深入浅出FFMPEG（一） SDK version 0.11

深入浅出FFMPEG

数字媒体处理的基本流程

认识FFMPEG

FFMPEG堪称自由软件中最完备的一套多媒体支持库，它几乎实现了所有当下常见的数据封装格式、多媒体传输协议以及音视频编解码器。因此，对于从事多媒体技术开发的工程师来说，深入研究FFMPEG成为一门必不可少的工作，可以这样说，FFMPEG之于多媒体开发工程师的重要性正如kernel之于嵌入式系统工程师一般。

几个小知识：

FFMPEG项目是由法国人Fabrice Bellard发起的，此人也是著名的CPU模拟器项目QEMU的发起者，同时还是圆周率算法纪录的保持者。

FF是Fast Forward的意思，翻译成中文是“快进”。

FFMPEG的LOGO是一个”Z字扫描”示意图，Z字扫描用于将图像的二维频域数据一维化，同时保证了一维化的数据具备良好的统计特性，从而提高其后要进行的一维熵编码的效率。

关于耻辱柱（Hall of Shame）：FFMpeg大部分代码遵循LGPL许可证，如果使用者对FFMpeg进行了修改，要求公布修改的源代码；有少部分代码遵循GPL许可证，要求使用者同时公开使用FFMpeg的软件的源代码。实际上，除去部分大的系统软件开发商（Microsoft、Apple等）以及某些著名的音视频服务提供商（Divx、Real等）提供的自有播放器之外，绝大部分第三方开发的播放器都离不开FFMpeg的支持，像Linux桌面环境中的开源播放器VLC、MPlayer，Windows下的KMPlayer、暴风影音以及Android下几乎全部第三方播放器都是基于FFMpeg的。也有许多看似具备自主技术的播放器，其实也都不声不响地使用了FFMpeg，这种行为被称为“盗窃”，参与“盗窃”的公司的名字则被刻在耻辱柱上，国产播放器暴风影音、QQ影音于2009年上榜。

一个简单的测试程序

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

#include <sys/time.h>

#include "libavutil/avstring.h"
#include "libavformat/avformat.h"
#include "libavdevice/avdevice.h"
#include "libavcodec/opt.h"
#include "libswscale/swscale.h"

#define DECODED_AUDIO_BUFFER_SIZE            192000

struct options
{
int streamId;
int frames;
int nodec;
int bplay;
int thread_count;
int64_t lstart;
char finput[256];
char foutput1[256];
char foutput2[256];
};

int parse_options(struct options *opts, int argc, char** argv)
{
int optidx;
char *optstr;

if (argc < 2) return -1;

opts->streamId = -1;
opts->lstart = -1;
opts->frames = -1;
opts->foutput1[0] = 0;
opts->foutput2[0] = 0;
opts->nodec = 0;
opts->bplay = 0;
opts->thread_count = 0;
strcpy(opts->finput, argv[1]);

optidx = 2;
while (optidx < argc)
{
optstr = argv[optidx++];
if (*optstr++ != '-') return -1;
switch (*optstr++)
{
case 's':  //< stream id
opts->streamId = atoi(optstr);
break;
case 'f':  //< frames
opts->frames = atoi(optstr);
break;
case 'k':  //< skipped
opts->lstart = atoll(optstr);
break;
case 'o':  //< output
strcpy(opts->foutput1, optstr);
strcat(opts->foutput1, ".mpg");
strcpy(opts->foutput2, optstr);
strcat(opts->foutput2, ".raw");
break;
case 'n': //decoding and output options
if (strcmp("dec", optstr) == 0)
opts->nodec = 1;
break;
case 'p':
opts->bplay = 1;
break;
case 't':
opts->thread_count = atoi(optstr);
break;
default:
return -1;
}
}

return 0;
}

void show_help(char* program)
{
printf("Simple FFMPEG test program\n");
printf("Usage: %s inputfile [-sstreamid [-fframes] [-kskipped] [-ooutput_filename(without extension)] [-p] [-tthread_count]]\n",
program);
return;
}

static void log_callback(void* ptr, int level, const char* fmt, va_list vl)
{
vfprintf(stdout, fmt, vl);
}

#include <sys/ioctl.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/soundcard.h>

