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在FFmpeg的libavcodec模块提供解析数据包和编解码功能。其中，av_parser_parse2()函数用来解析数据包，在使用av_read_frame()读取音视频帧时，会调用到该函数进行数据包解析。关于读取音视频帧的源码分析请查看：av_read_frame()文章。
目录
av_parser_parse2函数完整解析流程如下图所示：
一、av_parser_parse2源码分析
1、av_parser_parse2调用示例
av_parser_parse2()函数的声明位于libavcodec/avcodec.h，函数API调用示例如下：
while(in_len) {
len = av_parser_parse2(myparser, AVCodecContext, &data, &size,
in_data, in_len, pts, dts, pos);
in_data += len;
in_len -= len;
if(size)
decode_frame(data, size);
}2、av_parser_parse2函数源码
函数实现位于libavcodec/parser.c，具体代码如下：
int av_parser_parse2(AVCodecParserContext *s, AVCodecContext *avctx,
uint8_t **poutbuf, int *poutbuf_size,
const uint8_t *buf, int buf_size,
int64_t pts, int64_t dts, int64_t pos)
{
int index, i;
uint8_t dummy_buf[AV_INPUT_BUFFER_PADDING_SIZE];
if (!(s->flags & PARSER_FLAG_FETCHED_OFFSET)) {
s->next_frame_offset =
s->cur_offset = pos;
s->flags |= PARSER_FLAG_FETCHED_OFFSET;
}
if (buf_size == 0) {
/* padding is always necessary even if EOF, so we add it here */
memset(dummy_buf, 0, sizeof(dummy_buf));
buf = dummy_buf;
} else if (s->cur_offset + buf_size != s->cur_frame_end[s->cur_frame_start_index]) {
/* add a new packet descriptor */
i = (s->cur_frame_start_index + 1) & (AV_PARSER_PTS_NB - 1);
s->cur_frame_start_index = i;
s->cur_frame_offset[i] = s->cur_offset;
s->cur_frame_end[i] = s->cur_offset + buf_size;
s->cur_frame_pts[i] = pts;
s->cur_frame_dts[i] = dts;
s->cur_frame_pos[i] = pos;
}
if (s->fetch_timestamp) {
s->fetch_timestamp = 0;
s->last_pts = s->pts;
s->last_dts = s->dts;
s->last_pos = s->pos;
ff_fetch_timestamp(s, 0, 0, 0);
}
// 解析数据包，返回值可能为负数
index = s->parser->parser_parse(s, avctx, (const uint8_t **) poutbuf,
poutbuf_size, buf, buf_size);
// 更新文件指针
if (*poutbuf_size) {
s->frame_offset = s->next_frame_offset;
// 偏移到下一帧的位置
s->next_frame_offset = s->cur_offset + index;
s->fetch_timestamp = 1;
} else {
*poutbuf = NULL;
}
if (index < 0)
index = 0;
s->cur_offset += index;
return index;
}3、AVCodecParserContext解析器上下文
这里主要调用s->parser->parse()进行解析，s即为AVCodecParserContext 结构体，位于libavcodec/avcodec.h，具体如下：
typedef struct AVCodecParserContext {
void *priv_data;
struct AVCodecParser *parser;
int64_t frame_offset;
int64_t cur_offset;
int64_t next_frame_offset;
int pict_type;
int repeat_pict;
int64_t pts; // 当前帧pts
int64_t dts; // 当前帧dts
int64_t last_pts; // 上一帧pts
int64_t last_dts; // 上一帧dts
int fetch_timestamp;
int cur_frame_start_index;
int64_t cur_frame_offset[AV_PARSER_PTS_NB];
int64_t cur_frame_pts[AV_PARSER_PTS_NB];
int64_t cur_frame_dts[AV_PARSER_PTS_NB];
int flags;
int64_t offset;
int64_t cur_frame_end[AV_PARSER_PTS_NB];
// 关键帧标志，1代表关键帧，0代表非关键帧
int key_frame;
int dts_sync_point;
int dts_ref_dts_delta;
int pts_dts_delta;
int64_t cur_frame_pos[AV_PARSER_PTS_NB];
// 当前帧position位置
int64_t pos;
// 上一帧position位置
int64_t last_pos;
// 帧时长
int duration;
enum AVFieldOrder field_order;
enum AVPictureStructure picture_structure;
