSets *ps,
                                  int type, int size, void *logctx)
 {
@@ -285,6 +292,8 @@ static int decode_nal_sei_prefix(GetBitContext *gb, HEVCSEIContext *s, const HEV
         return decode_nal_sei_active_parameter_sets(s, gb, logctx);
     case HEVC_SEI_TYPE_USER_DATA_REGISTERED_ITU_T_T35:
         return decode_nal_sei_user_data_registered_itu_t_t35(s, gb, size);
+    case HEVC_SEI_TYPE_ALTERNATIVE_TRANSFER_CHARACTERISTICS:
+        return decode_nal_sei_alternative_transfer(&s->alternative_transfer, gb);
     default:
         av_log(logctx, AV_LOG_DEBUG, "Skipped PREFIX SEI %d\n", type);
         skip_bits_long(gb, 8 * size);

libavcodec/hevc_sei.h

@@ -56,6 +56,7 @@ typedef enum {
     HEVC_SEI_TYPE_REGION_REFRESH_INFO                  = 134,
     HEVC_SEI_TYPE_MASTERING_DISPLAY_INFO               = 137,
     HEVC_SEI_TYPE_CONTENT_LIGHT_LEVEL_INFO             = 144,
+    HEVC_SEI_TYPE_ALTERNATIVE_TRANSFER_CHARACTERISTICS = 147,
 } HEVC_SEI_Type;
 
 typedef struct HEVCSEIPictureHash {
@@ -100,6 +101,11 @@ typedef struct HEVCSEIContentLight {
     uint16_t max_pic_average_light_level;
 } HEVCSEIContentLight;
 
+typedef struct HEVCSEIAlternativeTransfer {
+    int present;
+    int preferred_transfer_characteristics;
+} HEVCSEIAlternativeTransfer;
+
 typedef struct HEVCSEIContext {
     HEVCSEIPictureHash picture_hash;
     HEVCSEIFramePacking frame_packing;
@@ -109,6 +115,7 @@ typedef struct HEVCSEIContext {
     HEVCSEIMasteringDisplay mastering_display;
     HEVCSEIContentLight content_light;
     int active_seq_parameter_set_id;
+    HEVCSEIAlternativeTransfer alternative_transfer;
 } HEVCSEIContext;
 
 struct HEVCParamSets;

libavcodec/hevcdec.c

@@ -2725,6 +2725,12 @@ static int set_side_data(HEVCContext *s)
         s->avctx->properties |= FF_CODEC_PROPERTY_CLOSED_CAPTIONS;
     }
 
+    if (s->sei.alternative_transfer.present &&
+        av_color_transfer_name(s->sei.alternative_transfer.preferred_transfer_characteristics) &&
+        s->sei.alternative_transfer.preferred_transfer_characteristics != AVCOL_TRC_UNSPECIFIED) {
+        s->avctx->color_trc = s->sei.alternative_transfer.preferred_transfer_characteristics;
+    }
+
     return 0;
 }