FFmpeg的H.264解码器源代码简单分析：解析器（Parser）部分
来源：互联网 发布：数据库审计系统作用 编辑：程序博客网 时间：2024/05/06 03:26
=====================================================
H.264源代码分析文章列表：
【编码 - x264】
x264源代码简单分析：概述
x264源代码简单分析：x264命令行工具（x264.exe）
x264源代码简单分析：编码器主干部分-1
x264源代码简单分析：编码器主干部分-2
x264源代码简单分析：x264_slice_write()
x264源代码简单分析：滤波（Filter）部分
x264源代码简单分析：宏块分析（Analysis）部分-帧内宏块（Intra）
x264源代码简单分析：宏块分析（Analysis）部分-帧间宏块（Inter）
x264源代码简单分析：宏块编码（Encode）部分
x264源代码简单分析：熵编码（Entropy Encoding）部分
FFmpeg与libx264接口源代码简单分析
【解码 - libavcodec H.264 解码器】
FFmpeg的H.264解码器源代码简单分析：概述
FFmpeg的H.264解码器源代码简单分析：解析器（Parser）部分
FFmpeg的H.264解码器源代码简单分析：解码器主干部分
FFmpeg的H.264解码器源代码简单分析：熵解码（EntropyDecoding）部分
FFmpeg的H.264解码器源代码简单分析：宏块解码（Decode）部分-帧内宏块（Intra）
FFmpeg的H.264解码器源代码简单分析：宏块解码（Decode）部分-帧间宏块（Inter）
FFmpeg的H.264解码器源代码简单分析：环路滤波（Loop Filter）部分
=====================================================
本文继续分析FFmpeg中libavcodec的H.264解码器（H.264 Decoder）。上篇文章概述了FFmpeg中H.264解码器的结构；从这篇文章开始，具体研究H.264解码器的源代码。本文分析H.264解码器中解析器（Parser）部分的源代码。这部分的代码用于分割H.264的NALU，并且解析SPS、PPS、SEI等信息。解析H.264码流（对应AVCodecParser结构体中的函数）和解码H.264码流（对应AVCodec结构体中的函数）的时候都会调用该部分的代码完成相应的功能。
函数调用关系图
解析器（Parser）部分的源代码在整个H.264解码器中的位置如下图所示。
单击查看更清晰的图片
解析器（Parser）部分的源代码的调用关系如下图所示。
单击查看更清晰的图片
图中简单列举了几个解析特定NALU的函数：
ff_h264_decode_nal()：解析NALU HeaderH.264解码器与H.264解析器最主要的不同的地方在于它调用了ff_h264_execute_decode_slices()函数进行了解码工作。这篇文章只分析H.264解析器的源代码，至于H.264解码器的源代码，则在后面几篇文章中再进行分析。
ff_h264_decode_seq_parameter_set()：解析SPS
ff_h264_decode_picture_parameter_set()：解析PPS
ff_h264_decode_sei()：解析SEI
ff_h264_decoder
ff_h264_decoder是FFmpeg的H.264解码器对应的AVCodec结构体。它的定义位于libavcodec\h264.c，如下所示。AVCodec ff_h264_decoder = { .name = "h264", .long_name = NULL_IF_CONFIG_SMALL("H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10"), .type = AVMEDIA_TYPE_VIDEO, .id = AV_CODEC_ID_H264, .priv_data_size = sizeof(H264Context), .init = ff_h264_decode_init, .close = h264_decode_end, .decode = h264_decode_frame, .capabilities = /*CODEC_CAP_DRAW_HORIZ_BAND |*/ CODEC_CAP_DR1 | CODEC_CAP_DELAY | CODEC_CAP_SLICE_THREADS | CODEC_CAP_FRAME_THREADS, .flush = flush_dpb, .init_thread_copy = ONLY_IF_THREADS_ENABLED(decode_init_thread_copy), .update_thread_context = ONLY_IF_THREADS_ENABLED(ff_h264_update_thread_context), .profiles = NULL_IF_CONFIG_SMALL(profiles), .priv_class = &h264_class,};从ff_h264_decoder的定义可以看出：解码器初始化的函数指针init()指向ff_h264_decode_init()函数，解码的函数指针decode()指向h264_decode_frame()函数，解码器关闭的函数指针close()指向h264_decode_end()函数。
有关H.264解码器这方面的源代码在以后的文章中再进行详细的分析。在这里我们只需要知道h264_decode_frame()内部调用了decode_nal_units()，而decode_nal_units()调用了和H.264解析器（Parser）有关的源代码就可以了。
ff_h264_parser
ff_h264_parser是FFmpeg的H.264解析器对应的AVCodecParser结构体。它的定义位于libavcodec\h264_parser.c，如下所示。AVCodecParser ff_h264_parser = { .codec_ids = { AV_CODEC_ID_H264 }, .priv_data_size = sizeof(H264Context), .parser_init = init, .parser_parse = h264_parse, .parser_close = close, .split = h264_split,};从ff_h264_parser的定义可以看出：AVCodecParser初始化的函数指针parser_init()指向init()函数；解析数据的函数指针parser_parse()指向h264_parse()函数；销毁的函数指针parser_close()指向close()函数。下面分别看看这些函数。
init() [对应于AVCodecParser-> parser_init()]
ff_h264_parser结构体中AVCodecParser的parser_init()指向init()函数。该函数完成了AVCodecParser的初始化工作。函数的定义很简单，如下所示。static av_cold int init(AVCodecParserContext *s){ H264Context *h = s->priv_data; h->thread_context[0] = h; h->slice_context_count = 1; ff_h264dsp_init(&h->h264dsp, 8, 1); return 0;}close() [对应于AVCodecParser-> parser_close()]
ff_h264_parser结构体中AVCodecParser的parser_close()指向close()函数。该函数完成了AVCodecParser的关闭工作。函数的定义也比较简单，如下所示。static void close(AVCodecParserContext *s){ H264Context *h = s->priv_data; ParseContext *pc = &h->parse_context; av_freep(&pc->buffer); ff_h264_free_context(h);}h264_parse() [对应于AVCodecParser-> parser_parse()]
ff_h264_parser结构体中AVCodecParser的parser_parse()指向h264_parse()函数。该函数完成了AVCodecParser的解析工作（在这里就是H.264码流的解析工作）。h264_parse()的定义位于libavcodec\h264_parser.c，如下所示。//解析H.264码流//输出一个完整的NAL，存储于poutbuf中static int h264_parse(AVCodecParserContext *s, AVCodecContext *avctx, const uint8_t **poutbuf, int *poutbuf_size, const uint8_t *buf, int buf_size){ H264Context *h = s->priv_data; ParseContext *pc = &h->parse_context; int next; //如果还没有解析过1帧，就调用这里解析extradata if (!h->got_first) { h->got_first = 1; if (avctx->extradata_size) { h->avctx = avctx; // must be done like in decoder, otherwise opening the parser, // letting it create extradata and then closing and opening again // will cause has_b_frames to be always set. // Note that estimate_timings_from_pts does exactly this. if (!avctx->has_b_frames) h->low_delay = 1; //解析AVCodecContext的extradata ff_h264_decode_extradata(h, avctx->extradata, avctx->extradata_size); } } //输入的数据是完整的一帧？ //这里通过设置flags的PARSER_FLAG_COMPLETE_FRAMES来确定 if (s->flags & PARSER_FLAG_COMPLETE_FRAMES) { //和缓存大小一样 next = buf_size; } else { //查找帧结尾（帧开始）位置 //以“起始码”为依据（0x000001或0x00000001） next = h264_find_frame_end(h, buf, buf_size); //组帧 if (ff_combine_frame(pc, next, &buf, &buf_size) < 0) { *poutbuf = NULL; *poutbuf_size = 0; return buf_size; } if (next < 0 && next != END_NOT_FOUND) { av_assert1(pc->last_index + next >= 0); h264_find_frame_end(h, &pc->buffer[pc->last_index + next], -next); // update state } } //解析NALU，从SPS、PPS、SEI等中获得一些基本信息。 //此时buf中存储的是完整的1帧数据 parse_nal_units(s, avctx, buf, buf_size); if (avctx->framerate.num) avctx->time_base = av_inv_q(av_mul_q(avctx->framerate, (AVRational){avctx->ticks_per_frame, 1})); if (h->sei_cpb_removal_delay >= 0) { s->dts_sync_point = h->sei_buffering_period_present; s->dts_ref_dts_delta = h->sei_cpb_removal_delay; s->pts_dts_delta = h->sei_dpb_output_delay; } else { s->dts_sync_point = INT_MIN; s->dts_ref_dts_delta = INT_MIN; s->pts_dts_delta = INT_MIN; } if (s->flags & PARSER_FLAG_ONCE) { s->flags &= PARSER_FLAG_COMPLETE_FRAMES; } //分割后的帧数据输出至poutbuf *poutbuf = buf; *poutbuf_size = buf_size; return next;}从源代码可以看出，h264_parse()主要完成了以下3步工作：
