我正在尝试录制电脑的屏幕并使用h264编码器对录制的帧进行编码 并将它们包装成一个mp4容器。我想这样做，因为这个超级用户链接https://superuser.com/questions/300897/what-is-a-codec-e-g-divx-and-how-does-it-differ-from-a-file-format-e-g-mp/300997#300997表明它允许在输出文件的大小和质量之间进行良好的权衡。
我正在处理的应用程序应允许用户录制几个小时的视频，并使最小输出文件大小具有良好的质量。
到目前为止我编写的代码允许我使用mpeg1video编码器记录和保存.mpg（容器）文件
运行：
ffmpeg -i test.mpg
输出文件上的给出以下输出：
[mpegvideo @ 028c7400] Estimating duration from bitrate, this may be inaccurate
Input #0, mpegvideo, from 'test.mpg':
Duration: 00:00:00.29, bitrate: 104857 kb/s
Stream #0:0: Video: mpeg1video, yuv420p(tv), 1366x768 [SAR 1:1 DAR 683:384], 104857 kb/s, 25 fps, 25 tbr, 1200k tbn, 25 tbc
我的输出有这些设置：
const char * filename="test.mpg";
int codec_id= AV_CODEC_ID_MPEG1VIDEO;
AVCodec *codec11;
AVCodecContext *outContext= NULL;
int got_output;
FILE *f;
AVPacket pkt;
uint8_t endcode[] = { 0, 0, 1, 0xb7 };
/* put sample parameters */
outContext->bit_rate = 400000;
/* resolution must be a multiple of two */
outContext->width=pCodecCtx->width;
outContext->height=pCodecCtx->height;
/* frames per second */
outContext->time_base.num=1;
outContext->time_base.den=25;
/* emit one intra frame every ten frames
* check frame pict_type before passing frame
* to encoder, if frame->pict_type is AV_PICTURE_TYPE_I
* then gop_size is ignored and the output of encoder
* will always be I frame irrespective to gop_size
*/
outContext->gop_size = 10;
outContext->max_b_frames = 1;
outContext->pix_fmt = AV_PIX_FMT_YUV420P;
当我将int codec_id = AV_CODEC_ID_MPEG1VIDEO更改为int codec_id = AV_CODEC_ID_H264时，我得到一个不与vlc一起播放的文件。
我读过写
uint8_t endcode[] = { 0, 0, 1, 0xb7 };
文件末尾的数组使您的文件成为合法的mpeg文件。它的编写方式如下：
fwrite(endcode, 1, sizeof(endcode), f);
fclose(f);
在我的代码中。将编码器更改为AV_CODEC_ID_H264时，我应该做同样的事情吗？
我正在使用这样的gdi输入捕获：
AVDictionary* options = NULL;
//Set some options
//grabbing frame rate
av_dict_set(&options,"framerate","30",0);
AVInputFormat *ifmt=av_find_input_format("gdigrab");
if(avformat_open_input(&pFormatCtx,"desktop",ifmt,&options)!=0){
printf("Couldn't open input stream.\n");
return -1;
}
我希望能够修改我的抓取速率以优化outptut文件大小 但是当我把它改为20时，我得到一个播放速度如此之快的视频。怎么做 我得到的视频以正常速度播放，帧速率为20 fps或者任意帧 较低的帧速率值？
录制时我在标准错误输出上得到以下输出：
[gdigrab @ 00cdb8e0] Capturing whole desktop as 1366x768x32 at (0,0)
Input #0, gdigrab, from '(null)':
Duration: N/A, start: 1420718663.655713, bitrate: 1006131 kb/s
Stream #0:0: Video: bmp, bgra, 1366x768, 1006131 kb/s, 29.97 tbr, 1000k tbn, 29.97 tbc
[swscaler @ 00d24120] Warning: data is not aligned! This can lead to a speedloss
[mpeg1video @ 00cdd160] AVFrame.format is not set
[mpeg1video @ 00cdd160] AVFrame.width or height is not set
[mpeg1video @ 00cdd160] AVFrame.format is not set
[mpeg1video @ 00cdd160] AVFrame.width or height is not set
[mpeg1video @ 00cdd160] AVFrame.format is not set
如何在代码中消除此错误？
总结： 1）如何编码包含在mp4容器中的h264视频？
2）如何以较低的帧速率捕捉并仍然播放 正常速度的编码视频？
3）如何设置格式（以及格式 - 取决于编解码器？） 和我写的帧的宽度和高度信息？
我正在使用的代码如下所示
extern "C"
{
#include "libavcodec/avcodec.h"
#include "libavformat/avformat.h"
#include "libswscale/swscale.h"
#include "libavdevice/avdevice.h"
#include <libavutil/opt.h>
#include <libavutil/channel_layout.h>
#include <libavutil/common.h>
#include <libavutil/imgutils.h>
#include <libavutil/mathematics.h>
#include <libavutil/samplefmt.h>
//SDL
#include "SDL.h"
#include "SDL_thread.h"
}
//Output YUV420P
#define OUTPUT_YUV420P 0
//'1' Use Dshow
//'0' Use GDIgrab
#define USE_DSHOW 0
int main(int argc, char* argv[])
{
//1.WE HAVE THE FORMAT CONTEXT
//THIS IS FROM THE DESKTOP GRAB STREAM.
