计划写2个有关FFmpeg的libavdevice类库的例子。上篇文章记录了一个基于FFmpeg的Libavdevice类库读取摄像头数据的例子。本篇文章记录一个基于FFmpeg的Libavdevice类库录制屏幕的例子。本文程序录制当前桌面内容并且解码显示出来。有关解码显示方面的代码本文不再详述，可以参考文章：
《100行代码实现最简单的基于FFMPEG+SDL的视频播放器（SDL1.x）》

抓屏方法

上篇文章记录了libavdevice的使用方法，本文不再重复。在Windows系统使用libavdevice抓取屏幕数据有两种方法：gdigrab和dshow。下文分别介绍。
1. gdigrab
gdigrab是FFmpeg专门用于抓取Windows桌面的设备。非常适合用于屏幕录制。它通过不同的输入URL支持两种方式的抓取：
（1）“desktop”：抓取整张桌面。或者抓取桌面中的一个特定的区域。
（2）“title={窗口名称}”：抓取屏幕中特定的一个窗口（目前中文窗口还有乱码问题）。
gdigrab另外还支持一些参数，用于设定抓屏的位置：
offset_x：抓屏起始点横坐标。
offset_y：抓屏起始点纵坐标。
video_size：抓屏的大小。
framerate：抓屏的帧率。
参考的代码如下：

//Use gdigrab
 AVDictionary* options = NULL;
 //Set some options
 //grabbing frame rate
 //av_dict_set(&options,"framerate","5",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.（无法打开输入流）\n");
  return -1;
  }

AVInputFormat *ifmt=av_find_input_format("dshow");
 if(avformat_open_input(&pFormatCtx,"video=screen-capture-recorder",ifmt,NULL)!=0){
  printf("Couldn‘t open input stream.（无法打开输入流）\n");
  return -1;
 }

注：上述两种抓屏方法也可以直接使用ffmpeg.exe的命令行完成，可以参考文章：

FFmpeg获取DirectShow设备数据（摄像头，录屏）

在Linux下可以使用x11grab抓屏，在这里不再详细叙述。

代码

下面直接贴上程序代码：

/**
 * 最简单的基于FFmpeg的AVDevice例子（屏幕录制）
 * Simplest FFmpeg Device (Screen Capture)
 *
 * 雷霄骅 Lei Xiaohua
 * leixiaohua1020@126.com
 * 中国传媒大学/数字电视技术
 * Communication University of China / Digital TV Technology
 * http://blog.csdn.net/leixiaohua1020
 *
 * 本程序实现了屏幕录制功能。可以录制并播放桌面数据。是基于FFmpeg
 * 的libavdevice类库最简单的例子。通过该例子，可以学习FFmpeg中
 * libavdevice类库的使用方法。
 * 本程序在Windows下可以使用2种方式录制屏幕：
 *  1.gdigrab: Win32下的基于GDI的屏幕录制设备。
 *             抓取桌面的时候，输入URL为“desktop”。
 *  2.dshow: 使用Directshow。注意需要安装额外的软件screen-capture-recorder
 * 在Linux下则可以使用x11grab录制屏幕。
 *
 * This software capture screen of computer. It‘s the simplest example
 * about usage of FFmpeg‘s libavdevice Library. 
 * It‘s suiltable for the beginner of FFmpeg.
 * This software support 2 methods to capture screen in Microsoft Windows:
 *  1.gdigrab: Win32 GDI-based screen capture device.
 *             Input URL in avformat_open_input() is "desktop".
 *  2.dshow: Use Directshow. Need to install screen-capture-recorder.
 * It use x11grab to capture screen in Linux.
 */


#include "stdafx.h"

extern "C"
{
#include "libavcodec/avcodec.h"
#include "libavformat/avformat.h"
#include "libswscale/swscale.h"
#include "libavdevice/avdevice.h"
//SDL
#include "sdl/SDL.h"
#include "sdl/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[])
{

	AVFormatContext	*pFormatCtx;
	int				i, videoindex;
	AVCodecContext	*pCodecCtx;
	AVCodec			*pCodec;
	
	av_register_all();
	avformat_network_init();
	pFormatCtx = avformat_alloc_context();
	