（1）如果是第一次解析，则首先调用ff_h264_decode_extradata()解析AVCodecContext的extradata（里面实际上存储了H.264的SPS、PPS）。下面分别看一下这3步中涉及到的函数：ff_h264_decode_extradata()，h264_find_frame_end()，parse_nal_units()。
（2）如果传入的flags 中包含PARSER_FLAG_COMPLETE_FRAMES，则说明传入的是完整的一帧数据，不作任何处理；如果不包含PARSER_FLAG_COMPLETE_FRAMES，则说明传入的不是完整的一帧数据而是任意一段H.264数据，则需要调用h264_find_frame_end()通过查找“起始码”（0x00000001或者0x000001）的方法，分离出完整的一帧数据。（3）调用parse_nal_units()完成了NALU的解析工作。
ff_h264_decode_extradata()
ff_h264_decode_extradata()用于解析AVCodecContext的extradata（里面实际上存储了H.264的SPS、PPS）。ff_h264_decode_extradata()的定义如下所示。//解析extradata//最常见的就是解析AVCodecContext的extradata。其中extradata实际上存储的就是SPS、PPSint ff_h264_decode_extradata(H264Context *h, const uint8_t *buf, int size){ AVCodecContext *avctx = h->avctx; int ret; if (!buf || size <= 0) return -1; if (buf[0] == 1) { int i, cnt, nalsize; const unsigned char *p = buf; //AVC1 描述:H.264 bitstream without start codes.是不带起始码0×00000001的。MKV/MOV/FLV中的H.264属于这种类型 //H264 描述:H.264 bitstream with start codes.是带有起始码0×00000001的。MPEGTS中的H.264，或者H.264裸流属于这种类型 h->is_avc = 1; //数据量太小 //随意测了一个视频 //SPS: 30 Byte //PPS: 6 Byte if (size < 7) { av_log(avctx, AV_LOG_ERROR, "avcC %d too short\n", size); return AVERROR_INVALIDDATA; } /* sps and pps in the avcC always have length coded with 2 bytes, * so put a fake nal_length_size = 2 while parsing them */ h->nal_length_size = 2; // Decode sps from avcC //解码SPS cnt = *(p + 5) & 0x1f; // Number of sps p += 6; for (i = 0; i < cnt; i++) { nalsize = AV_RB16(p) + 2; if(nalsize > size - (p-buf)) return AVERROR_INVALIDDATA; //解析 ret = decode_nal_units(h, p, nalsize, 1); if (ret < 0) { av_log(avctx, AV_LOG_ERROR, "Decoding sps %d from avcC failed\n", i); return ret; } p += nalsize; } // Decode pps from avcC //解码PPS cnt = *(p++); // Number of pps for (i = 0; i < cnt; i++) { nalsize = AV_RB16(p) + 2; if(nalsize > size - (p-buf)) return AVERROR_INVALIDDATA; ret = decode_nal_units(h, p, nalsize, 1); if (ret < 0) { av_log(avctx, AV_LOG_ERROR, "Decoding pps %d from avcC failed\n", i); return ret; } p += nalsize; } // Store right nal length size that will be used to parse all other nals h->nal_length_size = (buf[4] & 0x03) + 1; } else { h->is_avc = 0; //解析 ret = decode_nal_units(h, buf, size, 1); if (ret < 0) return ret; } return size;}从源代码中可以看出，ff_h264_decode_extradata()调用decode_nal_units()解析SPS、PPS信息。有关decode_nal_units()的源代码在后续文章中再进行分析。
h264_find_frame_end()
h264_find_frame_end()用于查找H.264码流中的“起始码”（start code）。在H.264码流中有两种起始码：0x000001和0x00000001。其中4Byte的长度的起始码最为常见。只有当一个完整的帧被编为多个slice的时候，包含这些slice的NALU才会使用3Byte的起始码。h264_find_frame_end()的定义位于libavcodec\h264_parser.c，如下所示。//查找帧结尾（帧开始）位置////几种状态state：//2 - 找到1个0//1 - 找到2个0//0 - 找到大于等于3个0//4 - 找到2个0和1个1，即001（即找到了起始码）//5 - 找到至少3个0和1个1，即0001等等（即找到了起始码）//7 - 初始化状态//>=8 - 找到2个Slice Header////关于起始码startcode的两种形式：3字节的0x000001和4字节的0x00000001//3字节的0x000001只有一种场合下使用，就是一个完整的帧被编为多个slice的时候，//包含这些slice的nalu使用3字节起始码。其余场合都是4字节的。//static int h264_find_frame_end(H264Context *h, const uint8_t *buf, int buf_size){ int i, j; uint32_t state; ParseContext *pc = &h->parse_context; int next_avc= h->is_avc ? 0 : buf_size;// mb_addr= pc->mb_addr - 1; state = pc->state; if (state > 13) state = 7; if (h->is_avc && !h->nal_length_size) av_log(h->avctx, AV_LOG_ERROR, "AVC-parser: nal length size invalid\n"); // //每次循环前进1个字节，读取该字节的值 //根据此前的状态state做不同的处理 //state取值为4,5代表找到了起始码 //类似于一个状态机，简单画一下状态转移图： // +-----+ // | | // v | // 7--(0)-->2--(0)-->1--(0)-->0-(0)-+ // ^ | | | // | (1) (1) (1) // | | | | // +--------+ v v // 4 5 // for (i = 0; i < buf_size; i++) { //超过了 if (i >= next_avc) { int nalsize = 0; i = next_avc; for (j = 0; j < h->nal_length_size; j++) nalsize = (nalsize << 8) | buf[i++]; if (nalsize <= 0 || nalsize > buf_size - i) { av_log(h->avctx, AV_LOG_ERROR, "AVC-parser: nal size %d remaining %d\n", nalsize, buf_size - i); return buf_size; } next_avc = i + nalsize; state = 5; } //初始state为7 if (state == 7) { //查找startcode的候选者？ //从一段内存中查找取值为0的元素的位置并返回 //增加i取值 i += h->h264dsp.startcode_find_candidate(buf + i, next_avc - i); //因为找到1个0，状态转换为2 if (i < next_avc) state = 2; } else if (state <= 2) { //找到0时候的state。包括1个0（状态2），2个0（状态1），或者3个及3个以上0（状态0）。 if (buf[i] == 1) //发现了一个1 state ^= 5; //状态转换关系：2->7, 1->4, 0->5。状态4代表找到了001，状态5代表找到了0001 else if (buf[i]) state = 7; //恢复初始 else //发现了一个0 state >>= 1; // 2->1, 1->0, 0->0 } else if (state <= 5) { //状态4代表找到了001，状态5代表找到了0001 //获取NALU类型 //NALU Header（1Byte）的后5bit int nalu_type = buf[i] & 0x1F; if (nalu_type == NAL_SEI || nalu_type == NAL_SPS || nalu_type == NAL_PPS || nalu_type == NAL_AUD) { //SPS，PPS，SEI类型的NALU if (pc->frame_start_found) { //如果之前已找到了帧头 i++; goto found; } } else if (nalu_type == NAL_SLICE || nalu_type == NAL_DPA || nalu_type == NAL_IDR_SLICE) { //表示有slice header的NALU //大于等于8的状态表示找到了两个帧头，但没有找到帧尾的状态 state += 8; continue; } //上述两个条件都不满足，回归初始状态（state取值7） state = 7; } else { h->parse_history[h->parse_history_count++]= buf[i]; if (h->parse_history_count>5) { unsigned int mb, last_mb= h->parse_last_mb; GetBitContext gb; init_get_bits(&gb, h->parse_history, 8*h->parse_history_count); h->parse_history_count=0; mb= get_ue_golomb_long(&gb); h->parse_last_mb= mb; if (pc->frame_start_found) { if (mb <= last_mb) goto found; } else pc->frame_start_found = 1; state = 7; } } } pc->state = state; if (h->is_avc) return next_avc; //没找到 return END_NOT_FOUND;found: pc->state = 7; pc->frame_start_found = 0; if (h->is_avc) return next_avc; //state=4时候，state & 5=4 //找到的是001（长度为3），i减小3+1=4，标识帧结尾 //state=5时候，state & 5=5 //找到的是0001（长度为4），i减小4+1=5，标识帧结尾 return i - (state & 5) - 5 * (state > 7);}从源代码可以看出，h264_find_frame_end()使用了一种类似于状态机的方式查找起始码。函数中的for()循环每执行一遍，状态机的状态就会改变一次。该状态机主要包含以下几种状态：