int output_picture_number;
int width;
int height;
int coded_width;
int coded_height;
// 视频为AVPixelFormat枚举类型，音频为AVSampleFormat枚举类型
int format;
} AVCodecParserContext;4、AVCodecParser解析器结构体
AVCodecParserContext有个参数是parser，为AVCodecParser结构体，具体如下：
typedef struct AVCodecParser {
int codec_ids[5];
int priv_data_size;
int (*parser_init)(AVCodecParserContext *s);
int (*parser_parse)(AVCodecParserContext *s,
AVCodecContext *avctx,
const uint8_t **poutbuf, int *poutbuf_size,
const uint8_t *buf, int buf_size);
void (*parser_close)(AVCodecParserContext *s);
int (*split)(AVCodecContext *avctx, const uint8_t *buf, int buf_size);
#if FF_API_NEXT
struct AVCodecParser *next;
#endif
} AVCodecParser;5、解析器列表
所有解析器声明在libavcodec/parsers.c中，编译时会自动生成parser_list.c，解析器以链表结构存储，解析器列表如下：
extern AVCodecParser ff_aac_parser;
extern AVCodecParser ff_aac_latm_parser;
extern AVCodecParser ff_ac3_parser;
extern AVCodecParser ff_adx_parser;
extern AVCodecParser ff_av1_parser;
extern AVCodecParser ff_avs2_parser;
extern AVCodecParser ff_avs3_parser;
extern AVCodecParser ff_bmp_parser;
extern AVCodecParser ff_cavsvideo_parser;
extern AVCodecParser ff_cook_parser;
extern AVCodecParser ff_cri_parser;
extern AVCodecParser ff_dca_parser;
extern AVCodecParser ff_dirac_parser;
extern AVCodecParser ff_dnxhd_parser;
extern AVCodecParser ff_dolby_e_parser;
extern AVCodecParser ff_dpx_parser;
extern AVCodecParser ff_dvaudio_parser;
extern AVCodecParser ff_dvbsub_parser;
extern AVCodecParser ff_dvdsub_parser;
extern AVCodecParser ff_dvd_nav_parser;
extern AVCodecParser ff_flac_parser;
extern AVCodecParser ff_g723_1_parser;
extern AVCodecParser ff_g729_parser;
extern AVCodecParser ff_gif_parser;
extern AVCodecParser ff_gsm_parser;
extern AVCodecParser ff_h261_parser;
extern AVCodecParser ff_h263_parser;
extern AVCodecParser ff_h264_parser;
extern AVCodecParser ff_hevc_parser;
extern AVCodecParser ff_ipu_parser;
extern AVCodecParser ff_jpeg2000_parser;
extern AVCodecParser ff_mjpeg_parser;
extern AVCodecParser ff_mlp_parser;
extern AVCodecParser ff_mpeg4video_parser;
extern AVCodecParser ff_mpegaudio_parser;
extern AVCodecParser ff_mpegvideo_parser;
extern AVCodecParser ff_opus_parser;
extern AVCodecParser ff_png_parser;
extern AVCodecParser ff_pnm_parser;
extern AVCodecParser ff_rv30_parser;
extern AVCodecParser ff_rv40_parser;
extern AVCodecParser ff_sbc_parser;
extern AVCodecParser ff_sipr_parser;
extern AVCodecParser ff_tak_parser;
extern AVCodecParser ff_vc1_parser;
extern AVCodecParser ff_vorbis_parser;
extern AVCodecParser ff_vp3_parser;
extern AVCodecParser ff_vp8_parser;
extern AVCodecParser ff_vp9_parser;
extern AVCodecParser ff_webp_parser;
extern AVCodecParser ff_xbm_parser;
extern AVCodecParser ff_xma_parser;二、h264解析器分析
1、 h264解析器结构体
以h264解析器为例，对应的解析器结构体为ff_h264_parser，位于libavcodec/h264_parser.c，结构体如下：
AVCodecParser ff_h264_parser = {
.codec_ids = { AV_CODEC_ID_H264 },
.priv_data_size = sizeof(H264ParseContext),
