./configure --prefix=/usr/local/3rdparty/sdl

make 
make install

#SDL ENVIRONMENT
export C_INCLUDE_PATH=/usr/local/3rdparty/sdl/include/SDL:$C_INCLUDE_PATH
export LD_LIBRARY_PATH=/usr/local/3rdparty/sdl/lib:$LD_LIBRARY_PATH
export PKG_CONFIG_PATH=/usr/local/3rdparty/sdl/lib/pkgconfig:$PKG_CONFIG_PATH

tutorial01: tutorial01.c
	gcc -o tutorial01 -g3 tutorial01.c -I${FFMPEG_INCLUDE} -I${SDL_INCLUDE} \
	-L${FFMPEG_LIB} -lavutil -lavformat -lavcodec -lswscale -lswresample -lz -lm \
	`sdl-config --cflags --libs`

clean:
	rm -rf tutorial01
	rm -rf *.ppm

./tutorial01 rtmp://58.200.131.2:1935/livetv/hunantv

// tutorial01.c
// Code based on a tutorial by Martin Bohme (boehme@inb.uni-luebeckREMOVETHIS.de)
// 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 breakpointlab@outlook.com

#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

//保存PPM文件
void SaveFrame(AVFrame *pFrame, int width, int height, int iFrame) {
	FILE *pFile;//定義文件對象
	char szFilename[32];//定義輸出文件名
	
	// Open file，打開文件
	sprintf(szFilename, "frame%d.ppm", iFrame);//格式化輸出文件名
	pFile = fopen(szFilename, "wb");//打開輸出文件
	if (pFile == NULL) {//檢查輸出文件是否打開成功
		return;
	}
 
	// Write header indicated how wide & tall the image is，向輸出文件中寫入文件頭
	fprintf(pFile, "P6\n%d %d\n255\n", width, height);
 
	// Write pixel data，write the file one line a time,一次一行循環(huán)寫入RGB24像素值
	int y;
	for (y = 0; y < height; y++) {
		fwrite(pFrame->data[0]+y*pFrame->linesize[0], 1, width*3, pFile);
	}
 
	// Close file，關閉文件
	fclose(pFile);
}

int main(int argc, char *argv[]) {
/*--------------參數定義-------------*/
	// Initalizing these to NULL prevents segfaults!
	AVFormatContext *pFormatCtx = NULL;//保存文件容器封裝信息及碼流參數的結構體
	AVCodecContext *pCodecCtxOrig = NULL;//解碼器上下文對象，解碼器依賴的相關環(huán)境、狀態(tài)、資源以及參數集的接口指針
	AVCodecContext *pCodecCtx = NULL;//編碼器上下文對象,用于PPM文件輸出
	AVCodec *pCodec = NULL;//保存編解碼器信息的結構體，提供編碼與解碼的公共接口，可以看作是編碼器與解碼器的一個全局變量
	AVPacket packet;//負責保存壓縮編碼數據相關信息的結構體,每幀圖像由一到多個packet包組成
	AVFrame *pFrame = NULL;//保存音視頻解碼后的數據，如狀態(tài)信息、編解碼器信息、宏塊類型表，QP表，運動矢量表等數據
	AVFrame *pFrameRGB = NULL;//保存輸出24-bit RGB的PPM文件數據
	struct SwsContext *sws_ctx = NULL;//描述轉換器參數的結構體

	int numBytes;//RGB24格式數據長度
	uint8_t *buffer = NULL;//解碼數據輸出緩存指針
	int i,videoStream;//循環(huán)變量，視頻流類型標號
	int frameFinished;//解碼操作是否成功標識

/*-------------參數初始化------------*/
	if (argc<2) {//檢查輸入參數個數是否正確
		printf("Please provide a movie file\n");
		return -1;
	}

	// Register all available formats and codecs，注冊所有ffmpeg支持的多媒體格式及編解碼器
	av_register_all();

	/*-----------------------
	 * Open video file，打開視頻文件，讀文件頭內容，取得文件容器的封裝信息及碼流參數并存儲在pFormatCtx中
	 * 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.
	}

	/*-----------------------
	 * 取得文件中保存的碼流信息，并填充到pFormatCtx->stream 字段
	 * 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，打印pFormatCtx中的碼流信息
	av_dump_format(pFormatCtx, 0, argv[1], 0);