7 - 初始化状态
2 - 找到1个0
1 - 找到2个0
0 - 找到大于等于3个0
4 - 找到2个0和1个1，即001（即找到了起始码）
5 - 找到至少3个0和1个1，即0001等等（即找到了起始码）
>=8 - 找到2个Slice Header
这些状态之间的状态转移图如下所示。图中粉红色代表初始状态，绿色代表找到“起始码”的状态。
如图所示，h264_find_frame_end()初始化时候位于状态“7”；当找到1个“0”之后，状态从“7”变为“2”；在状态“2”下，如果再次找到1个“0”，则状态变为“1”；在状态“1”下，如果找到“1”，则状态变换为“4”，表明找到了“0x000001”起始码；在状态“1”下，如果找到“0”，则状态变换为“0”；在状态“0”下，如果找到“1”，则状态变换为“5” ，表明找到了“0x000001”起始码。
其中，在查找数据中第1个“0”的时候，使用了H264DSPContext结构体中的startcode_find_candidate()函数。startcode_find_candidate()除了包含C语言版本的函数外，还包含了ARMV6等平台下经过汇编优化的函数（估计效率会比C语言版本函数高一些）。C语言版本的函数ff_startcode_find_candidate_c()的定义很简单，位于libavcodec\startcode.c，如下所示。
int ff_startcode_find_candidate_c(const uint8_t *buf, int size){ int i = 0; for (; i < size; i++) if (!buf[i]) break; return i;}parse_nal_units()
parse_nal_units()用于解析NALU，从SPS、PPS、SEI等中获得一些基本信息。在该函数中，根据NALU的不同，分别调用不同的函数进行具体的处理。parse_nal_units()的定义位于libavcodec\h264_parser.c，如下所示。/** * Parse NAL units of found picture and decode some basic information. * * @param s parser context. * @param avctx codec context. * @param buf buffer with field/frame data. * @param buf_size size of the buffer. *///解析NALU，从SPS、PPS、SEI等中获得一些基本信息。static inline int parse_nal_units(AVCodecParserContext *s, AVCodecContext *avctx, const uint8_t * const buf, int buf_size){ H264Context *h = s->priv_data; int buf_index, next_avc; unsigned int pps_id; unsigned int slice_type; int state = -1, got_reset = 0; const uint8_t *ptr; int q264 = buf_size >=4 && !memcmp("Q264", buf, 4); int field_poc[2]; /* set some sane default values */ s->pict_type = AV_PICTURE_TYPE_I; s->key_frame = 0; s->picture_structure = AV_PICTURE_STRUCTURE_UNKNOWN; h->avctx = avctx; ff_h264_reset_sei(h); h->sei_fpa.frame_packing_arrangement_cancel_flag = -1; if (!buf_size) return 0; buf_index = 0; next_avc = h->is_avc ? 0 : buf_size; for (;;) { int src_length, dst_length, consumed, nalsize = 0; if (buf_index >= next_avc) { nalsize = get_avc_nalsize(h, buf, buf_size, &buf_index); if (nalsize < 0) break; next_avc = buf_index + nalsize; } else { buf_index = find_start_code(buf, buf_size, buf_index, next_avc); if (buf_index >= buf_size) break; if (buf_index >= next_avc) continue; } src_length = next_avc - buf_index; //NALU Header (1 Byte) state = buf[buf_index]; switch (state & 0x1f) { case NAL_SLICE: case NAL_IDR_SLICE: // Do not walk the whole buffer just to decode slice header if ((state & 0x1f) == NAL_IDR_SLICE || ((state >> 5) & 0x3) == 0) { /* IDR or disposable slice * No need to decode many bytes because MMCOs shall not be present. */ if (src_length > 60) src_length = 60; } else { /* To decode up to MMCOs */ if (src_length > 1000) src_length = 1000; } break; } //解析NAL Header，获得nal_unit_type等信息 ptr = ff_h264_decode_nal(h, buf + buf_index, &dst_length, &consumed, src_length); if (!ptr || dst_length < 0) break; buf_index += consumed; //初始化GetBitContext //H264Context->gb //后面的解析都是从这里获取数据 init_get_bits(&h->gb, ptr, 8 * dst_length); switch (h->nal_unit_type) { case NAL_SPS: //解析SPS ff_h264_decode_seq_parameter_set(h); break; case NAL_PPS: //解析PPS ff_h264_decode_picture_parameter_set(h, h->gb.size_in_bits); break; case NAL_SEI: //解析SEI ff_h264_decode_sei(h); break; case NAL_IDR_SLICE: //如果是IDR Slice //赋值AVCodecParserContext的key_frame为1 s->key_frame = 1; h->prev_frame_num = 0; h->prev_frame_num_offset = 0; h->prev_poc_msb = h->prev_poc_lsb = 0; /* fall through */ case NAL_SLICE: //获取Slice的一些信息 //跳过first_mb_in_slice这一字段 get_ue_golomb_long(&h->gb); // skip first_mb_in_slice //获取帧类型（I,B,P） slice_type = get_ue_golomb_31(&h->gb); //赋值到AVCodecParserContext的pict_type（外部可以访问到） s->pict_type = golomb_to_pict_type[slice_type % 5]; //关键帧 if (h->sei_recovery_frame_cnt >= 0) { /* key frame, since recovery_frame_cnt is set */ //赋值AVCodecParserContext的key_frame为1 s->key_frame = 1; } //获取 PPS ID pps_id = get_ue_golomb(&h->gb); if (pps_id >= MAX_PPS_COUNT) { av_log(h->avctx, AV_LOG_ERROR, "pps_id %u out of range\n", pps_id); return -1; } if (!h->pps_buffers[pps_id]) { av_log(h->avctx, AV_LOG_ERROR, "non-existing PPS %u referenced\n", pps_id); return -1; } h->pps = *h->pps_buffers[pps_id]; if (!h->sps_buffers[h->pps.sps_id]) { av_log(h->avctx, AV_LOG_ERROR, "non-existing SPS %u referenced\n", h->pps.sps_id); return -1; } h->sps = *h->sps_buffers[h->pps.sps_id]; h->frame_num = get_bits(&h->gb, h->sps.log2_max_frame_num); if(h->sps.ref_frame_count <= 1 && h->pps.ref_count[0] <= 1 && s->pict_type == AV_PICTURE_TYPE_I) s->key_frame = 1; //获得“型”和“级” //赋值到AVCodecContext的profile和level avctx->profile = ff_h264_get_profile(&h->sps); avctx->level = h->sps.level_idc; if (h->sps.frame_mbs_only_flag) { h->picture_structure = PICT_FRAME; } else { if (get_bits1(&h->gb)) { // field_pic_flag h->picture_structure = PICT_TOP_FIELD + get_bits1(&h->gb); // bottom_field_flag } else { h->picture_structure = PICT_FRAME; } } if (h->nal_unit_type == NAL_IDR_SLICE) get_ue_golomb(&h->gb); /* idr_pic_id */ if (h->sps.poc_type == 0) { h->poc_lsb = get_bits(&h->gb, h->sps.log2_max_poc_lsb); if (h->pps.pic_order_present == 1 && h->picture_structure == PICT_FRAME) h->delta_poc_bottom = get_se_golomb(&h->gb); } if (h->sps.poc_type == 1 && !h->sps.delta_pic_order_always_zero_flag) { h->delta_poc[0] = get_se_golomb(&h->gb); if (h->pps.pic_order_present == 1 && h->picture_structure == PICT_FRAME) h->delta_poc[1] = get_se_golomb(&h->gb); } /* Decode POC of this picture. * The prev_ values needed for decoding POC of the next picture are not set here. */ field_poc[0] = field_poc[1] = INT_MAX; ff_init_poc(h, field_poc, &s->output_picture_number); /* Continue parsing to check if MMCO_RESET is present. * FIXME: MMCO_RESET could appear in non-first slice. * Maybe, we should parse all undisposable non-IDR slice of this * picture until encountering MMCO_RESET in a slice of it. */ if (h->nal_ref_idc && h->nal_unit_type != NAL_IDR_SLICE) { got_reset = scan_mmco_reset(s); if (got_reset < 0) return got_reset; } /* Set up the prev_ values for decoding POC of the next picture. */ h->prev_frame_num = got_reset ? 