AVFormatContext *pFormatCtx;
int i, videoindex;
AVCodecContext *pCodecCtx;
AVCodec *pCodec;
av_register_all();
avformat_network_init();
//ASSIGN STH TO THE FORMAT CONTEXT.
pFormatCtx = avformat_alloc_context();
//Register Device
avdevice_register_all();
//Windows
#ifdef _WIN32
#if USE_DSHOW
//Use dshow
//
//Need to Install screen-capture-recorder
//screen-capture-recorder
//Website: http://sourceforge.net/projects/screencapturer/
//
AVInputFormat *ifmt=av_find_input_format("dshow");
//if(avformat_open_input(&pFormatCtx,"video=screen-capture-recorder",ifmt,NULL)!=0){
if(avformat_open_input(&pFormatCtx,"video=UScreenCapture",ifmt,NULL)!=0){
printf("Couldn't open input stream.\n");
return -1;
}
#else
//Use gdigrab
AVDictionary* options = NULL;
//Set some options
//grabbing frame rate
av_dict_set(&options,"framerate","30",0);
//The distance from the left edge of the screen or desktop
//av_dict_set(&options,"offset_x","20",0);
//The distance from the top edge of the screen or desktop
//av_dict_set(&options,"offset_y","40",0);
//Video frame size. The default is to capture the full screen
//av_dict_set(&options,"video_size","640x480",0);
AVInputFormat *ifmt=av_find_input_format("gdigrab");
if(avformat_open_input(&pFormatCtx,"desktop",ifmt,&options)!=0){
printf("Couldn't open input stream.\n");
return -1;
}
#endif
#endif//FOR THE WIN32 THING.
if(avformat_find_stream_info(pFormatCtx,NULL)<0)
{
printf("Couldn't find stream information.\n");
return -1;
}
videoindex=-1;
for(i=0; i<pFormatCtx->nb_streams; i++)
if(pFormatCtx->streams[i]->codec->codec_type
==AVMEDIA_TYPE_VIDEO)
{
videoindex=i;
break;
}
if(videoindex==-1)
{
printf("Didn't find a video stream.\n");
return -1;
}
pCodecCtx=pFormatCtx->streams[videoindex]->codec;
pCodec=avcodec_find_decoder(pCodecCtx->codec_id);
if(pCodec==NULL)
{
printf("Codec not found.\n");
return -1;
}
if(avcodec_open2(pCodecCtx, pCodec,NULL)<0)
{
printf("Could not open codec.\n");
return -1;
}
//THIS IS WHERE YOU CONTROL THE FORMAT(THROUGH FRAMES).
AVFrame *pFrame;
pFrame=av_frame_alloc();
int ret, got_picture;
AVPacket *packet=(AVPacket *)av_malloc(sizeof(AVPacket));
//TRY TO INIT THE PACKET HERE
av_init_packet(packet);
//Output Information-----------------------------
printf("File Information---------------------\n");
av_dump_format(pFormatCtx,0,NULL,0);
printf("-------------------------------------------------\n");
//<<--FOR WRITING MPG FILES
//<<--START:PREPARE TO WRITE YOUR MPG FILE.