.parser_init = init,
.parser_parse = h264_parse,
.parser_close = h264_close,
.split = h264_split,
};2、 h264_parse函数源码
这里的parser_parse函数指针指向h264_parse，我们来看下h264_parse()的源码(位于libavcodec/h264_parser.c)：
static int h264_parse(AVCodecParserContext *s,
AVCodecContext *avctx,
const uint8_t **poutbuf, int *poutbuf_size,
const uint8_t *buf, int buf_size)
{
H264ParseContext *p = s->priv_data;
ParseContext *pc = &p->pc;
int next;
if (!p->got_first) {
p->got_first = 1;
if (avctx->extradata_size) {
// 解析额外数据：avcC，包含sps和pps等信息
ff_h264_decode_extradata(avctx->extradata, avctx->extradata_size,
&p->ps, &p->is_avc, &p->nal_length_size,
avctx->err_recognition, avctx);
}
}
if (s->flags & PARSER_FLAG_COMPLETE_FRAMES) {
next = buf_size;
} else {
// 找到帧结束符：FFmpeg使用下一帧的start_code作为当前帧结束符
next = h264_find_frame_end(p, buf, buf_size, avctx);
// 开始组帧
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(p, &pc->buffer[pc->last_index + next], -next, avctx);
}
}
// 解析NAL单元
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 (p->sei.picture_timing.cpb_removal_delay >= 0) {
s->dts_sync_point = p->sei.buffering_period.present;
s->dts_ref_dts_delta = p->sei.picture_timing.cpb_removal_delay;
s->pts_dts_delta = p->sei.picture_timing.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;
}
if (s->dts_sync_point >= 0) {
int64_t den = avctx->time_base.den * (int64_t)avctx->pkt_timebase.num;
if (den > 0) {
int64_t num = avctx->time_base.num * (int64_t)avctx->pkt_timebase.den;
if (s->dts != AV_NOPTS_VALUE) {
// 从stream中获取到dts，更新引用时间戳
p->reference_dts = s->dts - av_rescale(s->dts_ref_dts_delta, num, den);
} else if (p->reference_dts != AV_NOPTS_VALUE) {
// 基于引用时间戳计算dts
s->dts = p->reference_dts + av_rescale(s->dts_ref_dts_delta, num, den);
}
if (p->reference_dts != AV_NOPTS_VALUE && s->pts == AV_NOPTS_VALUE)
s->pts = s->dts + av_rescale(s->pts_dts_delta, num, den);
if (s->dts_sync_point > 0)
p->reference_dts = s->dts;
}
}
*poutbuf = buf;
*poutbuf_size = buf_size;
return next;
}由此可见，h264解析过程分为4个步骤：
- 调用ff_h264_decode_extradata()解析额外数据；
- 调用h264_find_frame_end()寻找帧结束符；
- 调用ff_combine_frame()执行组帧；
- 调用parse_nal_units()解析NAL单元；
3、ff_h264_decode_extradata函数
如果是第一帧，先解析extradata，对于h264编码，就是解析avcC，包含sps和pps参数。然后找到帧结束符，默认为下一帧的起始码start_code作为当前帧的结束符。接着解析NAL单元，更新dts时间戳。我们来看下解析extradata的代码(位于libavcodec/h264_parse.c)：
int ff_h264_decode_extradata(const uint8_t *data, int size, H264ParamSets *ps,
int *is_avc, int *nal_length_size,
int err_recognition, void *logctx)
{
int ret;
if (!data || size <= 0)
return -1;
if (data[0] == 1) {
int i, cnt, nalsize;
const uint8_t *p = data;
*is_avc = 1;
if (size < 7) {
return AVERROR_INVALIDDATA;
}
// 从avcC解析sps
cnt = *(p + 5) & 0x1f; // sps数量
p += 6;
for (i = 0; i < cnt; i++) {
nalsize = AV_RB16(p) + 2;
if (nalsize > size - (p - data))
return AVERROR_INVALIDDATA;
ret = decode_extradata_ps_mp4(p, nalsize, ps, err_recognition, logctx);
if (ret < 0) {
return ret;
}
p += nalsize;
}
// 从avcC解析pps
cnt = *(p++); // pps数量
