// 創(chuàng)建SDL窗口，并指定圖像尺寸
screen = SDL_SetVideoMode(pCodecCtx->width, pCodecCtx->height, 24, 0);

// 創(chuàng)建畫布對象
bmp = SDL_CreateYUVOverlay(pCodecCtx->width, pCodecCtx->height, SDL_YV12_OVERLAY, screen);

sws_scale(sws_ctx, (uint8_t const * const *)pFrame->data, pFrame->linesize, 0, pCodecCtx->height, pict.data, pict.linesize);

//Unlocks a previously locked overlay. An overlay must be unlocked before it can be displayed
SDL_UnlockYUVOverlay(bmp);

SDL_DisplayYUVOverlay(bmp, &rect);//圖像渲染

tutorial02: tutorial02.c
	gcc -o tutorial02 -g3 tutorial02.c -I${FFMPEG_INCLUDE} -I${SDL_INCLUDE} \
	-L${FFMPEG_LIB} -lavutil -lavformat -lavcodec -lswscale -lswresample -lz -lm \
	`sdl-config --cflags --libs`

clean:
	rm -rf tutorial02

./tutorial02 rtmp://58.200.131.2:1935/livetv/hunantv

// tutorial02.c
// A pedagogical video player that will stream through every video frame as fast as it can.
//
// This tutorial was written by Stephen Dranger (dranger@gmail.com).
//
// Code based on FFplay, Copyright (c) 2003 Fabrice Bellard, 
// and a tutorial by Martin Bohme (boehme@inb.uni-luebeckREMOVETHIS.de)
// Tested on Gentoo, CVS version 5/01/07 compiled with GCC 4.1.1
//
// Updates tested on:
// Mac OS X 10.11.6
// Apple LLVM version 8.0.0 (clang-800.0.38)
//
// Use 
//
// $ gcc -o tutorial02 tutorial02.c -lavutil -lavformat -lavcodec -lswscale -lz -lm `sdl-config --cflags --libs`
//
// to build (assuming libavutil/libavformat/libavcodec/libswscale are correctly installed your system).
//
// Run using
//
// $ tutorial02 myvideofile.mpg
//
// to play the video stream on your screen.


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

#include <SDL.h>
#include <SDL_thread.h>

#ifdef __MINGW32__
#undef main // Prevents SDL from overriding main().
#endif

#include <stdio.h>

int main(int argc, char *argv[]) {
/*--------------參數(shù)定義-------------*/
	AVFormatContext *pFormatCtx = NULL;//保存文件容器封裝信息及碼流參數(shù)的結(jié)構(gòu)體
	AVCodecContext *pCodecCtx = NULL;//解碼器上下文對象，解碼器依賴的相關(guān)環(huán)境、狀態(tài)、資源以及參數(shù)集的接口指針
	AVCodec *pCodec = NULL;//保存編解碼器信息的結(jié)構(gòu)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(gè)全局變量
	AVPacket packet;//負(fù)責(zé)保存壓縮編碼數(shù)據(jù)相關(guān)信息的結(jié)構(gòu)體,每幀圖像由一到多個(gè)packet包組成
	AVFrame *pFrame = NULL;//保存音視頻解碼后的數(shù)據(jù)，如狀態(tài)信息、編解碼器信息、宏塊類型表，QP表，運(yùn)動矢量表等數(shù)據(jù)
	struct SwsContext *sws_ctx = NULL;//描述轉(zhuǎn)換器參數(shù)的結(jié)構(gòu)體
	AVDictionary *optionsDict = NULL;
 
 	SDL_Surface *screen = NULL;//SDL繪圖窗口，A structure that contains a collection of pixels used in software blitting
 	SDL_Overlay *bmp = NULL;//SDL畫布
 	SDL_Rect rect;//SDL矩形對象
 	SDL_Event event;//SDL事件對象

 	int i, videoStream;//循環(huán)變量，視頻流類型標(biāo)號
 	int frameFinished;//解碼操作是否成功標(biāo)識
 
/*-------------參數(shù)初始化------------*/
	if (argc<2) {//檢查輸入?yún)?shù)個(gè)數(shù)是否正確
		fprintf(stderr, "Usage: test <file>\n");
		exit(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(nèi)容，取得文件容器的封裝信息及碼流參數(shù)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(biāo)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(biāo)號修改標(biāo)識，使之不為-1
			break;
		}
	}
	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(jù)流類型標(biāo)號從pFormatCtx->streams中取得視頻流對應(yīng)的解碼器上下文
	pCodecCtx = pFormatCtx->streams[videoStream]->codec;

	/*-----------------------
	 * Find the decoder for the video stream，根據(jù)視頻流對應(yīng)的解碼器上下文查找對應(yīng)的解碼器，返回對應(yīng)的解碼器(信息結(jié)構(gòu)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(pCodecCtx->codec_id);
	if (pCodec == NULL) {//檢查解碼器是否匹配
		fprintf(stderr, "Unsupported codec!\n");
		return -1; // Codec not found.
	}

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

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

	// Initialize SWS context for software scaling，設(shè)置圖像轉(zhuǎn)換像素格式為AV_PIX_FMT_YUV420P
	sws_ctx = sws_getContext(pCodecCtx->width, pCodecCtx->height, pCodecCtx->pix_fmt, pCodecCtx->width, pCodecCtx->height, AV_PIX_FMT_YUV420P, SWS_BILINEAR, NULL, NULL, NULL);