0 : h->frame_num; h->prev_frame_num_offset = got_reset ? 0 : h->frame_num_offset; if (h->nal_ref_idc != 0) { if (!got_reset) { h->prev_poc_msb = h->poc_msb; h->prev_poc_lsb = h->poc_lsb; } else { h->prev_poc_msb = 0; h->prev_poc_lsb = h->picture_structure == PICT_BOTTOM_FIELD ? 0 : field_poc[0]; } } //包含“场”概念的时候，先不管 if (h->sps.pic_struct_present_flag) { switch (h->sei_pic_struct) { case SEI_PIC_STRUCT_TOP_FIELD: case SEI_PIC_STRUCT_BOTTOM_FIELD: s->repeat_pict = 0; break; case SEI_PIC_STRUCT_FRAME: case SEI_PIC_STRUCT_TOP_BOTTOM: case SEI_PIC_STRUCT_BOTTOM_TOP: s->repeat_pict = 1; break; case SEI_PIC_STRUCT_TOP_BOTTOM_TOP: case SEI_PIC_STRUCT_BOTTOM_TOP_BOTTOM: s->repeat_pict = 2; break; case SEI_PIC_STRUCT_FRAME_DOUBLING: s->repeat_pict = 3; break; case SEI_PIC_STRUCT_FRAME_TRIPLING: s->repeat_pict = 5; break; default: s->repeat_pict = h->picture_structure == PICT_FRAME ? 1 : 0; break; } } else { s->repeat_pict = h->picture_structure == PICT_FRAME ? 1 : 0; } if (h->picture_structure == PICT_FRAME) { s->picture_structure = AV_PICTURE_STRUCTURE_FRAME; if (h->sps.pic_struct_present_flag) { switch (h->sei_pic_struct) { case SEI_PIC_STRUCT_TOP_BOTTOM: case SEI_PIC_STRUCT_TOP_BOTTOM_TOP: s->field_order = AV_FIELD_TT; break; case SEI_PIC_STRUCT_BOTTOM_TOP: case SEI_PIC_STRUCT_BOTTOM_TOP_BOTTOM: s->field_order = AV_FIELD_BB; break; default: s->field_order = AV_FIELD_PROGRESSIVE; break; } } else { if (field_poc[0] < field_poc[1]) s->field_order = AV_FIELD_TT; else if (field_poc[0] > field_poc[1]) s->field_order = AV_FIELD_BB; else s->field_order = AV_FIELD_PROGRESSIVE; } } else { if (h->picture_structure == PICT_TOP_FIELD) s->picture_structure = AV_PICTURE_STRUCTURE_TOP_FIELD; else s->picture_structure = AV_PICTURE_STRUCTURE_BOTTOM_FIELD; s->field_order = AV_FIELD_UNKNOWN; } return 0; /* no need to evaluate the rest */ } } if (q264) return 0; /* didn't find a picture! */ av_log(h->avctx, AV_LOG_ERROR, "missing picture in access unit with size %d\n", buf_size); return -1;}从源代码可以看出，parse_nal_units()主要做了以下几步处理：
（1）对于所有的NALU，都调用ff_h264_decode_nal解析NALU的Header，得到nal_unit_type等信息
（2）根据nal_unit_type的不同，调用不同的解析函数进行处理。例如：a)解析SPS的时候调用ff_h264_decode_seq_parameter_set()b)解析PPS的时候调用ff_h264_decode_picture_parameter_set()c)解析SEI的时候调用ff_h264_decode_sei()d)解析IDR Slice / Slice的时候，获取slice_type等一些信息。
ff_h264_decode_nal()
ff_h264_decode_nal()用于解析NAL Header，获得nal_unit_type等信息。该函数的定义位于libavcodec\h264.c，如下所示。//解析NAL Header，获得nal_unit_type等信息const uint8_t *ff_h264_decode_nal(H264Context *h, const uint8_t *src, int *dst_length, int *consumed, int length){ int i, si, di; uint8_t *dst; int bufidx; // src[0]&0x80; // forbidden bit // // 1 byte NALU头 // forbidden_zero_bit: 1bit // nal_ref_idc: 2bit // nal_unit_type: 5bit // nal_ref_idc指示NAL的优先级，取值0-3，值越高，代表NAL越重要 h->nal_ref_idc = src[0] >> 5; // nal_unit_type指示NAL的类型 h->nal_unit_type = src[0] & 0x1F; //后移1Byte src++; //未处理数据长度减1 length--; //起始码:0x000001 //保留:0x000002 //防止竞争:0x000003 //既表示NALU的开始，又表示NALU的结束 //STARTCODE_TEST这个宏在后面用到 //得到length //length是指当前NALU单元长度，这里不包括nalu头信息长度（即1个字节）#define STARTCODE_TEST \ if (i + 2 < length && src[i + 1] == 0 && src[i + 2] <= 3) { \ if (src[i + 2] != 3 && src[i + 2] != 0) { \ /* 取值为1或者2（保留用），为起始码。startcode, so we must be past the end */\ length = i; \ } \ break; \ }#if HAVE_FAST_UNALIGNED#define FIND_FIRST_ZERO \ if (i > 0 && !src[i]) \ i--; \ while (src[i]) \ i++#if HAVE_FAST_64BIT for (i = 0; i + 1 < length; i += 9) { if (!((~AV_RN64A(src + i) & (AV_RN64A(src + i) - 0x0100010001000101ULL)) & 0x8000800080008080ULL)) continue; FIND_FIRST_ZERO; STARTCODE_TEST; i -= 7; }#else for (i = 0; i + 1 < length; i += 5) { if (!((~AV_RN32A(src + i) & (AV_RN32A(src + i) - 0x01000101U)) & 0x80008080U)) continue; FIND_FIRST_ZERO; STARTCODE_TEST; i -= 3; }#endif#else for (i = 0; i + 1 < length; i += 2) { if (src[i]) continue; if (i > 0 && src[i - 1] == 0) i--; //起始码检测 STARTCODE_TEST; }#endif // use second escape buffer for inter data bufidx = h->nal_unit_type == NAL_DPC ? 1 : 0; av_fast_padded_malloc(&h->rbsp_buffer[bufidx], &h->rbsp_buffer_size[bufidx], length+MAX_MBPAIR_SIZE); dst = h->rbsp_buffer[bufidx]; if (!dst) return NULL; if(i>=length-1){ //no escaped 0 *dst_length= length; *consumed= length+1; //+1 for the header if(h->avctx->flags2 & CODEC_FLAG2_FAST){ return src; }else{ memcpy(dst, src, length); return dst; } } memcpy(dst, src, i); si = di = i; while (si + 2 < length) { // remove escapes (very rare 1:2^22) if (src[si + 2] > 3) { dst[di++] = src[si++]; dst[di++] = src[si++]; } else if (src[si] == 0 && src[si + 1] == 0 && src[si + 2] != 0) { if (src[si + 2] == 3) { // escape dst[di++] = 0; dst[di++] = 0; si += 3; continue; } else // next start code goto nsc; } dst[di++] = src[si++]; } while (si < length) dst[di++] = src[si++];nsc: memset(dst + di, 0, FF_INPUT_BUFFER_PADDING_SIZE); *dst_length = di; *consumed = si + 1; // +1 for the header /* FIXME store exact number of bits in the getbitcontext * (it is needed for decoding) */ return dst;}从源代码可以看出，ff_h264_decode_nal()首先从NALU Header（NALU第1个字节）中解析出了nal_ref_idc，nal_unit_type字段的值。然后函数进入了一个for()循环进行起始码检测。
