ffmpegプレーヤーは詳細解の枠組み構築過程を実現する。

// tutorial01.c
// Code based on a tutorial by Martin Bohme ([email protected])
// Tested on Gentoo, CVS version 5/01/07 compiled with GCC 4.1.1
// With updates from https://github.com/chelyaev/ffmpeg-tutorial
// Updates tested on:
// LAVC 54.59.100, LAVF 54.29.104, LSWS 2.1.101 
// on GCC 4.7.2 in Debian February 2015
//
// Updates tested on:
// Mac OS X 10.11.6
// Apple LLVM version 8.0.0 (clang-800.0.38)
//
// A small sample program that shows how to use libavformat and libavcodec to read video from a file.
//
// Use
//
// $ gcc -o tutorial01 tutorial01.c -lavutil -lavformat -lavcodec -lswscale -lz -lm
//
// to build (assuming libavutil/libavformat/libavcodec/libswscale are correctly installed your system).
//
// Run using
//
// $ tutorial01 myvideofile.mpg
//
// to write the first five frames from "myvideofile.mpg" to disk in PPM format.

// comment by [email protected]

#include <libavcodec/avcodec.h>
#include <libavformat/avformat.h>
#include <libswscale/swscale.h>
#include <libavutil/imgutils.h>

#include <stdio.h>

// compatibility with newer API
#if LIBAVCODEC_VERSION_INT < AV_VERSION_INT(55,28,1)
#define av_frame_alloc avcodec_alloc_frame
#define av_frame_free avcodec_free_frame
#endif

//  PPM  
void SaveFrame(AVFrame *pFrame, int width, int height, int iFrame) {
	FILE *pFile;//      
	char szFilename[32];//       
	
	// Open file，    
	sprintf(szFilename, "frame%d.ppm", iFrame);//        
	pFile = fopen(szFilename, "wb");//      
	if (pFile == NULL) {//            
		return;
	}
 
	// Write header indicated how wide & tall the image is，           
	fprintf(pFile, "P6
%d %d
255
", width, height);
 
	// Write pixel data，write the file one line a time,        RGB24   
	int y;
	for (y = 0; y < height; y++) {
		fwrite(pFrame->data[0]+y*pFrame->linesize[0], 1, width*3, pFile);
	}
 
	// Close file，    
	fclose(pFile);
}

int main(int argc, char *argv[]) {
/*--------------    -------------*/
	// Initalizing these to NULL prevents segfaults!
	AVFormatContext *pFormatCtx = NULL;//                   
	AVCodecContext *pCodecCtxOrig = NULL;//        ，          、  、            
	AVCodecContext *pCodecCtx = NULL;//        ,  PPM    
	AVCodec *pCodec = NULL;//            ，            ，                   
	AVPacket packet;//                  ,         packet   
	AVFrame *pFrame = NULL;//           ，     、      、     ，QP ，        
	AVFrame *pFrameRGB = NULL;//    24-bit RGB PPM    
	struct SwsContext *sws_ctx = NULL;//           

	int numBytes;//RGB24      
	uint8_t *buffer = NULL;//          
	int i,videoStream;//    ，       
	int frameFinished;//          

/*-------------     ------------*/
	if (argc<2) {//            
		printf("Please provide a movie file
");
		return -1;
	}

	// Register all available formats and codecs，    ffmpeg             
	av_register_all();

	/*-----------------------
	 * Open video file，      ，      ，                    pFormatCtx 
	 * read the file header and stores information about the file format in the AVFormatContext structure 
	 * The last three arguments are used to specify the file format, buffer size, and format options
	 * but by setting this to NULL or 0, libavformat will auto-detect these
	 -----------------------*/
	if (avformat_open_input(&pFormatCtx, argv[1], NULL, NULL) != 0) {
		return -1; // Couldn't open file.
	}

	/*-----------------------
	 *             ，    pFormatCtx->stream   
	 * check out & Retrieve the stream information in the file
	 * then populate pFormatCtx->stream with the proper information 
	 * pFormatCtx->streams is just an array of pointers, of size pFormatCtx->nb_streams
	 -----------------------*/
	if (avformat_find_stream_info(pFormatCtx, NULL) < 0) {
		return -1; // Couldn't find stream information.
	}

	// Dump information about file onto standard error，  pFormatCtx      
	av_dump_format(pFormatCtx, 0, argv[1], 0);

	// Find the first video stream.
	videoStream=-1;//           -1
	for (i=0;i<pFormatCtx->nb_streams;i++) {//               (   、   、    )
		if(pFormatCtx->streams[i]->codec->codec_type==AVMEDIA_TYPE_VIDEO) {//          
			videoStream = i;//             ，    -1
			break;//    
		}
	}
	if (videoStream==-1) {//            
		return -1; // Didn't find a video stream.
	}