#define OSS_DEVICE "/dev/dsp0"

struct audio_dsp
{
int audio_fd;
int channels;
int format;
int speed;
};
int map_formats(enum SampleFormat format)
{
switch(format)
{
case SAMPLE_FMT_U8:
return AFMT_U8;
case SAMPLE_FMT_S16:
return AFMT_S16_LE;
default:
return AFMT_U8;
}
}
int set_audio(struct audio_dsp* dsp)
{
if (dsp->audio_fd == -1)
{
printf("Invalid audio dsp id!\n");
return -1;
}

if (-1 == ioctl(dsp->audio_fd, SNDCTL_DSP_SETFMT, &dsp->format))
{
printf("Failed to set dsp format!\n");
return -1;
}

if (-1 == ioctl(dsp->audio_fd, SNDCTL_DSP_CHANNELS, &dsp->channels))
{
printf("Failed to set dsp format!\n");
return -1;
}

if (-1 == ioctl(dsp->audio_fd, SNDCTL_DSP_SPEED, &dsp->speed))
{
printf("Failed to set dsp format!\n");
return -1;
}
return 0;
}

int play_pcm(struct audio_dsp* dsp, unsigned char *buf, int size)
{
if (dsp->audio_fd == -1)
{
printf("Invalid audio dsp id!\n");
return -1;
}

if (-1 == write(dsp->audio_fd, buf, size))
{
printf("Failed to write audio dsp!\n");
return -1;
}

return 0;
}

#include <linux/fb.h>
#include <sys/mman.h>

#define FB_DEVICE "/dev/fb0"

enum pic_format
{
eYUV_420_Planer,
};
struct video_fb
{
int video_fd;
struct fb_var_screeninfo vinfo;
struct fb_fix_screeninfo finfo;
unsigned char *fbp;
AVFrame *frameRGB;
struct
{
int x;
int y;
} video_pos;
};

int open_video(struct video_fb *fb, int x, int y)
{
int screensize;
fb->video_fd = open(FB_DEVICE, O_WRONLY);
if (fb->video_fd == -1) return -1;

if (ioctl(fb->video_fd, FBIOGET_FSCREENINFO, &fb->finfo)) return -2;
if (ioctl(fb->video_fd, FBIOGET_VSCREENINFO, &fb->vinfo)) return -2;

printf("video device: resolution %dx%d, �pp\n", fb->vinfo.xres, fb->vinfo.yres, fb->vinfo.bits_per_pixel);
screensize = fb->vinfo.xres * fb->vinfo.yres * fb->vinfo.bits_per_pixel / 8;
fb->fbp = (unsigned char *) mmap(0, screensize, PROT_READ|PROT_WRITE, MAP_SHARED, fb->video_fd, 0);
if (fb->fbp == -1) return -3;

if (x >= fb->vinfo.xres || y >= fb->vinfo.yres)
{
return -4;
}
else
{
fb->video_pos.x = x;
fb->video_pos.y = y;
}

fb->frameRGB = avcodec_alloc_frame();
if (!fb->frameRGB) return -5;

return 0;
}
#if 0

int show_picture(struct video_fb *fb, AVFrame *frame, int width, int height, enum pic_format format)
{
struct SwsContext *sws;
int i;
unsigned char *dest;
unsigned char *src;

if (fb->video_fd == -1) return -1;
if ((fb->video_pos.x >= fb->vinfo.xres) || (fb->video_pos.y >= fb->vinfo.yres)) return -2;

if (fb->video_pos.x + width > fb->vinfo.xres)
{
width = fb->vinfo.xres - fb->video_pos.x;
}
if (fb->video_pos.y + height > fb->vinfo.yres)
{
height = fb->vinfo.yres - fb->video_pos.y;
}

if (format == PIX_FMT_YUV420P)
{
sws = sws_getContext(width, height, format, width, height, PIX_FMT_RGB32, SWS_FAST_BILINEAR, NULL, NULL, NULL);
if (sws == 0)
{
return -3;
}
if (sws_scale(sws, frame->data, frame->linesize, 0, height, fb->frameRGB->data, fb->frameRGB->linesize))
{
return -3;
}

dest = fb->fbp + (fb->video_pos.x+fb->vinfo.xoffset) * (fb->vinfo.bits_per_pixel/8) +(fb->video_pos.y+fb->vinfo.yoffset) * fb->finfo.line_length;
for (i = 0; i < height; i++)
{
memcpy(dest, src, width*4);
src += fb->frameRGB->linesize[0];
dest += fb->finfo.line_length;
}
}
return 0;
}
#endif
void close_video(struct video_fb *fb)
{
if (fb->video_fd != -1)
{
munmap(fb->fbp, fb->vinfo.xres * fb->vinfo.yres * fb->vinfo.bits_per_pixel / 8);
close(fb->video_fd);
fb->video_fd = -1;
}
}

