MP4中提取H.264码流


1.获取数据

ffmpeg读取mp4中的H264数据，并不能直接得到NALU，文件中也没有储存0x00000001的分隔符。下面这张图为packet.data中的数据



从图中可以发现，packet中的数据起始处没有分隔符(0x00000001), 也不是0x65、0x67、0x68、0x41等字节，所以可以肯定这不是标准的nalu。

其实，前4个字0x000032ce表示的是nalu的长度，从第5个字节开始才是nalu的数据。所以直接将前4个字节替换为0x00000001即可得到标准的nalu数据。

2.获取pps及sps

pps及sps不能从packet获得，而是保存在AVCodecContext的extradata数据域中。如下：



如何从extradata中解析出sps及pps呢？ffmpeg中提供了一个流过滤器"h264_mp4toannexb"完成这项工作，关键代码如下

[cpp]
view plaincopyprint?

//h264_mp4toannexb_bsf.c
static int h264_mp4toannexb_filter(AVBitStreamFilterContext *bsfc,

AVCodecContext *avctx, const char *args,

uint8_t **poutbuf, int *poutbuf_size,
const uint8_t *buf, int buf_size,

int keyframe) {
H264BSFContext *ctx = bsfc->priv_data;
uint8_t unit_type;
int32_t nal_size;
uint32_t cumul_size = 0;
const uint8_t *buf_end = buf + buf_size;

if (!avctx->extradata || avctx->extradata_size < 6) {

*poutbuf = (uint8_t*) buf;
*poutbuf_size = buf_size;
return 0;
}
//
//从extradata中分析出SPS、PPS
//

if (!ctx->extradata_parsed) {
uint16_t unit_size;
uint64_t total_size = 0;
uint8_t *out = NULL, unit_nb, sps_done = 0, sps_seen = 0, pps_seen = 0;

const uint8_t *extradata = avctx->extradata+4;
//跳过前4个字节
static const uint8_t nalu_header[4] = {0, 0, 0, 1};

ctx->length_size = (*extradata++ & 0x3) + 1; //用于指示表示编码数据长度所需字节数

if (ctx->length_size == 3)
return AVERROR(EINVAL);

unit_nb = *extradata++ & 0x1f;
if (!unit_nb) {
goto pps;
} else {
sps_seen = 1;
}
while (unit_nb--) {
void *tmp;
unit_size = AV_RB16(extradata);
total_size += unit_size+4;
if (total_size > INT_MAX - FF_INPUT_BUFFER_PADDING_SIZE ||

extradata+2+unit_size > avctx->extradata+avctx->extradata_size) {

av_free(out);
return AVERROR(EINVAL);
}
tmp = av_realloc(out, total_size + FF_INPUT_BUFFER_PADDING_SIZE);

if (!tmp) {
av_free(out);
return AVERROR(ENOMEM);
}
out = tmp;
memcpy(out+total_size-unit_size-4, nalu_header, 4);
memcpy(out+total_size-unit_size, extradata+2, unit_size);
extradata += 2+unit_size;
pps:
if (!unit_nb && !sps_done++) {
unit_nb = *extradata++;
if (unit_nb)
pps_seen = 1;
}
}
if(out)
memset(out + total_size, 0, FF_INPUT_BUFFER_PADDING_SIZE);
if (!sps_seen)
av_log(avctx, AV_LOG_WARNING, "Warning: SPS NALU missing or invalid. The resulting stream may not play.\n");

if (!pps_seen)
av_log(avctx, AV_LOG_WARNING, "Warning: PPS NALU missing or invalid. The resulting stream may not play.\n");

av_free(avctx->extradata);
avctx->extradata = out;
avctx->extradata_size = total_size;
ctx->first_idr = 1;
ctx->extradata_parsed = 1;
}
*poutbuf_size = 0;
*poutbuf = NULL;
do {
if (buf + ctx->length_size > buf_end)
goto fail; //buf为NULL时，以下代码将不再执行

//
//用于保存数据长度的字节数，是在分析原extradata计算出来的
//
if (ctx->length_size == 1) {
nal_size = buf[0];
} else if (ctx->length_size == 2) {

nal_size = AV_RB16(buf);
} else
nal_size = AV_RB32(buf);
buf += ctx->length_size;
unit_type = *buf & 0x1f;
if (buf + nal_size > buf_end || nal_size < 0)

goto fail;

if (ctx->first_idr && unit_type == 5) {

//
//copy IDR 帧时，需要将sps及pps一同拷贝

//
if (alloc_and_copy(poutbuf, poutbuf_size,

avctx->extradata, avctx->extradata_size,
buf, nal_size) < 0)
goto fail;
ctx->first_idr = 0;
} else {
//
//非IDR帧，没有sps及pps
if (alloc_and_copy(poutbuf, poutbuf_size,

NULL, 0,
buf, nal_size) < 0)
goto fail;
if (!ctx->first_idr && unit_type == 1)

ctx->first_idr = 1;
}
buf += nal_size;
cumul_size += nal_size + ctx->length_size;
} while (cumul_size < buf_size);
return 1;
fail:
av_freep(poutbuf);
*poutbuf_size = 0;
return AVERROR(EINVAL);

