SPS/PPS在ffmpeg的codec->extradata中获取
加载ffmpeg的日志模块
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av_log_set_level(AV_LOG_DEBUG);注册所有编解码器
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2/*register all formats and codec*/
av_register_all();打开视频文件并获取视频文件上下文
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11AVFormatContext *fmt_ctx = NULL;
/*open input media file, and allocate format context*/
if((err_code = avformat_open_input(&fmt_ctx, src_filename, NULL, NULL)) < 0){
av_strerror(err_code, errors, 1024);
av_log(NULL, AV_LOG_DEBUG, "Could not open source file: %s, %d(%s)\n",
src_filename,
err_code,
errors);
return -1;
}打印视频详细信息
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2/*dump input information*/
av_dump_format(fmt_ctx, 0, src_filename, 0);生成一个空的数据包，用于接收视频包
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4/*initialize packet*/
av_init_packet(&pkt);
pkt.data = NULL;
pkt.size = 0;获取视频流
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7/*find best video stream*/
video_stream_index = av_find_best_stream(fmt_ctx, AVMEDIA_TYPE_VIDEO, -1, -1, NULL, 0);
if (video_stream_index < 0)
{
av_log(NULL, AV_LOG_ERROR, "Could not find %s stream in input file %s\n", av_get_media_type_string(AVMEDIA_TYPE_VIDEO),src_filename);
return AVERROR(EINVAL);
}循环读取视频流中的包，进行处理
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10/*read frames from media file*/
while (av_read_frame(fmt_ctx, &pkt) >= 0)
{
if (pkt.stream_index == video_stream_index)
{
h264_mp4toannexb(fmt_ctx, &pkt, dst_fd);
}
//release pkt->data
av_packet_unref(&pkt);
}对包进行处理，提取帧数据进行写文件
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81int h264_mp4toannexb(AVFormatContext *fmt_ctx, AVPacket *in, FILE *dst_fd)
{
AVPacket *out = NULL;
AVPacket spspps_pkt;
int len;
uint8_t unit_type;
int32_t nal_size;
uint32_t cumul_size = 0;
const uint8_t *buf;
const uint8_t *buf_end;
int buf_size;
int ret = 0, i;
out = av_packet_alloc();
buf = in->data;
buf_size = in->size;
buf_end = in->data + in->size;
do
{
ret = AVERROR(EINVAL);
if (buf + 4 /*s->length_size*/ > buf_end)
goto fail;
//一个AVPacket中存的可能是一帧也可能是多帧
// nal_size是一个帧的具体大小，在帧的头部的前4个字节
for (nal_size = 0, i = 0; i < 4 /*s->length_size*/; i++)
nal_size = (nal_size << 8) | buf[i];
buf += 4; /*s->length_size;*/
//帧数据的第一个字节的后5位是这个帧的nal单元
/* nal: 7 -> sps
8 -> pps
key frame -> 5
normal frame -> 1 */
unit_type = *buf & 0x1f;
if (nal_size > buf_end - buf || nal_size < 0)
goto fail;
/* prepend only to the first type 5 NAL unit of an IDR picture, if no sps/pps are already present */
if (/*s->new_idr && */ unit_type == 5 /*&& !s->idr_sps_seen && !s->idr_pps_seen*/)
{
// 关键帧，需要获取sps/pps数据，并重新修改视频宽高比等参数
h264_extradata_to_annexb(fmt_ctx->streams[in->stream_index]->codec->extradata,
fmt_ctx->streams[in->stream_index]->codec->extradata_size,
&spspps_pkt,
AV_INPUT_BUFFER_PADDING_SIZE);
// 为数据增加特征码
if ((ret = alloc_and_copy(out,
spspps_pkt.data, spspps_pkt.size,
buf, nal_size)) < 0)
goto fail;
/*s->new_idr = 0;*/
/* if only SPS has been seen, also insert PPS */
}
else
{
if ((ret = alloc_and_copy(out, NULL, 0, buf, nal_size)) < 0)
goto fail;
}
len = fwrite(out->data, 1, out->size, dst_fd);
if (len != out->size)
{
av_log(NULL, AV_LOG_DEBUG, "warning, length of writed data isn't equal pkt.size(%d, %d)\n",
len,
out->size);
}
fflush(dst_fd);
next_nal:
buf += nal_size;
cumul_size += nal_size + 4; //s->length_size;
} while (cumul_size < buf_size);
fail:
av_packet_free(&out);
return ret;
}
获取SPS/PPS数据
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79int h264_extradata_to_annexb(const uint8_t *codec_extradata, const int codec_extradata_size, AVPacket *out_extradata, int padding)
{
uint16_t unit_size;
uint64_t total_size = 0;
uint8_t *out = NULL, unit_nb, sps_done = 0,
sps_seen = 0, pps_seen = 0, sps_offset = 0, pps_offset = 0;
const uint8_t *extradata = codec_extradata + 4;
static const uint8_t nalu_header[4] = {0, 0, 0, 1};
int length_size = (*extradata++ & 0x3) + 1; // retrieve length coded size, 用于指示表示编码数据长度所需字节数
sps_offset = pps_offset = -1;
/* retrieve sps and pps unit(s) */
unit_nb = *extradata++ & 0x1f; /* number of sps unit(s) */
if (!unit_nb)
{
goto pps;
}
else
{
sps_offset = 0;
sps_seen = 1;
}
while (unit_nb--)
{
int err;
unit_size = AV_RB16(extradata);
total_size += unit_size + 4;
if (total_size > INT_MAX - padding)
{
av_log(NULL, AV_LOG_ERROR,
"Too big extradata size, corrupted stream or invalid MP4/AVCC bitstream\n");
av_free(out);
return AVERROR(EINVAL);
}
if (extradata + 2 + unit_size > codec_extradata + codec_extradata_size)
{
av_log(NULL, AV_LOG_ERROR, "Packet header is not contained in global extradata, "
"corrupted stream or invalid MP4/AVCC bitstream\n");
av_free(out);
return AVERROR(EINVAL);
}
if ((err = av_reallocp(&out, total_size + padding)) < 0)
return err;
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++; /* number of pps unit(s) */
if (unit_nb)
{
pps_offset = total_size;
pps_seen = 1;
}
}
}
if (out)
memset(out + total_size, 0, padding);
if (!sps_seen)
av_log(NULL, AV_LOG_WARNING,
"Warning: SPS NALU missing or invalid. "
"The resulting stream may not play.\n");
if (!pps_seen)
av_log(NULL, AV_LOG_WARNING,
"Warning: PPS NALU missing or invalid. "
"The resulting stream may not play.\n");
out_extradata->data = out;
out_extradata->size = total_size;
return length_size;
}
为每一帧数据增加特征码
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28static int alloc_and_copy(AVPacket *out,
const uint8_t *sps_pps, uint32_t sps_pps_size,
const uint8_t *in, uint32_t in_size)
{
uint32_t offset = out->size;
uint8_t nal_header_size = offset ? 3 : 4;
int err;
err = av_grow_packet(out, sps_pps_size + in_size + nal_header_size);
if (err < 0)
return err;
if (sps_pps)
memcpy(out->data + offset, sps_pps, sps_pps_size);
memcpy(out->data + sps_pps_size + nal_header_size + offset, in, in_size);
if (!offset)
{
AV_WB32(out->data + sps_pps_size, 1);
}
else
{
(out->data + offset + sps_pps_size)[0] =
(out->data + offset + sps_pps_size)[1] = 0;
(out->data + offset + sps_pps_size)[2] = 1;
}
return 0;
}