起始码检测这里稍微有点复杂，其中包含了一个STARTCODE_TEST的宏。这个宏用于做具体的起始码的判断。这部分的代码还没有细看，以后有时间再进行补充。
ff_h264_decode_seq_parameter_set()
ff_h264_decode_seq_parameter_set()用于解析H.264码流中的SPS。该函数的定义位于libavcodec\h264_ps.c，如下所示。//解码SPSint ff_h264_decode_seq_parameter_set(H264Context *h){ int profile_idc, level_idc, constraint_set_flags = 0; unsigned int sps_id; int i, log2_max_frame_num_minus4; SPS *sps; //profile型，8bit //注意get_bits() profile_idc = get_bits(&h->gb, 8); constraint_set_flags |= get_bits1(&h->gb) << 0; // constraint_set0_flag constraint_set_flags |= get_bits1(&h->gb) << 1; // constraint_set1_flag constraint_set_flags |= get_bits1(&h->gb) << 2; // constraint_set2_flag constraint_set_flags |= get_bits1(&h->gb) << 3; // constraint_set3_flag constraint_set_flags |= get_bits1(&h->gb) << 4; // constraint_set4_flag constraint_set_flags |= get_bits1(&h->gb) << 5; // constraint_set5_flag skip_bits(&h->gb, 2); // reserved_zero_2bits //level级，8bit level_idc = get_bits(&h->gb, 8); //该SPS的ID号，该ID号将被picture引用 //注意：get_ue_golomb() sps_id = get_ue_golomb_31(&h->gb); if (sps_id >= MAX_SPS_COUNT) { av_log(h->avctx, AV_LOG_ERROR, "sps_id %u out of range\n", sps_id); return AVERROR_INVALIDDATA; } //赋值给这个结构体 sps = av_mallocz(sizeof(SPS)); if (!sps) return AVERROR(ENOMEM); //赋值 sps->sps_id = sps_id; sps->time_offset_length = 24; sps->profile_idc = profile_idc; sps->constraint_set_flags = constraint_set_flags; sps->level_idc = level_idc; sps->full_range = -1; memset(sps->scaling_matrix4, 16, sizeof(sps->scaling_matrix4)); memset(sps->scaling_matrix8, 16, sizeof(sps->scaling_matrix8)); sps->scaling_matrix_present = 0; sps->colorspace = 2; //AVCOL_SPC_UNSPECIFIED //Profile对应关系 if (sps->profile_idc == 100 || // High profile sps->profile_idc == 110 || // High10 profile sps->profile_idc == 122 || // High422 profile sps->profile_idc == 244 || // High444 Predictive profile sps->profile_idc == 44 || // Cavlc444 profile sps->profile_idc == 83 || // Scalable Constrained High profile (SVC) sps->profile_idc == 86 || // Scalable High Intra profile (SVC) sps->profile_idc == 118 || // Stereo High profile (MVC) sps->profile_idc == 128 || // Multiview High profile (MVC) sps->profile_idc == 138 || // Multiview Depth High profile (MVCD) sps->profile_idc == 144) { // old High444 profile //色度取样 //0代表单色 //1代表4:2:0 //2代表4:2:2 //3代表4:4:4 sps->chroma_format_idc = get_ue_golomb_31(&h->gb); if (sps->chroma_format_idc > 3U) { avpriv_request_sample(h->avctx, "chroma_format_idc %u", sps->chroma_format_idc); goto fail; } else if (sps->chroma_format_idc == 3) { sps->residual_color_transform_flag = get_bits1(&h->gb); if (sps->residual_color_transform_flag) { av_log(h->avctx, AV_LOG_ERROR, "separate color planes are not supported\n"); goto fail; } } //bit_depth_luma_minus8 //加8之后为亮度颜色深度 //该值取值范围应该在0到4之间。即颜色深度支持0-12bit sps->bit_depth_luma = get_ue_golomb(&h->gb) + 8; //加8之后为色度颜色深度 sps->bit_depth_chroma = get_ue_golomb(&h->gb) + 8; if (sps->bit_depth_chroma != sps->bit_depth_luma) { avpriv_request_sample(h->avctx, "Different chroma and luma bit depth"); goto fail; } if (sps->bit_depth_luma > 14U || sps->bit_depth_chroma > 14U) { av_log(h->avctx, AV_LOG_ERROR, "illegal bit depth value (%d, %d)\n", sps->bit_depth_luma, sps->bit_depth_chroma); goto fail; } sps->transform_bypass = get_bits1(&h->gb); decode_scaling_matrices(h, sps, NULL, 1, sps->scaling_matrix4, sps->scaling_matrix8); } else { //默认 sps->chroma_format_idc = 1; sps->bit_depth_luma = 8; sps->bit_depth_chroma = 8; } //log2_max_frame_num_minus4为另一个句法元素frame_num服务 //fram_num的解码函数是ue（v），函数中的v 在这里指定： // v = log2_max_frame_num_minus4 + 4 //从另一个角度看，这个句法元素同时也指明了frame_num 的所能达到的最大值： // MaxFrameNum = 2^( log2_max_frame_num_minus4 + 4 ) log2_max_frame_num_minus4 = get_ue_golomb(&h->gb); if (log2_max_frame_num_minus4 < MIN_LOG2_MAX_FRAME_NUM - 4 || log2_max_frame_num_minus4 > MAX_LOG2_MAX_FRAME_NUM - 4) { av_log(h->avctx, AV_LOG_ERROR, "log2_max_frame_num_minus4 out of range (0-12): %d\n", log2_max_frame_num_minus4); goto fail; } sps->log2_max_frame_num = log2_max_frame_num_minus4 + 4; //pic_order_cnt_type 指明了poc (picture order count) 的编码方法 //poc标识图像的播放顺序。 //由于H.264使用了B帧预测，使得图像的解码顺序并不一定等于播放顺序，但它们之间存在一定的映射关系 //poc 可以由frame-num 通过映射关系计算得来，也可以索性由编码器显式地传送。 //H.264 中一共定义了三种poc 的编码方法 sps->poc_type = get_ue_golomb_31(&h->gb); //3种poc的编码方法 if (sps->poc_type == 0) { // FIXME #define unsigned t = get_ue_golomb(&h->gb); if (t>12) { av_log(h->avctx, AV_LOG_ERROR, "log2_max_poc_lsb (%d) is out of range\n", t); goto fail; } sps->log2_max_poc_lsb = t + 4; } else if (sps->poc_type == 1) { // FIXME #define sps->delta_pic_order_always_zero_flag = get_bits1(&h->gb); sps->offset_for_non_ref_pic = get_se_golomb(&h->gb); sps->offset_for_top_to_bottom_field = get_se_golomb(&h->gb); sps->poc_cycle_length = get_ue_golomb(&h->gb); if ((unsigned)sps->poc_cycle_length >= FF_ARRAY_ELEMS(sps->offset_for_ref_frame)) { av_log(h->avctx, AV_LOG_ERROR, "poc_cycle_length overflow %d\n", sps->poc_cycle_length); goto fail; } for (i = 0; i < sps->poc_cycle_length; i++) sps->offset_for_ref_frame[i] = get_se_golomb(&h->gb); } else if (sps->poc_type != 2) { av_log(h->avctx, AV_LOG_ERROR, "illegal POC type %d\n", sps->poc_type); goto fail; } //num_ref_frames 指定参考帧队列可能达到的最大长度，解码器依照这个句法元素的值开辟存储区，这个存储区用于存放已解码的参考帧， //H.264 规定最多可用16 个参考帧，因此最大值为16。 