for (i = 0; i < cnt; i++) {
nalsize = AV_RB16(p) + 2;
if (nalsize > size - (p - data))
return AVERROR_INVALIDDATA;
ret = decode_extradata_ps_mp4(p, nalsize, ps, err_recognition, logctx);
if (ret < 0) {
return ret;
}
p += nalsize;
}
// 存储正确的NAL长度
*nal_length_size = (data[4] & 0x03) + 1;
} else {
*is_avc = 0;
ret = decode_extradata_ps(data, size, ps, 0, logctx);
if (ret < 0)
return ret;
}
return size;
}4、 h264_find_frame_end函数
接着调用h264_find_frame_end()找到帧结束符，代码位于libavcodec/h264_parser.c，具体如下：
static int h264_find_frame_end(H264ParseContext *p, const uint8_t *buf,
int buf_size, void *logctx)
{
int i, j;
uint32_t state;
ParseContext *pc = &p->pc;
int next_avc = p->is_avc ? 0 : buf_size;
state = pc->state;
if (state > 13)
state = 7;
for (i = 0; i < buf_size; i++) {
if (i >= next_avc) {
int nalsize = 0;
i = next_avc;
for (j = 0; j < p->nal_length_size; j++)
nalsize = (nalsize << 8) | buf[i++];
if (nalsize <= 0 || nalsize > buf_size - i) {
return buf_size;
}
next_avc = i + nalsize;
state = 5;
}
if (state == 7) {
i += p->h264dsp.startcode_find_candidate(buf + i, next_avc - i);
if (i < next_avc)
state = 2;
} else if (state <= 2) {
if (buf[i] == 1)
state ^= 5;
else if (buf[i])
state = 7;
else
state >>= 1;
} else if (state <= 5) {
int nalu_type = buf[i] & 0x1F;
if (nalu_type == H264_NAL_SEI || nalu_type == H264_NAL_SPS ||
nalu_type == H264_NAL_PPS || nalu_type == H264_NAL_AUD) {
if (pc->frame_start_found) {
i++;
goto found;
}
} else if (nalu_type == H264_NAL_SLICE || nalu_type == H264_NAL_DPA ||
nalu_type == H264_NAL_IDR_SLICE) {
state += 8;
continue;
}
state = 7;
} else {
unsigned int mb, last_mb = p->parse_last_mb;
GetBitContext gb;
p->parse_history[p->parse_history_count++] = buf[i];
init_get_bits(&gb, p->parse_history, 8*p->parse_history_count);
mb= get_ue_golomb_long(&gb);
if (get_bits_left(&gb) > 0 || p->parse_history_count > 5) {
p->parse_last_mb = mb;
if (pc->frame_start_found) {
if (mb <= last_mb) {
i -= p->parse_history_count - 1;
p->parse_history_count = 0;
goto found;
}
} else
pc->frame_start_found = 1;
p->parse_history_count = 0;
state = 7;
}
}
}
pc->state = state;
if (p->is_avc)
return next_avc;
return END_NOT_FOUND;
found:
pc->state = 7;
pc->frame_start_found = 0;
if (p->is_avc)
return next_avc;
return i - (state & 5);
}5、ff_combine_frame函数
找到帧结束符后，调用ff_combine_frame()进行组帧，拼接成一个完整的音视频帧。代码位于libavcodec/parser.c，具体如下：
int ff_combine_frame(ParseContext *pc, int next, const uint8_t **buf, int *buf_size)
{
/* Copy overread bytes from last frame into buffer. */
for (; pc->overread > 0; pc->overread--)
pc->buffer[pc->index++] = pc->buffer[pc->overread_index++];
if (next > *buf_size)
return AVERROR(EINVAL);
/* flush remaining if EOF */
if (!*buf_size && next == END_NOT_FOUND)
next = 0;
pc->last_index = pc->index;
/* copy into buffer end return */
if (next == END_NOT_FOUND) {
void *new_buffer = av_fast_realloc(pc->buffer, &pc->buffer_size,
*buf_size + pc->index +
AV_INPUT_BUFFER_PADDING_SIZE);