}

//h264_mp4toannexb_bsf.c static int h264_mp4toannexb_filter(AVBitStreamFilterContext<wbr> *bsfc, AVCodecContext *avctx, const char *args, uint8_t **poutbuf, int *poutbuf_size, const uint8_t *buf, int buf_size, int keyframe) { H264BSFContext *ctx = bsfc->priv_data; uint8_t unit_type; int32_t nal_size; uint32_t cumul_size = 0; const uint8_t *buf_end = buf + buf_size; if (!avctx->extradata || avctx->extradata_size < 6) { *poutbuf = (uint8_t*) buf; *poutbuf_size = buf_size; return 0; } // //从extradata中分析出SPS、PPS // if (!ctx->extradata_parsed) { uint16_t unit_size; uint64_t total_size = 0; uint8_t *out = NULL, unit_nb, sps_done = 0, sps_seen = 0, pps_seen = 0; const uint8_t *extradata = avctx->extradata+4; //跳过前4个字节 static const uint8_t nalu_header[4] = {0, 0, 0, 1}; ctx->length_size = (*extradata++ & 0x3) + 1; //用于指示表示编码数据长度所需字节数 if (ctx->length_size == 3) return AVERROR(EINVAL); unit_nb = *extradata++ & 0x1f; if (!unit_nb) { goto pps; } else { sps_seen = 1; } while (unit_nb--) { void *tmp; unit_size = AV_RB16(extradata); total_size += unit_size+4; if (total_size > INT_MAX - FF_INPUT_BUFFER_PADDING_SIZE || extradata+2+unit_size > avctx->extradata+avctx->extradata_size) { av_free(out); return AVERROR(EINVAL); } tmp = av_realloc(out, total_size + FF_INPUT_BUFFER_PADDING_SIZE); if (!tmp) { av_free(out); return AVERROR(ENOMEM); } out = tmp; memcpy(out+total_size-unit_size-4, nalu_header, 4); memcpy(out+total_size-unit_size, extradata+2, unit_size); extradata += 2+unit_size; pps: if (!unit_nb && !sps_done++) { unit_nb = *extradata++; if (unit_nb) pps_seen = 1; } } if(out) memset(out + total_size, 0, FF_INPUT_BUFFER_PADDING_SIZE); if (!sps_seen) av_log(avctx, AV_LOG_WARNING, "Warning: SPS NALU missing or invalid. The resulting stream may not play.\n"); if (!pps_seen) av_log(avctx, AV_LOG_WARNING, "Warning: PPS NALU missing or invalid. The resulting stream may not play.\n"); av_free(avctx->extradata); avctx->extradata = out; avctx->extradata_size = total_size; ctx->first_idr = 1; ctx->extradata_parsed = 1; } *poutbuf_size = 0; *poutbuf = NULL; do { if (buf + ctx->length_size > buf_end) goto fail; //buf为NULL时，以下代码将不再执行 // //用于保存数据长度的字节数，是在分析原extradata计算出来的 // if (ctx->length_size == 1) { nal_size = buf[0]; } else if (ctx->length_size == 2) { nal_size = AV_RB16(buf); } else nal_size = AV_RB32(buf); buf += ctx->length_size; unit_type = *buf & 0x1f; if (buf + nal_size > buf_end || nal_size < 0) goto fail; if (ctx->first_idr && unit_type == 5) { // //copy IDR 帧时，需要将sps及pps一同拷贝 // if (alloc_and_copy(poutbuf, poutbuf_size, avctx->extradata, avctx->extradata_size, buf, nal_size) < 0) goto fail; ctx->first_idr = 0; } else { // //非IDR帧，没有sps及pps if (alloc_and_copy(poutbuf, poutbuf_size, NULL, 0, buf, nal_size) < 0) goto fail; if (!ctx->first_idr && unit_type == 1) ctx->first_idr = 1; } buf += nal_size; cumul_size += nal_size + ctx->length_size; } while (cumul_size < buf_size); return 1; fail: av_freep(poutbuf); *poutbuf_size = 0; return AVERROR(EINVAL); }</wbr>

一般情况下，extradata中包含一个sps、一个pps 的nalu, 从上面的代码中容易看出extradata的数据格式。分析后的sps及pps依然储存在extradata域中，并添加了起始符。从代码中还可以看出，上面的函数会将sps、pps及packet中的数据，都copy到poutbuf指示的内存中，如果不需要copy到指定内存，直接给buf参数传入空值即可。

3.使用ffmpeg的流过滤器获取sps及pps

流过滤器"h264_mp4toannexb", 在av_register_all()函数中会被注册。用法示例如下：

[cpp]
view plaincopyprint?

int ParseH264ExtraDataInMp4(int
stream_id)
{
uint8_t *dummy = NULL;
int dummy_size;
AVBitStreamFilterContext* bsfc = av_bitstream_filter_init("h264_mp4toannexb");

if(bsfc == NULL)
{
return -1;
}
av_bitstream_filter_filter(
bsfc, format_ctx_->streams[stream_id]->codec, NULL, &dummy, &dummy_size, NULL, 0, 0);

int ParseH264ExtraDataInMp4(int stream_id) { uint8_t *dummy = NULL; int dummy_size; AVBitStreamFilterContext<wbr>* bsfc = av_bitstream_filter_init("h264_mp4toannexb"); if(bsfc == NULL) { return -1; } av_bitstream_filter_filter( bsfc, format_ctx_->streams[stream_id]->codec, NULL, &dummy, &dummy_size, NULL, 0, 0);</wbr>

[cpp]
view plaincopyprint?

av_bitstream_filter_close(bsfc);
return 0;
}

av_bitstream_filter_close(bsfc); return 0; }