sps->ref_frame_count = get_ue_golomb_31(&h->gb); if (h->avctx->codec_tag == MKTAG('S', 'M', 'V', '2')) sps->ref_frame_count = FFMAX(2, sps->ref_frame_count); if (sps->ref_frame_count > H264_MAX_PICTURE_COUNT - 2 || sps->ref_frame_count > 16U) { av_log(h->avctx, AV_LOG_ERROR, "too many reference frames %d\n", sps->ref_frame_count); goto fail; } sps->gaps_in_frame_num_allowed_flag = get_bits1(&h->gb); //加1后为图像宽（以宏块为单位） //以像素为单位图像宽度（亮度）：width=mb_width*16 sps->mb_width = get_ue_golomb(&h->gb) + 1; //加1后为图像高（以宏块为单位） //以像素为单位图像高度（亮度）：height=mb_height*16 sps->mb_height = get_ue_golomb(&h->gb) + 1; //检查一下 if ((unsigned)sps->mb_width >= INT_MAX / 16 || (unsigned)sps->mb_height >= INT_MAX / 16 || av_image_check_size(16 * sps->mb_width, 16 * sps->mb_height, 0, h->avctx)) { av_log(h->avctx, AV_LOG_ERROR, "mb_width/height overflow\n"); goto fail; } sps->frame_mbs_only_flag = get_bits1(&h->gb); if (!sps->frame_mbs_only_flag) sps->mb_aff = get_bits1(&h->gb); else sps->mb_aff = 0; sps->direct_8x8_inference_flag = get_bits1(&h->gb);#ifndef ALLOW_INTERLACE if (sps->mb_aff) av_log(h->avctx, AV_LOG_ERROR, "MBAFF support not included; enable it at compile-time.\n");#endif //裁剪输出，没研究过 sps->crop = get_bits1(&h->gb); if (sps->crop) { int crop_left = get_ue_golomb(&h->gb); int crop_right = get_ue_golomb(&h->gb); int crop_top = get_ue_golomb(&h->gb); int crop_bottom = get_ue_golomb(&h->gb); int width = 16 * sps->mb_width; int height = 16 * sps->mb_height * (2 - sps->frame_mbs_only_flag); if (h->avctx->flags2 & CODEC_FLAG2_IGNORE_CROP) { av_log(h->avctx, AV_LOG_DEBUG, "discarding sps cropping, original " "values are l:%d r:%d t:%d b:%d\n", crop_left, crop_right, crop_top, crop_bottom); sps->crop_left = sps->crop_right = sps->crop_top = sps->crop_bottom = 0; } else { int vsub = (sps->chroma_format_idc == 1) ? 1 : 0; int hsub = (sps->chroma_format_idc == 1 || sps->chroma_format_idc == 2) ? 1 : 0; int step_x = 1 << hsub; int step_y = (2 - sps->frame_mbs_only_flag) << vsub; if (crop_left & (0x1F >> (sps->bit_depth_luma > 8)) && !(h->avctx->flags & CODEC_FLAG_UNALIGNED)) { crop_left &= ~(0x1F >> (sps->bit_depth_luma > 8)); av_log(h->avctx, AV_LOG_WARNING, "Reducing left cropping to %d " "chroma samples to preserve alignment.\n", crop_left); } if (crop_left > (unsigned)INT_MAX / 4 / step_x || crop_right > (unsigned)INT_MAX / 4 / step_x || crop_top > (unsigned)INT_MAX / 4 / step_y || crop_bottom> (unsigned)INT_MAX / 4 / step_y || (crop_left + crop_right ) * step_x >= width || (crop_top + crop_bottom) * step_y >= height ) { av_log(h->avctx, AV_LOG_ERROR, "crop values invalid %d %d %d %d / %d %d\n", crop_left, crop_right, crop_top, crop_bottom, width, height); goto fail; } sps->crop_left = crop_left * step_x; sps->crop_right = crop_right * step_x; sps->crop_top = crop_top * step_y; sps->crop_bottom = crop_bottom * step_y; } } else { sps->crop_left = sps->crop_right = sps->crop_top = sps->crop_bottom = sps->crop = 0; } sps->vui_parameters_present_flag = get_bits1(&h->gb); if (sps->vui_parameters_present_flag) { int ret = decode_vui_parameters(h, sps); if (ret < 0) goto fail; } if (!sps->sar.den) sps->sar.den = 1; //Debug的时候可以输出一些信息 if (h->avctx->debug & FF_DEBUG_PICT_INFO) { static const char csp[4][5] = { "Gray", "420", "422", "444" }; av_log(h->avctx, AV_LOG_DEBUG, "sps:%u profile:%d/%d poc:%d ref:%d %dx%d %s %s crop:%u/%u/%u/%u %s %s %"PRId32"/%"PRId32" b%d reo:%d\n", sps_id, sps->profile_idc, sps->level_idc, sps->poc_type, sps->ref_frame_count, sps->mb_width, sps->mb_height, sps->frame_mbs_only_flag ? "FRM" : (sps->mb_aff ? "MB-AFF" : "PIC-AFF"), sps->direct_8x8_inference_flag ? "8B8" : "", sps->crop_left, sps->crop_right, sps->crop_top, sps->crop_bottom, sps->vui_parameters_present_flag ? "VUI" : "", csp[sps->chroma_format_idc], sps->timing_info_present_flag ? sps->num_units_in_tick : 0, sps->timing_info_present_flag ? sps->time_scale : 0, sps->bit_depth_luma, sps->bitstream_restriction_flag ? sps->num_reorder_frames : -1 ); } sps->new = 1; av_free(h->sps_buffers[sps_id]); h->sps_buffers[sps_id] = sps; return 0;fail: av_free(sps); return -1;}解析SPS源代码并不是很有“技术含量”。只要参考ITU-T的《H.264标准》就可以理解了，不再做过多详细的分析。
ff_h264_decode_picture_parameter_set()
ff_h264_decode_picture_parameter_set()用于解析H.264码流中的PPS。该函数的定义位于libavcodec\h264_ps.c，如下所示。//解码PPSint ff_h264_decode_picture_parameter_set(H264Context *h, int bit_length){//获取PPS ID unsigned int pps_id = get_ue_golomb(&h->gb); PPS *pps; SPS *sps; int qp_bd_offset; int bits_left; if (pps_id >= MAX_PPS_COUNT) { av_log(h->avctx, AV_LOG_ERROR, "pps_id %u out of range\n", pps_id); return AVERROR_INVALIDDATA; } //解析后赋值给PPS这个结构体 pps = av_mallocz(sizeof(PPS)); if (!pps) return AVERROR(ENOMEM); //该PPS引用的SPS的ID pps->sps_id = get_ue_golomb_31(&h->gb); if ((unsigned)pps->sps_id >= MAX_SPS_COUNT || !h->sps_buffers[pps->sps_id]) { av_log(h->avctx, AV_LOG_ERROR, "sps_id %u out of range\n", pps->sps_id); goto fail; } sps = h->sps_buffers[pps->sps_id]; qp_bd_offset = 6 * (sps->bit_depth_luma - 8); if (sps->bit_depth_luma > 14) { av_log(h->avctx, AV_LOG_ERROR, "Invalid luma bit depth=%d\n", sps->bit_depth_luma); goto fail; } else if (sps->bit_depth_luma == 11 || sps->bit_depth_luma == 13) { av_log(h->avctx, AV_LOG_ERROR, "Unimplemented luma bit depth=%d\n", sps->bit_depth_luma); goto fail; } //entropy_coding_mode_flag //0表示熵编码使用CAVLC，1表示熵编码使用CABAC pps->cabac = get_bits1(&h->gb); pps->pic_order_present = get_bits1(&h->gb); pps->slice_group_count = get_ue_golomb(&h->gb) + 1; if (pps->slice_group_count > 1) { pps->mb_slice_group_map_type = get_ue_golomb(&h->gb); av_log(h->avctx, AV_LOG_ERROR, "FMO