Cutting principle
It is to cut all the streams in the multimedia file.
logical flow
On the basis of multimedia format conversion, each required stream is cropped.
Note: Each dts and pts after clipping are relative to the start time of clipping.
Implementation code
Intercept mp4 file, start time is 60s, end time is 120s
#include <stdio.h>
#include <stdlib.h>
#include <libavutil/log.h>
#include <libavutil/timestamp.h>
#include <libavutil/avutil.h>
#include <libavformat/avformat.h>
static void log_packet(AVFormatContext* fmtCtx, const AVPacket* pkt, int64_t pts_start, int64_t dts_start) {
// = &fmtCtx->streams[pkt->stream_index]->time_base;
av_log(fmtCtx,
AV_LOG_INFO,
"pts:%s dts:%s pts_diff:%lld dts_diff:%lld stream_idx:%d pts_start:%lld dts_start:%lld\n",
av_ts2str(pkt->pts),
av_ts2str(pkt->dts),
pkt->pts - pts_start,
pkt->dts - dts_start,
pkt->stream_index,
pts_start,
dts_start);
}
void AVCrop() {
int ret = -1;
int idx = -1;
int stream_idx = 0;
int i = 0;
//1. 处理一些参数；
char* src;
char* dst;
double starttime = 0;
double endtime = 0;
int* stream_map = NULL;
int64_t* dts_start_time = NULL;
int64_t* pts_start_time = NULL;
AVFormatContext* pFmtCtx = NULL;
AVFormatContext* oFmtCtx = NULL;
const AVOutputFormat* outFmt = NULL;
AVPacket pkt;
av_log_set_level(AV_LOG_DEBUG);
/*src = "F:\\test_data\\jiuzhe_summer.mp4";
dst = "F:\\test_data\\crop_jiuzhe_summer.mp4";*/
src = "F:\\test_data\\jiuzhe_summer.mp4";
dst = "F:\\test_data\\crop_jiuzhe_summer2.mp4";
starttime = atof("60"); // 字符串类型转double
endtime = atof("120");
//2. 打开多媒体文件
if ((ret = avformat_open_input(&pFmtCtx, src, NULL, NULL)) < 0) {
av_log(NULL, AV_LOG_ERROR, "%s\n", av_err2str(ret));
exit(-1);
}
//4. 打开目的文件的上下文
avformat_alloc_output_context2(&oFmtCtx, NULL, NULL, dst);
if (!oFmtCtx) {
av_log(NULL, AV_LOG_ERROR, "NO MEMORY!\n");
goto _ERROR;
}
stream_map = av_calloc(pFmtCtx->nb_streams, sizeof(int));
if (!stream_map) {
av_log(NULL, AV_LOG_ERROR, "NO MEMORY!\n");
goto _ERROR;
}
for (i = 0; i < pFmtCtx->nb_streams; i++) {
AVStream* outStream = NULL;
AVStream* inStream = pFmtCtx->streams[i];
AVCodecParameters* inCodecPar = inStream->codecpar;
if (inCodecPar->codec_type != AVMEDIA_TYPE_AUDIO &&
inCodecPar->codec_type != AVMEDIA_TYPE_VIDEO &&
inCodecPar->codec_type != AVMEDIA_TYPE_SUBTITLE) {
stream_map[i] = -1;
continue;
}
stream_map[i] = stream_idx++;
//5. 为目的文件，创建一个新的视频流
outStream = avformat_new_stream(oFmtCtx, NULL);
if (!outStream) {
av_log(oFmtCtx, AV_LOG_ERROR, "NO MEMORY!\n");
goto _ERROR;
}
avcodec_parameters_copy(outStream->codecpar, inStream->codecpar);
outStream->codecpar->codec_tag = 0;
}
//绑定
ret = avio_open2(&oFmtCtx->pb, dst, AVIO_FLAG_WRITE, NULL, NULL);
if (ret < 0) {
av_log(oFmtCtx, AV_LOG_ERROR, "%s", av_err2str(ret));
goto _ERROR;
}
//7. 写多媒体文件头到目的文件
ret = avformat_write_header(oFmtCtx, NULL);
if (ret < 0) {
av_log(oFmtCtx, AV_LOG_ERROR, "%s", av_err2str(ret));
goto _ERROR;
}
//
/*
* 告诉ffmpeg什么时候截取时间.