if (!new_buffer) {
pc->index = 0;
return AVERROR(ENOMEM);
}
pc->buffer = new_buffer;
memcpy(&pc->buffer[pc->index], *buf, *buf_size);
pc->index += *buf_size;
return -1;
}
*buf_size =
pc->overread_index = pc->index + next;
if (pc->index) {
void *new_buffer = av_fast_realloc(pc->buffer, &pc->buffer_size,
next + pc->index +
AV_INPUT_BUFFER_PADDING_SIZE);
if (!new_buffer) {
pc->overread_index =
pc->index = 0;
return AVERROR(ENOMEM);
}
pc->buffer = new_buffer;
if (next > -AV_INPUT_BUFFER_PADDING_SIZE)
memcpy(&pc->buffer[pc->index], *buf,
next + AV_INPUT_BUFFER_PADDING_SIZE);
pc->index = 0;
*buf = pc->buffer;
}
if (next < -8) {
pc->overread += -8 - next;
next = -8;
}
/* store overread bytes */
for (; next < 0; next++) {
pc->state = pc->state << 8 | pc->buffer[pc->last_index + next];
pc->state64 = pc->state64 << 8 | pc->buffer[pc->last_index + next];
pc->overread++;
}
return 0;
}6、parse_nal_units
最后是调用parse_nal_units()来解析NAL单元，位于libavcodec/h264_parser.c，具体代码如下：
static inline int parse_nal_units(AVCodecParserContext *s,
AVCodecContext *avctx,
const uint8_t * const buf, int buf_size)
{
H264ParseContext *p = s->priv_data;
H2645RBSP rbsp = { NULL };
H2645NAL nal = { NULL };
int buf_index, next_avc;
unsigned int pps_id;
unsigned int slice_type;
int state = -1, got_reset = 0;
int q264 = buf_size >=4 && !memcmp("Q264", buf, 4);
int field_poc[2];
int ret;
s->pict_type = AV_PICTURE_TYPE_I;
s->key_frame = 0;
s->picture_structure = AV_PICTURE_STRUCTURE_UNKNOWN;
ff_h264_sei_uninit(&p->sei);
p->sei.frame_packing.arrangement_cancel_flag = -1;
if (!buf_size)
return 0;
av_fast_padded_malloc(&rbsp.rbsp_buffer, &rbsp.rbsp_buffer_alloc_size, buf_size);
if (!rbsp.rbsp_buffer)
return AVERROR(ENOMEM);
buf_index = 0;
next_avc = p->is_avc ? 0 : buf_size;
for (;;) {
const SPS *sps;
int src_length, consumed, nalsize = 0;
if (buf_index >= next_avc) {
// 获取NAL长度
nalsize = get_nalsize(p->nal_length_size, buf, buf_size, &buf_index, avctx);
if (nalsize < 0)
break;
next_avc = buf_index + nalsize;
} else {
// 找到起始码
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;
state = buf[buf_index];
switch (state & 0x1f) {
case H264_NAL_SLICE:
case H264_NAL_IDR_SLICE:
// 只解析slice条带的头部
if ((state & 0x1f) == H264_NAL_IDR_SLICE || ((state >> 5) & 0x3) == 0) {
if (src_length > 60)
src_length = 60;
} else {
if (src_length > 1000)
src_length = 1000;
}
break;
}
// 抽取rbsp序列
consumed = ff_h2645_extract_rbsp(buf + buf_index, src_length, &rbsp, &nal, 1);
if (consumed < 0)
break;
buf_index += consumed;
ret = init_get_bits8(&nal.gb, nal.data, nal.size);
if (ret < 0)
goto fail;
get_bits1(&nal.gb);
nal.ref_idc = get_bits(&nal.gb, 2);
nal.type = get_bits(&nal.gb, 5);
switch (nal.type) {
case H264_NAL_SPS: // SPS序列参数集
ff_h264_decode_seq_parameter_set(&nal.gb, avctx, &p->ps, 0);
break;
case H264_NAL_PPS: // PPS图像参数集
ff_h264_decode_picture_parameter_set(&nal.gb, avctx, &p->ps,
nal.size_bits);
break;
case H264_NAL_SEI: // 补充增强信息