	// Get a pointer to the codec context for the video stream，        pFormatCtx->streams               
	pCodecCtxOrig = pFormatCtx->streams[videoStream]->codec;
	/*-----------------------
	 * Find the decoder for the video stream，                      ，        (     )
	 * The stream's information about the codec is in what we call the "codec context.
	 * This contains all the information about the codec that the stream is using
	 -----------------------*/
	pCodec = avcodec_find_decoder(pCodecCtxOrig->codec_id);
	if (pCodec == NULL) {//         
		fprintf(stderr, "Unsupported codec!
");
		return -1; // Codec not found.
	}

	// Copy context，           ，             
	pCodecCtx = avcodec_alloc_context3(pCodec);//  AVCodecContext     pCodecCtx
	if (avcodec_copy_context(pCodecCtx, pCodecCtxOrig) != 0) {//           
		fprintf(stderr, "Couldn't copy codec context");
		return -1; // Error copying codec context.
	}

	// Open codec，     
	if (avcodec_open2(pCodecCtx, pCodec, NULL) < 0) {
		return -1; // Could not open codec.
	}

	// Allocate video frame，                        (                   )
	pFrame = av_frame_alloc();

	// Allocate an AVFrame structure，   PPM                  
	pFrameRGB = av_frame_alloc();
	if (pFrameRGB == NULL) {//           
		return -1;
	}

	// Determine required buffer size and allocate buffer，                 
	numBytes = av_image_get_buffer_size(AV_PIX_FMT_RGB24, pCodecCtx->width, pCodecCtx->height, 1);
	buffer = (uint8_t *) av_malloc(numBytes * sizeof(uint8_t));//     RGB24        

	// Assign appropriate parts of buffer to image planes in pFrameRGB Note that pFrameRGB is an AVFrame, but AVFrame is a superset of AVPicture
	//  AVFrame        ， out_buffer    pFrameYUV->data     
	av_image_fill_arrays(pFrameRGB->data, pFrameRGB->linesize, buffer, AV_PIX_FMT_RGB24, pCodecCtx->width, pCodecCtx->height, 1);
 
	// Initialize SWS context for software scaling，           AV_PIX_FMT_RGB24
	sws_ctx = sws_getContext(pCodecCtx->width, pCodecCtx->height, pCodecCtx->pix_fmt, pCodecCtx->width, pCodecCtx->height, AV_PIX_FMT_RGB24, SWS_BILINEAR, NULL, NULL, NULL);

/*--------------    -------------*/
	i = 0;// Read frames(2 packet) and save first five frames to disk,
	/*-----------------------
	 * read in a packet and store it in the AVPacket struct
	 * ffmpeg allocates the internal data for us,which is pointed to by packet.data
	 * this is freed by the av_free_packet()
	 -----------------------*/
	while (av_read_frame(pFormatCtx, &packet) >= 0) {//                         ，    AVPacket     
		// Is this a packet from the video stream，       
		if (packet.stream_index == videoStream) {
		 /*-----------------------
	 		 * Decode video frame，         ，  frameFinished   true
			 *            packet           frame，        packet  
	 		 * avcodec_decode_video2()             frameFinished  
			 * Technically a packet can contain partial frames or other bits of data
			 * ffmpeg's parser ensures that the packets we get contain either complete or multiple frames
			 * convert the packet to a frame for us and set frameFinisned for us when we have the next frame
	 	 	 -----------------------*/
			avcodec_decode_video2(pCodecCtx, pFrame, &frameFinished, &packet);

			// Did we get a video frame，             
			if (frameFinished) {
				// Convert the image from its native format to RGB，//          RGB24  
				sws_scale(sws_ctx, (uint8_t const * const *) pFrame->data, pFrame->linesize, 0, pCodecCtx->height, pFrameRGB->data, pFrameRGB->linesize);

				if (++i <= 5) {// Save the frame to disk，  5         
					SaveFrame(pFrameRGB, pCodecCtx->width, pCodecCtx->height, i);
				}
			}
		}
		// Free the packet that was allocated by av_read_frame，  AVPacket           
		av_packet_unref(&packet);
	}

/*--------------    -------------*/
	// Free the RGB image buffer
	av_free(buffer);
	av_frame_free(&pFrameRGB);

	// Free the YUV frame.
	av_frame_free(&pFrame);

	// Close the codecs.
	avcodec_close(pCodecCtx);
	avcodec_close(pCodecCtxOrig);

	// Close the video file.
	avformat_close_input(&pFormatCtx);

	return 0;
}