* 有两种情况：
* 1.对于音频来说
* 音频是一个流 数据，你说截取到哪里就从哪里截取。
* 2.对于视频来说
* 视频是由I、B、P帧组成，在指定时间截取的帧可能是一个B帧或P帧。由于没有I帧，会导致之前的画面的显示不清楚
* 所以要从前或者从后找到I帧，再进行解析。AVSEEK_FLAG_BACKWARD就是向后找。
*/
ret = av_seek_frame(pFmtCtx, -1, starttime * AV_TIME_BASE, AVSEEK_FLAG_BACKWARD);
if (ret < 0) {
av_log(oFmtCtx, AV_LOG_ERROR, "%s", av_err2str(ret));
goto _ERROR;
}
// 创建数组，用来记录每一路流起始的pts和dts
dts_start_time = av_calloc(pFmtCtx->nb_streams, sizeof(int64_t));
for (int t = 0; t < pFmtCtx->nb_streams; t++) {
dts_start_time[t] = -1;
}
pts_start_time = av_calloc(pFmtCtx->nb_streams, sizeof(int64_t));
for (int t = 0; t < pFmtCtx->nb_streams; t++) {
pts_start_time[t] = -1;
}
//8. 从源多媒体文件中读取音频/视频/字幕数据到目的文件中
while (av_read_frame(pFmtCtx, &pkt) >= 0) {
AVStream* inStream, * outStream;
// 该流之前没有记录过，记录它的起始dts和pts
if (dts_start_time[pkt.stream_index] == -1 && pkt.dts >= 0) {
dts_start_time[pkt.stream_index] = pkt.dts;
}
if (pts_start_time[pkt.stream_index] == -1 && pkt.pts >= 0) {
pts_start_time[pkt.stream_index] = pkt.pts;
}
inStream = pFmtCtx->streams[pkt.stream_index];
// 如果大于结束时间，则截取完毕
if (av_q2d(inStream->time_base) * pkt.pts > endtime) {
av_log(oFmtCtx, AV_LOG_INFO, "success!\n");
break;
}
// 该数据包不是需要的多媒体流，不要 继续循环读取
if (stream_map[pkt.stream_index] < 0) {
av_packet_unref(&pkt);
continue;
}
printf("pkt.pts=%lld, pkt.dts=%lld\n", pkt.pts, pkt.dts);
//log_packet(pFmtCtx, &pkt, pts_start_time[pkt.stream_index], dts_start_time[pkt.stream_index]);
// 截取的dts和pts现在是相对于起始时间的，所以要减去每一路的起始时间
pkt.pts = pkt.pts - pts_start_time[pkt.stream_index];
pkt.dts = pkt.dts - dts_start_time[pkt.stream_index];
// 对于视频来说，视频的pts一定大于等于dts。
if (pkt.dts > pkt.pts) {
pkt.pts = pkt.dts;
}
pkt.stream_index = stream_map[pkt.stream_index];
outStream = oFmtCtx->streams[pkt.stream_index];
av_packet_rescale_ts(&pkt, inStream->time_base, outStream->time_base);
pkt.pos = -1;
av_interleaved_write_frame(oFmtCtx, &pkt);
av_packet_unref(&pkt);
}
//9. 写多媒体文件尾到文件中
av_write_trailer(oFmtCtx);
//10. 将申请的资源释放掉
_ERROR:
if (pFmtCtx) {
avformat_close_input(&pFmtCtx);
pFmtCtx = NULL;
}
if (oFmtCtx->pb) {
avio_close(oFmtCtx->pb);
}
if (oFmtCtx) {
avformat_free_context(oFmtCtx);
oFmtCtx = NULL;
}
if (stream_map) {
av_free(stream_map);
}
if (dts_start_time) {
av_free(dts_start_time);
}
if (pts_start_time) {
av_free(pts_start_time);
}
printf("hello, world!\n");
}