ff_h264_sei_decode(&p->sei, &nal.gb, &p->ps, avctx);
break;
case H264_NAL_IDR_SLICE: //IDR立即刷新
s->key_frame = 1;
p->poc.prev_frame_num = 0;
p->poc.prev_frame_num_offset = 0;
p->poc.prev_poc_msb =
p->poc.prev_poc_lsb = 0;
case H264_NAL_SLICE: // slice条带
get_ue_golomb_long(&nal.gb);
slice_type = get_ue_golomb_31(&nal.gb);
s->pict_type = ff_h264_golomb_to_pict_type[slice_type % 5];
if (p->sei.recovery_point.recovery_frame_cnt >= 0) {
s->key_frame = 1;
}
pps_id = get_ue_golomb(&nal.gb);
if (pps_id >= MAX_PPS_COUNT) {
goto fail;
}
if (!p->ps.pps_list[pps_id]) {
goto fail;
}
av_buffer_unref(&p->ps.pps_ref);
p->ps.pps = NULL;
p->ps.sps = NULL;
p->ps.pps_ref = av_buffer_ref(p->ps.pps_list[pps_id]);
if (!p->ps.pps_ref)
goto fail;
p->ps.pps = (const PPS*)p->ps.pps_ref->data;
p->ps.sps = p->ps.pps->sps;
sps = p->ps.sps;
// 检测未标记的关键帧
if (p->ps.sps->ref_frame_count <= 1 && p->ps.pps->ref_count[0] <= 1 && s->pict_type == AV_PICTURE_TYPE_I)
s->key_frame = 1;
p->poc.frame_num = get_bits(&nal.gb, sps->log2_max_frame_num);
s->coded_width = 16 * sps->mb_width;
s->coded_height = 16 * sps->mb_height;
s->width = s->coded_width - (sps->crop_right + sps->crop_left);
s->height = s->coded_height - (sps->crop_top + sps->crop_bottom);
if (s->width <= 0 || s->height <= 0) {
s->width = s->coded_width;
s->height = s->coded_height;
}
// 根据色度的idc来推测像素格式
switch (sps->bit_depth_luma) {
case 9:
if (sps->chroma_format_idc == 3) s->format = AV_PIX_FMT_YUV444P9;
else if (sps->chroma_format_idc == 2) s->format = AV_PIX_FMT_YUV422P9;
else s->format = AV_PIX_FMT_YUV420P9;
break;
case 10:
if (sps->chroma_format_idc == 3) s->format = AV_PIX_FMT_YUV444P10;
else if (sps->chroma_format_idc == 2) s->format = AV_PIX_FMT_YUV422P10;
else s->format = AV_PIX_FMT_YUV420P10;
break;
case 8:
if (sps->chroma_format_idc == 3) s->format = AV_PIX_FMT_YUV444P;
else if (sps->chroma_format_idc == 2) s->format = AV_PIX_FMT_YUV422P;
else s->format = AV_PIX_FMT_YUV420P;
break;
default:
s->format = AV_PIX_FMT_NONE;
}
avctx->profile = ff_h264_get_profile(sps);
avctx->level = sps->level_idc;
if (sps->frame_mbs_only_flag) {
p->picture_structure = PICT_FRAME;
} else {
if (get_bits1(&nal.gb)) {
p->picture_structure = PICT_TOP_FIELD + get_bits1(&nal.gb);
} else {
p->picture_structure = PICT_FRAME;
}
}
if (nal.type == H264_NAL_IDR_SLICE)
get_ue_golomb_long(&nal.gb); /* idr_pic_id */
if (sps->poc_type == 0) {
p->poc.poc_lsb = get_bits(&nal.gb, sps->log2_max_poc_lsb);
if (p->ps.pps->pic_order_present == 1 &&
p->picture_structure == PICT_FRAME)
p->poc.delta_poc_bottom = get_se_golomb(&nal.gb);
}
if (sps->poc_type == 1 &&
!sps->delta_pic_order_always_zero_flag) {
p->poc.delta_poc[0] = get_se_golomb(&nal.gb);
if (p->ps.pps->pic_order_present == 1 &&
p->picture_structure == PICT_FRAME)
p->poc.delta_poc[1] = get_se_golomb(&nal.gb);
}
field_poc[0] = field_poc[1] = INT_MAX;
ret = ff_h264_init_poc(field_poc, &s->output_picture_number, sps,
&p->poc, p->picture_structure, nal.ref_idc);