not supported\n"); switch (pps->mb_slice_group_map_type) { case 0:#if 0 | for (i = 0; i <= num_slice_groups_minus1; i++) | | | | run_length[i] |1 |ue(v) |#endif break; case 2:#if 0 | for (i = 0; i < num_slice_groups_minus1; i++) { | | | | top_left_mb[i] |1 |ue(v) | | bottom_right_mb[i] |1 |ue(v) | | } | | |#endif break; case 3: case 4: case 5:#if 0 | slice_group_change_direction_flag |1 |u(1) | | slice_group_change_rate_minus1 |1 |ue(v) |#endif break; case 6:#if 0 | slice_group_id_cnt_minus1 |1 |ue(v) | | for (i = 0; i <= slice_group_id_cnt_minus1; i++)| | | | slice_group_id[i] |1 |u(v) |#endif break; } } //num_ref_idx_l0_active_minus1 加１后指明目前参考帧队列的长度，即有多少个参考帧 //读者可能还记得在SPS中有句法元素num_ref_frames 也是跟参考帧队列有关，它们的区 //别是num_ref_frames 指明参考帧队列的最大值， 解码器用它的值来分配内存空间； //num_ref_idx_l0_active_minus1 指明在这个队列中当前实际的、已存在的参考帧数目，这从它的名字 //“active”中也可以看出来。 pps->ref_count[0] = get_ue_golomb(&h->gb) + 1; pps->ref_count[1] = get_ue_golomb(&h->gb) + 1; if (pps->ref_count[0] - 1 > 32 - 1 || pps->ref_count[1] - 1 > 32 - 1) { av_log(h->avctx, AV_LOG_ERROR, "reference overflow (pps)\n"); goto fail; } //P Slice 是否使用加权预测？ pps->weighted_pred = get_bits1(&h->gb); //B Slice 是否使用加权预测？ pps->weighted_bipred_idc = get_bits(&h->gb, 2); //QP初始值。读取后需要加26 pps->init_qp = get_se_golomb(&h->gb) + 26 + qp_bd_offset; //SP和SI的QP初始值（没怎么见过这两种帧） pps->init_qs = get_se_golomb(&h->gb) + 26 + qp_bd_offset; pps->chroma_qp_index_offset[0] = get_se_golomb(&h->gb); pps->deblocking_filter_parameters_present = get_bits1(&h->gb); pps->constrained_intra_pred = get_bits1(&h->gb); pps->redundant_pic_cnt_present = get_bits1(&h->gb); pps->transform_8x8_mode = 0; // contents of sps/pps can change even if id doesn't, so reinit h->dequant_coeff_pps = -1; memcpy(pps->scaling_matrix4, h->sps_buffers[pps->sps_id]->scaling_matrix4, sizeof(pps->scaling_matrix4)); memcpy(pps->scaling_matrix8, h->sps_buffers[pps->sps_id]->scaling_matrix8, sizeof(pps->scaling_matrix8)); bits_left = bit_length - get_bits_count(&h->gb); if (bits_left > 0 && more_rbsp_data_in_pps(h, pps)) { pps->transform_8x8_mode = get_bits1(&h->gb); decode_scaling_matrices(h, h->sps_buffers[pps->sps_id], pps, 0, pps->scaling_matrix4, pps->scaling_matrix8); // second_chroma_qp_index_offset pps->chroma_qp_index_offset[1] = get_se_golomb(&h->gb); } else { pps->chroma_qp_index_offset[1] = pps->chroma_qp_index_offset[0]; } build_qp_table(pps, 0, pps->chroma_qp_index_offset[0], sps->bit_depth_luma); build_qp_table(pps, 1, pps->chroma_qp_index_offset[1], sps->bit_depth_luma); if (pps->chroma_qp_index_offset[0] != pps->chroma_qp_index_offset[1]) pps->chroma_qp_diff = 1; if (h->avctx->debug & FF_DEBUG_PICT_INFO) { av_log(h->avctx, AV_LOG_DEBUG, "pps:%u sps:%u %s slice_groups:%d ref:%u/%u %s qp:%d/%d/%d/%d %s %s %s %s\n", pps_id, pps->sps_id, pps->cabac ? "CABAC" : "CAVLC", pps->slice_group_count, pps->ref_count[0], pps->ref_count[1], pps->weighted_pred ? "weighted" : "", pps->init_qp, pps->init_qs, pps->chroma_qp_index_offset[0], pps->chroma_qp_index_offset[1], pps->deblocking_filter_parameters_present ? "LPAR" : "", pps->constrained_intra_pred ? "CONSTR" : "", pps->redundant_pic_cnt_present ? "REDU" : "", pps->transform_8x8_mode ? "8x8DCT" : ""); } av_free(h->pps_buffers[pps_id]); h->pps_buffers[pps_id] = pps; return 0;fail: av_free(pps); return -1;}和解析SPS类似，解析PPS源代码并不是很有“技术含量”。只要参考ITU-T的《H.264标准》就可以理解，不再做过多详细的分析。
ff_h264_decode_sei()
ff_h264_decode_sei()用于解析H.264码流中的SEI。该函数的定义位于libavcodec\h264_sei.c，如下所示。//SEI补充增强信息单元int ff_h264_decode_sei(H264Context *h){ while (get_bits_left(&h->gb) > 16 && show_bits(&h->gb, 16)) { int type = 0; unsigned size = 0; unsigned next; int ret = 0; do { if (get_bits_left(&h->gb) < 8) return AVERROR_INVALIDDATA; type += show_bits(&h->gb, 8); } while (get_bits(&h->gb, 8) == 255); do { if (get_bits_left(&h->gb) < 8) return AVERROR_INVALIDDATA; size += show_bits(&h->gb, 8); } while (get_bits(&h->gb, 8) == 255); if (h->avctx->debug&FF_DEBUG_STARTCODE) av_log(h->avctx, AV_LOG_DEBUG, "SEI %d len:%d\n", type, size); if (size > get_bits_left(&h->gb) / 8) { av_log(h->avctx, AV_LOG_ERROR, "SEI type %d size %d truncated at %d\n", type, 8*size, get_bits_left(&h->gb)); return AVERROR_INVALIDDATA; } next = get_bits_count(&h->gb) + 8 * size; switch (type) { case SEI_TYPE_PIC_TIMING: // Picture timing SEI ret = decode_picture_timing(h); if (ret < 0) return ret; break; case SEI_TYPE_USER_DATA_ITU_T_T35: if (decode_user_data_itu_t_t35(h, size) < 0) return -1; break; //x264的编码参数信息一般都会存储在USER_DATA_UNREGISTERED //其他种类的SEI见得很少 case SEI_TYPE_USER_DATA_UNREGISTERED: ret = decode_unregistered_user_data(h, size); if (ret < 0) return ret; break; case SEI_TYPE_RECOVERY_POINT: ret = decode_recovery_point(h); if (ret < 0) return ret; break; case SEI_TYPE_BUFFERING_PERIOD: ret = decode_buffering_period(h); if (ret < 0) return ret; break; case SEI_TYPE_FRAME_PACKING: ret = decode_frame_packing_arrangement(h); if (ret < 0) return ret; break; case SEI_TYPE_DISPLAY_ORIENTATION: ret = decode_display_orientation(h); if (ret < 0) return ret; break; default: av_log(h->avctx, AV_LOG_DEBUG, "unknown SEI type %d\n", type); } skip_bits_long(&h->gb, next - get_bits_count(&h->gb)); // FIXME check bits here align_get_bits(&h->gb); } return 0;}在《H.264官方标准》中，SEI的种类是非常多的。在ff_h264_decode_sei()中包含以下种类的SEI：
SEI_TYPE_BUFFERING_PERIOD其中的大部分种类的SEI信息我并没有接触过。唯一接触比较多的就是SEI_TYPE_USER_DATA_UNREGISTERED类型的信息了。使用X264编码视频的时候，会自动将配置信息以SEI_TYPE_USER_DATA_UNREGISTERED（用户数据未注册SEI）的形式写入码流。
SEI_TYPE_PIC_TIMING
SEI_TYPE_USER_DATA_ITU_T_T35
SEI_TYPE_USER_DATA_UNREGISTERED
SEI_TYPE_RECOVERY_POINT
