FFmpeg FU-A封包說明

+-+-+-+-+-+-+-+-+?????????? |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];//第一個字節(jié)是FU indicator
??? type = nal & 0x1f;//取后面五個字節(jié)，

??? /* 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)