- 用ffmpeg處理視頻時,有時需要從視頻裏提取某個時間的一幀視頻數據,這時需要用到ffmpeg的一個關鍵函數,av_seek_frame。
av_seek_frame原型如下:
int av_seek_frame(AVFormatContext *s, int stream_index, int64_t timestamp, int flags){}
參數1: s操作上下文;
參數2: stream_index 流索引,當流索引爲-1時,會選擇一個默認流,時間戳會從以AV_TIME_BASE爲單位
參數3: timestamp將要定位處的時間戳
參數4: flags功能flag
AVSEEK_FLAG_BACKWARD seek到請求的時間戳之前最近的關鍵幀
AVSEEK_FLAG_BYTE 基於字節位置的查找
AVSEEK_FLAG_ANY 是可以seek到任意幀,注意不一定是關鍵幀,因此使用時可能會導致花屏
AVSEEK_FLAG_FRAME 是基於幀數量快進
- 示例很簡單,工程代碼如下:
保存文件可以爲調試代碼用,函數如下
int saveAsJPEG(AVFrame* pFrame, int width, int height, int index)
{
char out_file[256] = {0};
sprintf_s(out_file, sizeof(out_file), "%s%d.jpg", "", index);
AVFormatContext* pFormatCtx = avformat_alloc_context();
pFormatCtx->oformat = av_guess_format("mjpeg", nullptr, nullptr);
if( avio_open(&pFormatCtx->pb, out_file, AVIO_FLAG_READ_WRITE) < 0)
{
printf("Couldn't open output file.");
return -1;
}
AVStream* pAVStream = avformat_new_stream(pFormatCtx, 0);
if( pAVStream == nullptr )
{
return -1;
}
AVCodecContext* pCodecCtx = pAVStream->codec;
pCodecCtx->codec_id = pFormatCtx->oformat->video_codec;
pCodecCtx->codec_type = AVMEDIA_TYPE_VIDEO;
pCodecCtx->pix_fmt = AV_PIX_FMT_YUVJ420P;
pCodecCtx->width = width;
pCodecCtx->height = height;
pCodecCtx->time_base.num = 1;
pCodecCtx->time_base.den = 25;
//打印輸出相關信息
av_dump_format(pFormatCtx, 0, out_file, 1);
//================================== 查找編碼器 ==================================//
AVCodec* pCodec = avcodec_find_encoder(pCodecCtx->codec_id);
if( !pCodec )
{
printf("Codec not found.");
return -1;
}
if( avcodec_open2(pCodecCtx, pCodec, nullptr) < 0 )
{
printf("Could not open codec.");
return -1;
}
//================================Write Header ===============================//
avformat_write_header(pFormatCtx, nullptr);
int y_size = pCodecCtx->width * pCodecCtx->height;
AVPacket pkt;
av_new_packet(&pkt, y_size * 3);
//
int got_picture = 0;
int ret = avcodec_encode_video2(pCodecCtx, &pkt, pFrame, &got_picture);
if( ret < 0 )
{
printf("Encode Error.\n");
return -1;
}
if( got_picture == 1 )
{
pkt.stream_index = pAVStream->index;
ret = av_write_frame(pFormatCtx, &pkt);
}
av_free_packet(&pkt);
av_write_trailer(pFormatCtx);
if( pAVStream )
{
avcodec_close(pAVStream->codec);
}
avio_close(pFormatCtx->pb);
avformat_free_context(pFormatCtx);
return 0;
}
main函數如下:
int main()
{
//std::cout << "Hello World!" << std::endl;
AVFormatContext *pFormatCtx = avformat_alloc_context();
int res;
res = avformat_open_input(&pFormatCtx, "test.mp4", nullptr, nullptr);
if (res) {
return 0;
}
avformat_find_stream_info(pFormatCtx, nullptr);
int videoStream = -1;
for(int i=0; i<pFormatCtx->nb_streams; i++) {
if(pFormatCtx->streams[i]->codec->codec_type == AVMEDIA_TYPE_VIDEO) {
videoStream=i;
break;
}
}
if(videoStream == -1) {
return 0;
}
AVCodecContext *pCodecCtxOrig = nullptr;
// Get a pointer to the codec context for the video stream
pCodecCtxOrig = pFormatCtx->streams[videoStream]->codec;
AVCodec *pCodec = nullptr;
// Find the decoder for the video stream
pCodec = avcodec_find_decoder(pCodecCtxOrig->codec_id);
if(pCodec == nullptr) {
fprintf(stderr, "Unsupported codec!\n");
return 0; // Codec not found
}
AVCodecContext *pCodecCtx = nullptr;
// Copy context
pCodecCtx = avcodec_alloc_context3(pCodec);
if(avcodec_copy_context(pCodecCtx, pCodecCtxOrig) != 0) {
fprintf(stderr, "Couldn't copy codec context");
return 0; // Error copying codec context
}
// Open codec
if(avcodec_open2(pCodecCtx, pCodec, nullptr)<0) {
return 0;// Could not open codec
}
AVFrame *pFrameRGB = nullptr;
pFrameRGB = av_frame_alloc();
res = av_seek_frame(pFormatCtx, -1, 10 * AV_TIME_BASE, AVSEEK_FLAG_BACKWARD);//10(second)
if (res<0) {
return 0;
}
AVPacket packet;
while(1) {
av_read_frame(pFormatCtx, &packet);
if(packet.stream_index == videoStream) {
res = avcodec_send_packet(pCodecCtx, &packet);
int gotPicture = avcodec_receive_frame(pCodecCtx, pFrameRGB); //gotPicture = 0 success, a frame was returned
if(gotPicture == 0) {
SwsContext* swsContext = sws_getContext(pCodecCtx->width, pCodecCtx->height, pCodecCtx->pix_fmt, pCodecCtx->width, pCodecCtx->height, AV_PIX_FMT_RGB24,
SWS_BICUBIC, nullptr, nullptr, nullptr);
AVFrame* frameRGB = av_frame_alloc();
avpicture_alloc((AVPicture*)frameRGB, AV_PIX_FMT_RGB24, pCodecCtx->width, pCodecCtx->height);
sws_scale(swsContext, pFrameRGB->data, pFrameRGB->linesize, 0, pCodecCtx->height, frameRGB->data, frameRGB->linesize);
saveAsJPEG(pFrameRGB, pCodecCtx->width, pCodecCtx->height, 10);
avformat_close_input(&pFormatCtx);
return 0;
}
}
}
return 0;
}
如果在Qt工程裏使用,則可以很簡單的用QImage來保存數據,具體講SaveAsJPEG()這一行代碼替換爲:
QImage image(frameRGB->data[0], pCodecCtx->width, pCodecCtx->height, frameRGB->linesize[0], QImage::Format_RGB888);
image.save("10.jpg");
需要包含頭文件
extern "C" {
#include <libavcodec/avcodec.h>
#include <libavformat/avformat.h>
#include <libavformat/avio.h>
#include <libavutil/file.h>
#include "libswresample/swresample.h"
#include <libavutil/frame.h>
#include <libavutil/mem.h>
#include <libavutil/imgutils.h>
#include <libavutil/samplefmt.h>
#include <libavutil/timestamp.h>
#include <libavformat/avformat.h>
#include <libswscale/swscale.h>
}