	//SDL_Init initialize the Event Handling, File I/O, and Threading subsystems，初始化SDL 
	if (SDL_Init(SDL_INIT_VIDEO | SDL_INIT_AUDIO | SDL_INIT_TIMER)) {//initialize the video audio & timer subsystem 
		fprintf(stderr, "Could not initialize SDL - %s\n", SDL_GetError());//tell the library what features we're going to use
		exit(1);
	}

	// Make a screen to put our video,在SDL2.0中SDL_SetVideoMode及SDL_Overlay已經(jīng)棄用，改為SDL_CreateWindow及SDL_CreateRenderer創(chuàng)建窗口及著色器
#ifndef __DARWIN__
	screen = SDL_SetVideoMode(pCodecCtx->width, pCodecCtx->height, 24, 0);//創(chuàng)建SDL窗口，并指定圖像尺寸
#else
	screen = SDL_SetVideoMode(pCodecCtx->width, pCodecCtx->height, 24, 0);//創(chuàng)建SDL窗口，并指定圖像尺寸
#endif
	if (!screen) {//檢查SDL窗口是否創(chuàng)建成功
		fprintf(stderr, "SDL: could not set video mode - exiting\n");
		exit(1);
	}
 	SDL_WM_SetCaption(argv[1],0);//用輸入文件名設(shè)置SDL窗口標(biāo)題

	// Allocate a place to put our YUV image on that screen，創(chuàng)建畫布對象
	bmp = SDL_CreateYUVOverlay(pCodecCtx->width, pCodecCtx->height, SDL_YV12_OVERLAY, screen);

/*--------------循環(huán)解碼-------------*/
	i = 0;// Read frames 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) {//從文件中依次讀取每個(gè)圖像編碼數(shù)據(jù)包，并存儲在AVPacket數(shù)據(jù)結(jié)構(gòu)中
		// Is this a packet from the video stream，檢查數(shù)據(jù)包類型
		if (packet.stream_index == videoStream) {
		 /*-----------------------
	 		 * Decode video frame，解碼完整的一幀數(shù)據(jù)，并將frameFinished設(shè)置為true
			 * 可能無法通過只解碼一個(gè)packet就獲得一個(gè)完整的視頻幀frame，可能需要讀取多個(gè)packet才行
	 		 * avcodec_decode_video2()會在解碼到完整的一幀時(shí)設(shè)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) {
				SDL_LockYUVOverlay(bmp);//locks the overlay for direct access to pixel data，原子操作，保護(hù)像素緩沖區(qū)臨界資源

				AVFrame pict;//保存轉(zhuǎn)換為AV_PIX_FMT_YUV420P格式的視頻幀
				pict.data[0] = bmp->pixels[0];//將轉(zhuǎn)碼后的圖像與畫布的像素緩沖器關(guān)聯(lián)
				pict.data[1] = bmp->pixels[2];
				pict.data[2] = bmp->pixels[1];

				pict.linesize[0] = bmp->pitches[0];//將轉(zhuǎn)碼后的圖像掃描行長度與畫布像素緩沖區(qū)的掃描行長度相關(guān)聯(lián)
				pict.linesize[1] = bmp->pitches[2];//linesize-Size, in bytes, of the data for each picture/channel plane
				pict.linesize[2] = bmp->pitches[1];//For audio, only linesize[0] may be set

				// Convert the image into YUV format that SDL uses，將解碼后的圖像轉(zhuǎn)換為AV_PIX_FMT_YUV420P格式，并賦值到pict對象
				sws_scale(sws_ctx, (uint8_t const * const *)pFrame->data, pFrame->linesize, 0, pCodecCtx->height, pict.data, pict.linesize);

				SDL_UnlockYUVOverlay(bmp);//Unlocks a previously locked overlay. An overlay must be unlocked before it can be displayed

				//設(shè)置矩形顯示區(qū)域
				rect.x = 0;
				rect.y = 0;
				rect.w = pCodecCtx->width;
				rect.h = pCodecCtx->height;
				SDL_DisplayYUVOverlay(bmp, &rect);//圖像渲染
			}
		}
 
		// Free the packet that was allocated by av_read_frame，釋放AVPacket數(shù)據(jù)結(jié)構(gòu)中編碼數(shù)據(jù)指針
		av_packet_unref(&packet);
		
		/*-------------------------
		 * 在每次循環(huán)中從SDL后臺隊(duì)列取事件并填充到SDL_Event對象中
		 * SDL的事件系統(tǒng)使得你可以接收用戶的輸入，從而完成一些控制操作
		 * SDL_PollEvent() is the favored way of receiving system events 
		 * since it can be done from the main loop and does not suspend the main loop
		 * while waiting on an event to be posted
		 * poll for events right after we finish processing a packet
		 ------------------------*/
		SDL_PollEvent(&event);
		switch (event.type) {//檢查SDL事件對象
			case SDL_QUIT://退出事件
				printf("SDL_QUIT\n");
				SDL_Quit();//退出操作
				exit(0);//結(jié)束進(jìn)程
				break;
			default:
				break;
		}//end for switch
	}//end for while
/*--------------參數(shù)撤銷-------------*/ 
	// Free the YUV frame.
	av_free(pFrame);

	// Close the codec.
	avcodec_close(pCodecCtx);

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

	return 0;
}