ffmpeg 如何探测网络流格式/如何从内存中获取数据，ffmpeg探测

ffmpeg 如何探测网络流格式/如何从内存中获取数据，ffmpeg探测

一般ffmpeg都是直接从文件中读取或者从网络流中读取，比如rtp://xx.xx.xx.xx:xxxx。

事实上也支持从内存中获取。

函数avio_alloc_context()实现该功能。

AVIOContext *avio_alloc_context(
unsigned char *buffer,
int buffer_size,
int write_flag,
void *opaque,
int (*read_packet)(void *opaque, uint8_t *buf, int buf_size),//重写该函数，指定从内存中读取的方法，将buf_size字节大小的数据保存到buf
int (*write_packet)(void *opaque, uint8_t *buf, int buf_size),//对应的这是写内存的函数
int64_t (*seek)(void *opaque, int64_t offset, int whence));

对于探测网络媒体流个格式，也可以用此种方法，先接收数据，然后探测。

下面贴出代码：

/*
*author tongli
*/
extern "C"{
#include "libavformat/avformat.h"
#include "libavcodec/avcodec.h"
#include "libavutil/avutil.h"
}
#define BUF_SIZE 4096*500

FILE* file;
//实现read_packet函数，从文件中读取模拟的是从内存中获取，同样可以实现为接收网络流
int read_packet(void *opaque, uint8_t *buf, int buf_size)
{
int n = 0;
if (!feof(file)){
n = fread(buf, 1, buf_size, file);
return n;
}else
return -1;
}

int main(int argc, char** argv)
{
file = fopen("2.mp4", "rb");
if (file == NULL)
return -1;
av_register_all();
AVIOContext* pb = NULL;
AVInputFormat* piFmt = NULL;
AVInputFormat* pFmt = NULL;

uint8_t* buf = (uint8_t*)av_mallocz(sizeof(uint8_t)* BUF_SIZE);

pb = avio_alloc_context(buf, BUF_SIZE, 0, NULL, read_packet, NULL, NULL);
if (av_probe_input_buffer(pb, &piFmt, "", NULL, 0, 0) < 0)//探测从内存中获取到的媒体流的格式
{
fprintf(stderr, "probe format failed\n");
return -1;
}
else{
fprintf(stdout, "format:%s[%s]\n", piFmt->name, piFmt->long_name);
}
return 0;
}

下面实现一个简单的例子，从内存中读取，然后播放。

<pre name="code" class="cpp">/*
*author tongli
*/
#include <stdio.h>
#include <direct.h>
#include <io.h>
extern "C"{
#include "libavformat/avformat.h"
#include "libavcodec/avcodec.h"
#include "libavutil/avutil.h"
#include "libswscale/swscale.h"
#include "libavformat/avio.h"
#include "sdl/SDL.h"
}
#define BUF_SIZE 4096 * 500
FILE* file;

int read_data(void *opaque, uint8_t *buf, int buf_size)
{
int n = 0;
if (!feof(file)){
n = fread(buf, 1, buf_size, file);
return n;
}
else
return -1;
}

int main(int argc, char* argv[])
{
av_register_all();
//file = fopen("h2.dat", "rb");
file = fopen("3.mp4", "rb+");
if (file == NULL)
return -1;
AVFormatContext	*pFormatCtx;
int				i, videoindex;
AVCodecContext	*pCodecCtx;
AVCodec			*pCodec;
AVIOContext* pb = NULL;
AVInputFormat* piFmt = NULL;

uint8_t* buf = (uint8_t*)av_mallocz(sizeof(uint8_t)* BUF_SIZE);

pb = avio_alloc_context(buf, BUF_SIZE, 0, NULL, read_data, NULL, NULL);
if (av_probe_input_buffer(pb, &piFmt, "", NULL, 0, 0) < 0)
{
fprintf(stderr, "probe format failed\n");
return -1;
}
else{
fprintf(stdout, "format:%s[%s]\n", piFmt->name, piFmt->long_name);
}
pFormatCtx = avformat_alloc_context();
pFormatCtx->pb = pb;