	// Find the first video stream.
	videoStream=-1;//視頻流類型標號初始化為-1
	for (i=0;i<pFormatCtx->nb_streams;i++) {//遍歷文件中包含的所有流媒體類型(視頻流、音頻流、字幕流等)
		if(pFormatCtx->streams[i]->codec->codec_type==AVMEDIA_TYPE_VIDEO) {//若文件中包含有視頻流
			videoStream = i;//用視頻流類型的標號修改標識，使之不為-1
			break;//退出循環(huán)
		}
	}
	if (videoStream==-1) {//檢查文件中是否存在視頻流
		return -1; // Didn't find a video stream.
	}

	// Get a pointer to the codec context for the video stream，根據流類型標號從pFormatCtx->streams中取得視頻流對應的解碼器上下文
	pCodecCtxOrig = pFormatCtx->streams[videoStream]->codec;
	/*-----------------------
	 * Find the decoder for the video stream，根據視頻流對應的解碼器上下文查找對應的解碼器，返回對應的解碼器(信息結構體)
	 * 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) {//檢查解碼器是否匹配
		fprintf(stderr, "Unsupported codec!\n");
		return -1; // Codec not found.
	}

	// Copy context，復制編解碼器上下文對象，用于保存從視頻流中抽取的幀
	pCodecCtx = avcodec_alloc_context3(pCodec);//創(chuàng)建AVCodecContext結構體對象pCodecCtx
	if (avcodec_copy_context(pCodecCtx, pCodecCtxOrig) != 0) {//復制編解碼器上下文對象
		fprintf(stderr, "Couldn't copy codec context");
		return -1; // Error copying codec context.
	}

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

	// Allocate video frame，為解碼后的視頻信息結構體分配空間并完成初始化操作(結構體中的圖像緩存按照下面兩步手動安裝)
	pFrame = av_frame_alloc();

	// Allocate an AVFrame structure，為轉換PPM文件的結構體分配空間并完成初始化操作
	pFrameRGB = av_frame_alloc();
	if (pFrameRGB == NULL) {//檢查初始化操作是否成功
		return -1;
	}

	// Determine required buffer size and allocate buffer，根據像素格式及圖像尺寸計算內存大小
	numBytes = av_image_get_buffer_size(AV_PIX_FMT_RGB24, pCodecCtx->width, pCodecCtx->height, 1);
	buffer = (uint8_t *) av_malloc(numBytes * sizeof(uint8_t));//為轉換后的RGB24圖像配置緩存空間

	// Assign appropriate parts of buffer to image planes in pFrameRGB Note that pFrameRGB is an AVFrame, but AVFrame is a superset of AVPicture
	// 為AVFrame對象安裝圖像緩存，將out_buffer緩存掛到pFrameYUV->data指針結構上
	av_image_fill_arrays(pFrameRGB->data, pFrameRGB->linesize, buffer, AV_PIX_FMT_RGB24, pCodecCtx->width, pCodecCtx->height, 1);
 
	// Initialize SWS context for software scaling，設置圖像轉換像素格式為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);

/*--------------循環(huán)解碼-------------*/
	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) {//從視頻文件或網絡流媒體中依次讀取每個圖像編碼數據包，并存儲在AVPacket數據結構中
		// Is this a packet from the video stream，檢查數據包類型
		if (packet.stream_index == videoStream) {
		 /*-----------------------
	 		 * Decode video frame，解碼完整的一幀數據，并將frameFinished設置為true
			 * 可能無法通過只解碼一個packet就獲得一個完整的視頻幀frame，可能需要讀取多個packet才行
	 		 * avcodec_decode_video2()會在解碼到完整的一幀時設置frameFinished為真
			 * 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，檢查是否解碼出完整一幀圖像
			if (frameFinished) {
				// Convert the image from its native format to RGB，//將解碼后的圖像轉換為RGB24格式
				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，將前5幀圖像存儲到磁盤上
					SaveFrame(pFrameRGB, pCodecCtx->width, pCodecCtx->height, i);
				}
			}
		}
		// Free the packet that was allocated by av_read_frame，釋放AVPacket數據結構中編碼數據指針
		av_packet_unref(&packet);
	}

/*--------------參數撤銷-------------*/
	// 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;
}