SEI_TYPE_FRAME_PACKING
SEI_TYPE_DISPLAY_ORIENTATION
从ff_h264_decode_sei()的定义可以看出，该函数根据不同的SEI类型调用不同的解析函数。当SEI类型为SEI_TYPE_USER_DATA_UNREGISTERED的时候，就会调用decode_unregistered_user_data()函数。
decode_unregistered_user_data()的定义如下所示。从代码可以看出该函数只是简单的提取了X264的版本信息。
//x264的编码参数信息一般都会存储在USER_DATA_UNREGISTEREDstatic int decode_unregistered_user_data(H264Context *h, int size){ uint8_t user_data[16 + 256]; int e, build, i; if (size < 16) return AVERROR_INVALIDDATA; for (i = 0; i < sizeof(user_data) - 1 && i < size; i++) user_data[i] = get_bits(&h->gb, 8); //user_data内容示例：x264 core 118 //int sscanf(const char *buffer,const char *format,[argument ]...); //sscanf会从buffer里读进数据，依照format的格式将数据写入到argument里。 user_data[i] = 0; e = sscanf(user_data + 16, "x264 - core %d", &build); if (e == 1 && build > 0) h->x264_build = build; if (e == 1 && build == 1 && !strncmp(user_data+16, "x264 - core 0000", 16)) h->x264_build = 67; if (h->avctx->debug & FF_DEBUG_BUGS) av_log(h->avctx, AV_LOG_DEBUG, "user data:\"%s\"\n", user_data + 16); for (; i < size; i++) skip_bits(&h->gb, 8); return 0;}解析Slice Header
对于包含图像压缩编码的Slice，解析器（Parser）并不进行解码处理，而是简单提取一些Slice Header中的信息。该部分的代码并没有写成一个函数，而是直接写到了parse_nal_units()里面，截取出来如下所示。case NAL_IDR_SLICE: //如果是IDR Slice //赋值AVCodecParserContext的key_frame为1 s->key_frame = 1; h->prev_frame_num = 0; h->prev_frame_num_offset = 0; h->prev_poc_msb = h->prev_poc_lsb = 0; /* fall through */ case NAL_SLICE: //获取Slice的一些信息 //跳过first_mb_in_slice这一字段 get_ue_golomb_long(&h->gb); // skip first_mb_in_slice //获取帧类型（I,B,P） slice_type = get_ue_golomb_31(&h->gb); //赋值到AVCodecParserContext的pict_type（外部可以访问到） s->pict_type = golomb_to_pict_type[slice_type % 5]; //关键帧 if (h->sei_recovery_frame_cnt >= 0) { /* key frame, since recovery_frame_cnt is set */ //赋值AVCodecParserContext的key_frame为1 s->key_frame = 1; } //获取 PPS ID pps_id = get_ue_golomb(&h->gb); if (pps_id >= MAX_PPS_COUNT) { av_log(h->avctx, AV_LOG_ERROR, "pps_id %u out of range\n", pps_id); return -1; } if (!h->pps_buffers[pps_id]) { av_log(h->avctx, AV_LOG_ERROR, "non-existing PPS %u referenced\n", pps_id); return -1; } h->pps = *h->pps_buffers[pps_id]; if (!h->sps_buffers[h->pps.sps_id]) { av_log(h->avctx, AV_LOG_ERROR, "non-existing SPS %u referenced\n", h->pps.sps_id); return -1; } h->sps = *h->sps_buffers[h->pps.sps_id]; h->frame_num = get_bits(&h->gb, h->sps.log2_max_frame_num); if(h->sps.ref_frame_count <= 1 && h->pps.ref_count[0] <= 1 && s->pict_type == AV_PICTURE_TYPE_I) s->key_frame = 1; //获得“型”和“级” //赋值到AVCodecContext的profile和level avctx->profile = ff_h264_get_profile(&h->sps); avctx->level = h->sps.level_idc; if (h->sps.frame_mbs_only_flag) { h->picture_structure = PICT_FRAME; } else { if (get_bits1(&h->gb)) { // field_pic_flag h->picture_structure = PICT_TOP_FIELD + get_bits1(&h->gb); // bottom_field_flag } else { h->picture_structure = PICT_FRAME; } } if (h->nal_unit_type == NAL_IDR_SLICE) get_ue_golomb(&h->gb); /* idr_pic_id */ if (h->sps.poc_type == 0) { h->poc_lsb = get_bits(&h->gb, h->sps.log2_max_poc_lsb); if (h->pps.pic_order_present == 1 && h->picture_structure == PICT_FRAME) h->delta_poc_bottom = get_se_golomb(&h->gb); } if (h->sps.poc_type == 1 && !h->sps.delta_pic_order_always_zero_flag) { h->delta_poc[0] = get_se_golomb(&h->gb); if (h->pps.pic_order_present == 1 && h->picture_structure == PICT_FRAME) h->delta_poc[1] = get_se_golomb(&h->gb); } /* Decode POC of this picture. * The prev_ values needed for decoding POC of the next picture are not set here. */ field_poc[0] = field_poc[1] = INT_MAX; ff_init_poc(h, field_poc, &s->output_picture_number); /* Continue parsing to check if MMCO_RESET is present. * FIXME: MMCO_RESET could appear in non-first slice. * Maybe, we should parse all undisposable non-IDR slice of this * picture until encountering MMCO_RESET in a slice of it. */ if (h->nal_ref_idc && h->nal_unit_type != NAL_IDR_SLICE) { got_reset = scan_mmco_reset(s); if (got_reset < 0) return got_reset; } /* Set up the prev_ values for decoding POC of the next picture. */ h->prev_frame_num = got_reset ? 0 : h->frame_num; h->prev_frame_num_offset = got_reset ? 0 : h->frame_num_offset; if (h->nal_ref_idc != 0) { if (!got_reset) { h->prev_poc_msb = h->poc_msb; h->prev_poc_lsb = h->poc_lsb; } else { h->prev_poc_msb = 0; h->prev_poc_lsb = h->picture_structure == PICT_BOTTOM_FIELD ? 0 : field_poc[0]; } }可以看出该部分代码提取了根据NALU Header、Slice Header中的信息赋值了一些字段，比如说AVCodecParserContext中的key_frame、pict_type，H264Context中的sps、pps、frame_num等等。
雷霄骅
leixiaohua1020@126.com
http://blog.csdn.net/leixiaohua1020
5 0
- FFmpeg的H.264解码器源代码简单分析：解析器（Parser）部分
- FFmpeg的H.264解码器源代码简单分析：解析器（Parser）部分
- FFmpeg的H.264解码器源代码简单分析：解析器（Parser）部分
- FFmpeg的H.264解码器源代码简单分析：解析器（Parser）部分
- FFmpeg的HEVC解码器源代码简单分析：解析器（Parser）部分
- FFmpeg的H.264解码器源代码简单分析：解码器主干部分
- FFmpeg的H.264解码器源代码简单分析：熵解码（Entropy Decoding）部分
- FFmpeg的H.264解码器源代码简单分析：环路滤波（Loop Filter）部分
- FFmpeg的H.264解码器源代码简单分析：概述
- FFmpeg的H.264解码器源代码简单分析：概述
- FFmpeg的H.264解码器源代码简单分析
- FFmpeg的H.264解码器源代码简单分析：概述
- FFmpeg的H.264解码器源代码简单分析：宏块解码（Decode）部分-帧内宏块（Intra）
- FFmpeg的H.264解码器源代码简单分析：宏块解码（Decode）部分-帧间宏块（Inter）
- FFmpeg的HEVC解码器源代码简单分析：解码器主干部分
- FFmpeg的HEVC解码器源代码简单分析：CTU解码（CTU Decode）部分-PU
- FFmpeg的HEVC解码器源代码简单分析：CTU解码（CTU Decode）部分-TU
- FFmpeg的HEVC解码器源代码简单分析：概述
- jetty服务器数据源配置JNDI-Oracle,MySQL,SQLServer,DB2等
- Activity动态增加Fragment
- python深入学习--decorator强大的装饰器
- BeanUtil为javaBean注入属性支持类型 以及个人的实现方法
- opencv画直线圆椭圆矩形
- FFmpeg的H.264解码器源代码简单分析：解析器（Parser）部分
- Android开源项目--分类汇总
- hdu 5202
- java程序向指定邮箱发邮件
- dedecms标签大全
- Android--通信技术
- SQL Server 2005的几个新功能
- SQL Server CET 通用表表达式 之 精典递归
- Android简介