const char * filename="test.mpg";
int codec_id= AV_CODEC_ID_MPEG1VIDEO;
AVCodec *codec11;
AVCodecContext *outContext= NULL;
int got_output;
FILE *f;
AVPacket pkt;
uint8_t endcode[] = { 0, 0, 1, 0xb7 };
printf("Encode video file %s\n", filename);
/* find the mpeg1 video encoder */
codec11 = avcodec_find_encoder((AVCodecID)codec_id);
if (!codec11) {
fprintf(stderr, "Codec not found\n");
exit(1);
}
outContext = avcodec_alloc_context3(codec11);
if (!outContext) {
fprintf(stderr, "Could not allocate video codec context\n");
exit(1);
}
/* put sample parameters */
outContext->bit_rate = 400000;
/* resolution must be a multiple of two */
outContext->width=pCodecCtx->width;
outContext->height=pCodecCtx->height;
/* frames per second */
outContext->time_base.num=1;
outContext->time_base.den=25;
/* emit one intra frame every ten frames
* check frame pict_type before passing frame
* to encoder, if frame->pict_type is AV_PICTURE_TYPE_I
* then gop_size is ignored and the output of encoder
* will always be I frame irrespective to gop_size
*/
outContext->gop_size = 10;
outContext->max_b_frames = 1;
outContext->pix_fmt = AV_PIX_FMT_YUV420P;
if (codec_id == AV_CODEC_ID_H264)
av_opt_set(outContext->priv_data, "preset", "slow", 0);
/* open it */
if (avcodec_open2(outContext, codec11, NULL) < 0) {
fprintf(stderr, "Could not open codec\n");
exit(1);
}
f = fopen(filename, "wb");
if (!f) {
fprintf(stderr, "Could not open %s\n", filename);
exit(1);
}
AVFrame *outframe = av_frame_alloc();
int nbytes = avpicture_get_size(outContext->pix_fmt,
outContext->width,
outContext->height);
uint8_t* outbuffer = (uint8_t*)av_malloc(nbytes);
//ASSOCIATE THE FRAME TO THE ALLOCATED BUFFER.
avpicture_fill((AVPicture*)outframe, outbuffer,
AV_PIX_FMT_YUV420P,
outContext->width, outContext->height);
SwsContext* swsCtx_ ;
swsCtx_= sws_getContext(pCodecCtx->width,
pCodecCtx->height,
pCodecCtx->pix_fmt,
outContext->width, outContext->height,
outContext->pix_fmt,
SWS_BICUBIC, NULL, NULL, NULL);
//HERE WE START PULLING PACKETS FROM THE SPECIFIED FORMAT CONTEXT.
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.\n");
return -1;
}
if(got_picture)
{
sws_scale(swsCtx_, pFrame->data, pFrame->linesize,
0, pCodecCtx->height, outframe->data,
outframe->linesize);
av_init_packet(&pkt);
pkt.data = NULL; // packet data will be allocated by the encoder
pkt.size = 0;
ret = avcodec_encode_video2(outContext, &pkt, outframe, &got_output);
if (ret < 0) {
fprintf(stderr, "Error encoding frame\n");
exit(1);
}
if (got_output) {
printf("Write frame %3d (size=%5d)\n", i, pkt.size);
fwrite(pkt.data, 1, pkt.size, f);
av_free_packet(&pkt);
}
}
}
av_free_packet(packet);
}//THE LOOP TO PULL PACKETS FROM THE FORMAT CONTEXT ENDS HERE.
//
/* get the delayed frames */
for (got_output = 1; got_output; i++) {
//fflush(stdout);
ret = avcodec_encode_video2(outContext, &pkt, NULL, &got_output);
if (ret < 0) {
fprintf(stderr, "Error encoding frame\n");
exit(1);
}
if (got_output) {
printf("Write frame %3d (size=%5d)\n", i, pkt.size);
fwrite(pkt.data, 1, pkt.size, f);
av_free_packet(&pkt);
}
}
/* add sequence end code to have a real mpeg file */
fwrite(endcode, 1, sizeof(endcode), f);
fclose(f);
avcodec_close(outContext);
av_free(outContext);
//av_freep(&frame->data[0]);
//av_frame_free(&frame);
//THIS WAS ADDED LATER
av_free(outbuffer);
avcodec_close(pCodecCtx);
avformat_close_input(&pFormatCtx);
return 0;
}
感谢您的时间。
答案 0 :(得分：1)
可以以正常速度录制和播放。
AVDictionary* options = NULL;
av_dict_set( &options, "preset", "veryslow", 0 );
可以使用以下预设：
{
"ultrafast",
"superfast",
"veryfast",
"faster",
"fast",
"medium",
"slow",
"slower",
"veryslow",
"placebo", 0
}
设置合适的预设。