int main(int argc, char **argv)
{
AVFormatContext* pCtx = 0;
AVCodecContext *pCodecCtx = 0;
AVCodec *pCodec = 0;
AVPacket packet;
AVFrame *pFrame = 0;
FILE *fpo1 = NULL;
FILE *fpo2 = NULL;
int nframe;
int err;
int got_picture;
int picwidth, picheight, linesize;
unsigned char *pBuf;
int i;
int64_t timestamp;
struct options opt;
int usefo = 0;
struct audio_dsp dsp;
int dusecs;
float usecs1 = 0;
float usecs2 = 0;
struct timeval elapsed1, elapsed2;
int decoded = 0;

av_register_all();

av_log_set_callback(log_callback);
av_log_set_level(50);

if (parse_options(&opt, argc, argv) < 0 || (strlen(opt.finput) == 0))
{
show_help(argv[0]);
return 0;
}

err = avformat_open_input(&pCtx, opt.finput, 0, 0);
if (err < 0)
{
printf("\n->(avformat_open_input)\tERROR:\t%d\n", err);
goto fail;
}
err = avformat_find_stream_info(pCtx, 0);
if (err < 0)
{
printf("\n->(avformat_find_stream_info)\tERROR:\t%d\n", err);
goto fail;
}
if (opt.streamId < 0)
{
av_dump_format(pCtx, 0, pCtx->filename, 0);
goto fail;
}
else
{
printf("\n extra data in Stream %d (�):", opt.streamId, pCtx->streams[opt.streamId]->codec->extradata_size);
for (i = 0; i < pCtx->streams[opt.streamId]->codec->extradata_size; i++)
{
if (i%16 == 0) printf("\n");
printf("%2x  ", pCtx->streams[opt.streamId]->codec->extradata[i]);
}
}

if (strlen(opt.foutput1) && strlen(opt.foutput2))
{
fpo1 = fopen(opt.foutput1, "wb");
fpo2 = fopen(opt.foutput2, "wb");
if (!fpo1 || !fpo2)
{
printf("\n->error opening output files\n");
goto fail;
}
usefo = 1;
}
else
{
usefo = 0;
}

if (opt.streamId >= pCtx->nb_streams)
{
printf("\n->StreamId\tERROR\n");
goto fail;
}

if (opt.lstart > 0)
{
err = av_seek_frame(pCtx, opt.streamId, opt.lstart, AVSEEK_FLAG_ANY);
if (err < 0)
{
printf("\n->(av_seek_frame)\tERROR:\t%d\n", err);
goto fail;
}
}

if (!opt.nodec)
{

pCodecCtx = pCtx->streams[opt.streamId]->codec;

if (opt.thread_count <= 16 && opt.thread_count > 0 )
{
pCodecCtx->thread_count = opt.thread_count;
pCodecCtx->thread_type = FF_THREAD_FRAME;
}
pCodec = avcodec_find_decoder(pCodecCtx->codec_id);
if (!pCodec)
{
printf("\n->can not find codec!\n");
goto fail;
}
err = avcodec_open2(pCodecCtx, pCodec, 0);
if (err < 0)
{
printf("\n->(avcodec_open)\tERROR:\t%d\n", err);
goto fail;
}
pFrame = avcodec_alloc_frame();

if (opt.bplay)
{

dsp.audio_fd = open(OSS_DEVICE, O_WRONLY);
if (dsp.audio_fd == -1)
{
printf("\n-> can not open audio device\n");
goto fail;
}
dsp.channels = pCodecCtx->channels;
dsp.speed = pCodecCtx->sample_rate;
dsp.format = map_formats(pCodecCtx->sample_fmt);
if (set_audio(&dsp) < 0)
{
printf("\n-> can not set audio device\n");
goto fail;
}

