libavcodec: factor out SEI generation for A53 captions

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/utils.c

@@ -4178,3 +4178,46 @@ int avcodec_parameters_to_context(AVCodecContext *codec,
 
     return 0;
 }
+
+int ff_alloc_a53_sei(const AVFrame *frame, size_t prefix_len,
+                     void **data, size_t *sei_size)
+{
+    AVFrameSideData *side_data = NULL;
+    uint8_t *sei_data;
+
+    if (frame)
+        side_data = av_frame_get_side_data(frame, AV_FRAME_DATA_A53_CC);
+
+    if (!side_data) {
+        *data = NULL;
+        return 0;
+    }
+
+    *sei_size = side_data->size + 11;
+    *data = av_mallocz(*sei_size + prefix_len);
+    if (!*data)
+        return AVERROR(ENOMEM);
+    sei_data = (uint8_t*)*data + prefix_len;
+
+    // country code
+    sei_data[0] = 181;
+    sei_data[1] = 0;
+    sei_data[2] = 49;
+
+    /**
+     * 'GA94' is standard in North America for ATSC, but hard coding
+     * this style may not be the right thing to do -- other formats
+     * do exist. This information is not available in the side_data
+     * so we are going with this right now.
+     */
+    AV_WL32(sei_data + 3, MKTAG('G', 'A', '9', '4'));
+    sei_data[7] = 3;
+    sei_data[8] = ((side_data->size/3) & 0x1f) | 0x40;
+    sei_data[9] = 0;
+
+    memcpy(sei_data + 10, side_data->data, side_data->size);
+
+    sei_data[side_data->size+10] = 255;
+
+    return 0;
+}