	//Open File
	//char filepath[]="src01_480x272_22.h265";
	//avformat_open_input(&pFormatCtx,filepath,NULL,NULL)

	//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){
		printf("Couldn‘t open input stream.（无法打开输入流）\n");
		return -1;
	}
#else
	//Use gdigrab
	AVDictionary* options = NULL;
	//Set some options
	//grabbing frame rate
	//av_dict_set(&options,"framerate","5",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.（无法打开输入流）\n");
		return -1;
	}

#endif
#else
	//Linux
	AVDictionary* options = NULL;
	//Set some options
	//grabbing frame rate
	//av_dict_set(&options,"framerate","5",0);
	//Make the grabbed area follow the mouse
	//av_dict_set(&options,"follow_mouse","centered",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("x11grab");
	//Grab at position 10,20
	if(avformat_open_input(&pFormatCtx,":0.0+10,20",ifmt,&options)!=0){
		printf("Couldn‘t open input stream.（无法打开输入流）\n");
		return -1;
	}
#endif

	if(avformat_find_stream_info(pFormatCtx,NULL)<0)
	{
		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++) 
		if(pFormatCtx->streams[i]->codec->codec_type==AVMEDIA_TYPE_VIDEO)
		{
			videoindex=i;
			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=avcodec_alloc_frame();
	pFrameYUV=avcodec_alloc_frame();
	uint8_t *out_buffer=(uint8_t *)av_malloc(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;
	} 
	int screen_w=640,screen_h=360;
	const SDL_VideoInfo *vi = SDL_GetVideoInfo();
	//Half of the Desktop‘s width and height.
	screen_w = vi->current_w/2;
	screen_h = vi->current_h/2;
	SDL_Surface *screen; 
	screen = SDL_SetVideoMode(screen_w, screen_h, 0,0);

	if(!screen) {  
		printf("SDL: could not set video mode - exiting:%s\n",SDL_GetError());  
		return -1;
	}
	SDL_Overlay *bmp; 
	bmp = SDL_CreateYUVOverlay(pCodecCtx->width, pCodecCtx->height,SDL_YV12_OVERLAY, screen); 
	SDL_Rect rect;
	//SDL End------------------------
	int ret, got_picture;

	AVPacket *packet=(AVPacket *)av_malloc(sizeof(AVPacket));
	//Output Information-----------------------------
	printf("File Information（文件信息）---------------------\n");
	av_dump_format(pFormatCtx,0,NULL,0);
	printf("-------------------------------------------------\n");

#if OUTPUT_YUV420P 
    FILE *fp_yuv=fopen("output.yuv","wb+");  
#endif  

	struct SwsContext *img_convert_ctx;
	img_convert_ctx = sws_getContext(pCodecCtx->width, pCodecCtx->height, 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);
				
#if OUTPUT_YUV420P
				int y_size=pCodecCtx->width*pCodecCtx->height;  
				fwrite(pFrameYUV->data[0],1,y_size,fp_yuv);    //Y 
				fwrite(pFrameYUV->data[1],1,y_size/4,fp_yuv);  //U
				fwrite(pFrameYUV->data[2],1,y_size/4,fp_yuv);  //V
#endif
				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 = screen_w;    
				rect.h = screen_h;  
				SDL_DisplayYUVOverlay(bmp, &rect); 
				//Delay 40ms
				SDL_Delay(40);
			}
		}
		av_free_packet(packet);
	}
	sws_freeContext(img_convert_ctx);

#if OUTPUT_YUV420P 
    fclose(fp_yuv);
#endif 

	SDL_Quit();

	av_free(out_buffer);
	av_free(pFrameYUV);
	avcodec_close(pCodecCtx);
	avformat_close_input(&pFormatCtx);

	return 0;
}

结果

 

#define OUTPUT_YUV420P 0

注：
 本工程包含两个基于FFmpeg的libavdevice的例子：
 simplest_ffmpeg_grabdesktop：屏幕录制。
 simplest_ffmpeg_readcamera：读取摄像头。

最简单的基于FFmpeg的AVDevice例子（屏幕录制）