if (avformat_open_input(&pFormatCtx, "", piFmt, NULL) != 0){//iformat，priv_data赋值，pb, nbstreams,streams为null
printf("Couldn't open input stream.（无法打开输入流）\n");
return -1;
}
if (avformat_find_stream_info(pFormatCtx, NULL)<0)//nbstreams,streams赋值， pb还是为null
{
printf("Couldn't find stream information.（无法获取流信息）\n");
return -1;
}
videoindex = -1;
for (i = 0; i<pFormatCtx->nb_streams; i++) //一般情况下，一个媒体只有两个流，视频和音频流即streams[0],stream[1]
if (pFormatCtx->streams[i]->codec->codec_type == AVMEDIA_TYPE_VIDEO)
{//找到视频流
videoindex = i;//在nb_streams视频流的索引
break;
}
if (videoindex == -1)
{
printf("Didn't find a video stream.（没有找到视频流）\n");
return -1;
}
pCodecCtx = pFormatCtx->streams[videoindex]->codec;
pCodec = avcodec_find_decoder(pCodecCtx->codec_id);
if (pCodec == NULL)
{
printf("Codec not found.（没有找到解码器）\n");
return -1;
}
if (avcodec_open2(pCodecCtx, pCodec, NULL)<0)
{
printf("Could not open codec.（无法打开解码器）\n");
return -1;
}
AVFrame	*pFrame, *pFrameYUV;
pFrame = av_frame_alloc();
pFrameYUV = av_frame_alloc();

uint8_t *out_buffer;
out_buffer = new uint8_t[avpicture_get_size(PIX_FMT_YUV420P, pCodecCtx->width, pCodecCtx->height)];

avpicture_fill((AVPicture *)pFrameYUV, out_buffer, PIX_FMT_YUV420P, pCodecCtx->width, pCodecCtx->height);
//------------SDL----------------
if (SDL_Init(SDL_INIT_VIDEO | SDL_INIT_AUDIO | SDL_INIT_TIMER)) {
printf("Could not initialize SDL - %s\n", SDL_GetError());
return -1;
}
SDL_Surface *screen;
screen = SDL_SetVideoMode(pCodecCtx->width, pCodecCtx->height, 0, 0);
if (!screen) {
printf("SDL: could not set video mode - exiting\n");
return -1;
}
SDL_Overlay *bmp;
bmp = SDL_CreateYUVOverlay(pCodecCtx->width, pCodecCtx->height, SDL_YV12_OVERLAY, screen);
SDL_Rect rect;
//---------------
int ret, got_picture;
int y_size = pCodecCtx->width * pCodecCtx->height;

AVPacket *packet = (AVPacket *)av_malloc(sizeof(AVPacket));
av_new_packet(packet, y_size);

struct SwsContext *img_convert_ctx;
img_convert_ctx = sws_getContext(pCodecCtx->width, pCodecCtx->height,
AV_PIX_FMT_YUVJ420P/*pCodecCtx->pix_fmt*/, pCodecCtx->width, pCodecCtx->height,
PIX_FMT_YUV420P, SWS_BICUBIC, NULL, NULL, NULL);
//------------------------------
while (av_read_frame(pFormatCtx, packet) >= 0)
{
if (packet->stream_index == videoindex)
{
ret = avcodec_decode_video2(pCodecCtx, pFrame, &got_picture, packet);
if (ret < 0)
{
printf("Decode Error.（解码错误）\n");
return -1;
}
if (got_picture)
{
sws_scale(img_convert_ctx, (const uint8_t* const*)pFrame->data, pFrame->linesize, 0, pCodecCtx->height, pFrameYUV->data, pFrameYUV->linesize);

SDL_LockYUVOverlay(bmp);
bmp->pixels[0] = pFrameYUV->data[0];
bmp->pixels[2] = pFrameYUV->data[1];
bmp->pixels[1] = pFrameYUV->data[2];
bmp->pitches[0] = pFrameYUV->linesize[0];
bmp->pitches[2] = pFrameYUV->linesize[1];
bmp->pitches[1] = pFrameYUV->linesize[2];
SDL_UnlockYUVOverlay(bmp);
rect.x = 0;
rect.y = 0;
rect.w = pCodecCtx->width;
rect.h = pCodecCtx->height;
SDL_DisplayYUVOverlay(bmp, &rect);
//延时40ms
SDL_Delay(40);
}
}
av_free_packet(packet);
}
sws_freeContext(img_convert_ctx);

delete[] out_buffer;
av_free(pFrameYUV);
avcodec_close(pCodecCtx);
avformat_close_input(&pFormatCtx);

return 0;
}