}
}

nframe = 0;
while(nframe < opt.frames || opt.frames == -1)
{
gettimeofday(&elapsed1, NULL);
err = av_read_frame(pCtx, &packet);
if (err < 0)
{
printf("\n->(av_read_frame)\tERROR:\t%d\n", err);
break;
}
gettimeofday(&elapsed2, NULL);
dusecs = (elapsed2.tv_sec - elapsed1.tv_sec)*1000000 + (elapsed2.tv_usec - elapsed1.tv_usec);
usecs2 += dusecs;
timestamp = av_rescale_q(packet.dts, pCtx->streams[packet.stream_index]->time_base, (AVRational){1, AV_TIME_BASE});
printf("\nFrame No ] stream#%d\tsize mB, timestamp:%6lld, dts:%6lld, pts:%6lld, ", nframe++, packet.stream_index, packet.size,
timestamp, packet.dts, packet.pts);

if (packet.stream_index == opt.streamId)
{
#if 0
for (i = 0; i < 16;  i++)
{
if (i%16 == 0) printf("\n pktdata: ");
printf("%2x  ", packet.data[i]);
}
printf("\n");
#endif
if (usefo)
{
fwrite(packet.data, packet.size, 1, fpo1);
fflush(fpo1);
}

if (pCtx->streams[opt.streamId]->codec->codec_type == AVMEDIA_TYPE_VIDEO && !opt.nodec)
{
picheight = pCtx->streams[opt.streamId]->codec->height;
picwidth = pCtx->streams[opt.streamId]->codec->width;

gettimeofday(&elapsed1, NULL);
avcodec_decode_video2(pCodecCtx, pFrame, &got_picture, &packet);
decoded++;
gettimeofday(&elapsed2, NULL);
dusecs = (elapsed2.tv_sec - elapsed1.tv_sec)*1000000 + (elapsed2.tv_usec - elapsed1.tv_usec);
usecs1 += dusecs;

if (got_picture)
{
printf("[Video: type %d, ref %d, pts %lld, pkt_pts %lld, pkt_dts %lld]",
pFrame->pict_type, pFrame->reference, pFrame->pts, pFrame->pkt_pts, pFrame->pkt_dts);

if (pCtx->streams[opt.streamId]->codec->pix_fmt == PIX_FMT_YUV420P)
{
if (usefo)
{
linesize = pFrame->linesize[0];
pBuf = pFrame->data[0];
for (i = 0; i < picheight; i++)
{
fwrite(pBuf, picwidth, 1, fpo2);
pBuf += linesize;
}

linesize = pFrame->linesize[1];
pBuf = pFrame->data[1];
for (i = 0; i < picheight/2; i++)
{
fwrite(pBuf, picwidth/2, 1, fpo2);
pBuf += linesize;
}

linesize = pFrame->linesize[2];
pBuf = pFrame->data[2];
for (i = 0; i < picheight/2; i++)
{
fwrite(pBuf, picwidth/2, 1, fpo2);
pBuf += linesize;
}
fflush(fpo2);
}

if (opt.bplay)
{

}
}
}
av_free_packet(&packet);
}
else if (pCtx->streams[opt.streamId]->codec->codec_type == AVMEDIA_TYPE_AUDIO && !opt.nodec)
{
int got;

gettimeofday(&elapsed1, NULL);
avcodec_decode_audio4(pCodecCtx, pFrame, &got, &packet);
decoded++;
gettimeofday(&elapsed2, NULL);
dusecs = (elapsed2.tv_sec - elapsed1.tv_sec)*1000000 + (elapsed2.tv_usec - elapsed1.tv_usec);
usecs1 += dusecs;

if (got)
{
printf("[Audio: ]B raw data, decoding time: %d]", pFrame->linesize[0], dusecs);
if (usefo)
{
fwrite(pFrame->data[0],  pFrame->linesize[0], 1, fpo2);
fflush(fpo2);
}
if (opt.bplay)
{
play_pcm(&dsp, pFrame->data[0],  pFrame->linesize[0]);
}
}
}
}
}

if (!opt.nodec && pCodecCtx)
{
avcodec_close(pCodecCtx);
}

printf("\n%d frames parsed, average %.2f us per frame\n", nframe, usecs2/nframe);
printf("%d frames decoded, average %.2f us per frame\n", decoded, usecs1/decoded);

fail:
if (pCtx)
{
avformat_close_input(&pCtx);
}
if (fpo1)
{
fclose(fpo1);
}
if (fpo2)
{
fclose(fpo2);
}
if (!pFrame)
{
av_free(pFrame);
}
if (!usefo && (dsp.audio_fd != -1))
{
close(dsp.audio_fd);
}
return 0;
}

这一小段代码可以实现的功能包括：

打开一个多媒体文件并获取基本的媒体信息。

获取编码器句柄。

根据给定的时间标签进行一个跳转。

读取数据帧。

解码音频帧或者视频帧。

关闭多媒体文件。

这些功能足以支持一个功能强大的多媒体播放器，因为最复杂的解复用、解码、数据分析过程已经在FFMpeg内部实现了，需要关注的仅剩同步问题。