if (ret < 0)
goto fail;
if (nal.ref_idc && nal.type != H264_NAL_IDR_SLICE) {
got_reset = scan_mmco_reset(s, &nal.gb, avctx);
if (got_reset < 0)
goto fail;
}
p->poc.prev_frame_num = got_reset ? 0 : p->poc.frame_num;
p->poc.prev_frame_num_offset = got_reset ? 0 : p->poc.frame_num_offset;
if (nal.ref_idc != 0) {
if (!got_reset) {
p->poc.prev_poc_msb = p->poc.poc_msb;
p->poc.prev_poc_lsb = p->poc.poc_lsb;
} else {
p->poc.prev_poc_msb = 0;
p->poc.prev_poc_lsb =
p->picture_structure == PICT_BOTTOM_FIELD ? 0 : field_poc[0];
}
}
if (p->sei.picture_timing.present) {
// 处理补充增强信息
ret = ff_h264_sei_process_picture_timing(&p->sei.picture_timing,
sps, avctx);
if (ret < 0) {
p->sei.picture_timing.present = 0;
}
}
if (sps->pic_struct_present_flag && p->sei.picture_timing.present) {
switch (p->sei.picture_timing.pic_struct) {
case H264_SEI_PIC_STRUCT_TOP_FIELD:
case H264_SEI_PIC_STRUCT_BOTTOM_FIELD:
s->repeat_pict = 0;
break;
case H264_SEI_PIC_STRUCT_FRAME:
case H264_SEI_PIC_STRUCT_TOP_BOTTOM:
case H264_SEI_PIC_STRUCT_BOTTOM_TOP:
s->repeat_pict = 1;
break;
case H264_SEI_PIC_STRUCT_TOP_BOTTOM_TOP:
case H264_SEI_PIC_STRUCT_BOTTOM_TOP_BOTTOM:
s->repeat_pict = 2;
break;
case H264_SEI_PIC_STRUCT_FRAME_DOUBLING:
s->repeat_pict = 3;
break;
case H264_SEI_PIC_STRUCT_FRAME_TRIPLING:
s->repeat_pict = 5;
break;
default:
s->repeat_pict = p->picture_structure == PICT_FRAME ? 1 : 0;
break;
}
} else {
s->repeat_pict = p->picture_structure == PICT_FRAME ? 1 : 0;
}
if (p->picture_structure == PICT_FRAME) {
s->picture_structure = AV_PICTURE_STRUCTURE_FRAME;
if (sps->pic_struct_present_flag && p->sei.picture_timing.present) {
switch (p->sei.picture_timing.pic_struct) {
case H264_SEI_PIC_STRUCT_TOP_BOTTOM:
case H264_SEI_PIC_STRUCT_TOP_BOTTOM_TOP:
s->field_order = AV_FIELD_TT;
break;
case H264_SEI_PIC_STRUCT_BOTTOM_TOP:
case H264_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 (p->picture_structure == PICT_TOP_FIELD)
s->picture_structure = AV_PICTURE_STRUCTURE_TOP_FIELD;
else
s->picture_structure = AV_PICTURE_STRUCTURE_BOTTOM_FIELD;
if (p->poc.frame_num == p->last_frame_num &&
p->last_picture_structure != AV_PICTURE_STRUCTURE_UNKNOWN &&
p->last_picture_structure != AV_PICTURE_STRUCTURE_FRAME &&
p->last_picture_structure != s->picture_structure) {
if (p->last_picture_structure == AV_PICTURE_STRUCTURE_TOP_FIELD)
s->field_order = AV_FIELD_TT;
else
s->field_order = AV_FIELD_BB;
} else {
s->field_order = AV_FIELD_UNKNOWN;
}
p->last_picture_structure = s->picture_structure;
p->last_frame_num = p->poc.frame_num;
}
av_freep(&rbsp.rbsp_buffer);
return 0;
}
}
if (q264) {
av_freep(&rbsp.rbsp_buffer);
return 0;
}
av_log(avctx, AV_LOG_ERROR, "missing picture in access unit with size %d\n", buf_size);
fail:
av_freep(&rbsp.rbsp_buffer);
return -1;
}解析NAL单元流程主要是根据NAL类型来解析：SPS、PPS、IDR、SLICE、SEI。如果是slice类型，会检测未标记的关键帧、根据色度的idc来推测像素格式、处理补充增强信息。
至此，FFmpeg的解析数据包流程分析完毕。
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