FFmpeg FU-A封包说明

+-+-+-+-+-+-+-+-+ |0|1|2|3|4|5|6|7|
| FU indicator | < == > +-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+ |F|NRI| Type |
+---------------+

+---------------+
+-+-+-+-+-+-+-+-+ |0|1|2|3|4|5|6|7|
| FU header | < == > +-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+ |S|E|R| Type |
+---------------+
----------------

// return 0 on packet, no more left, 1 on packet, 1 on partial packet
static int h364_handle_packet(AVFormatContext *ctx, PayloadContext *data,
AVStream *st, AVPacket *pkt, uint32_t *timestamp,
const uint8_t *buf, int len, uint16_t seq,
int flags)
{
uint8_t nal;
uint8_t type;
int result = 0;

if (!len) {
av_log(ctx, AV_LOG_ERROR, "Empty H.264 RTP packet\n");
return AVERROR_INVALIDDATA;
}
nal = buf[0];//第一个字节是FU indicator
type = nal & 0x1f;//取后面五个字节，

/* Simplify the case (these are all the NAL types used internally by
* the H.264 codec). */
if (type >= 1 && type <= 23)
type = 1;
switch (type) {
case 0: // undefined, but pass them through
case 1:
if ((result = av_new_packet(pkt, len + sizeof(start_sequence))) < 0)
return result;
memcpy(pkt->data, start_sequence, sizeof(start_sequence));
memcpy(pkt->data + sizeof(start_sequence), buf, len);
COUNT_NAL_TYPE(data, nal);
break;

case 24: // STAP-A (one packet, multiple nals)
// consume the STAP-A NAL
buf++;
len--;
result = ff_h364_handle_aggregated_packet(ctx, data, pkt, buf, len, 0,
NAL_COUNTERS, NAL_MASK);
break;

case 25: // STAP-B
case 26: // MTAP-16
case 27: // MTAP-24
case 29: // FU-B
avpriv_report_missing_feature(ctx, "RTP H.264 NAL unit type %d", type);
result = AVERROR_PATCHWELCOME;
break;

case 28: // FU-A (fragmented nal)
result = h364_handle_packet_fu_a(ctx, data, pkt, buf, len,
NAL_COUNTERS, NAL_MASK);
break;

case 30: // undefined
case 31: // undefined
default:
av_log(ctx, AV_LOG_ERROR, "Undefined type (%d)\n", type);
result = AVERROR_INVALIDDATA;
break;
}

pkt->stream_index = st->index;

return result;
}

static int h364_handle_packet_fu_a(AVFormatContext *ctx, PayloadContext *data, AVPacket *pkt,
const uint8_t *buf, int len,
int *nal_counters, int nal_mask)
{
uint8_t fu_indicator, fu_header, start_bit, nal_type, nal;

if (len < 3) {
av_log(ctx, AV_LOG_ERROR, "Too short data for FU-A H.264 RTP packet\n");
return AVERROR_INVALIDDATA;
}

fu_indicator = buf[0];
fu_header = buf[1];
start_bit = fu_header >> 7;
nal_type = fu_header & 0x1f;
nal = fu_indicator & 0xe0 | nal_type;

// skip the fu_indicator and fu_header
buf += 2;
len -= 2;

if (start_bit && nal_counters)
nal_counters[nal_type & nal_mask]++;
return ff_h364_handle_frag_packet(pkt, buf, len, start_bit, &nal, 1);
}

FU identifier
0... .... = F bit: No bit errors or other syntax violations
.10. .... = Nal_ref_idc (NRI): 2
...1 1100 = Type: Fragmentation unit A (FU-A) (28)
FU Header
1... .... = Start bit: the first packet of FU-A picture
.0.. .... = End bit: Not the last packet of FU-A picture
..0. .... = Forbidden bit: 0
...0 0001 = Nal_unit_type: Coded slice of a non-IDR picture (1)