基于FFmpeg4.0+SDL2.0实现简单的纯视频播放器

本文使用FFmpeg4.0.2+SDL2.0实现一个简单的纯视频(无音频)播放器，代码尽量使用新的函数，注释完整。该播放器支持空格键暂停/播放和调整窗口大小。注：SDL.h可以不放在extern “C”中，因为SDL本身进行了C++程序的判断处理。另外av_register_all()和avformat_network_init()都已经不再是必须调用的函数了，前者弃用，后者可以自动调用。更加完整的播放器工程参见GitHub上本人对ffplay的简单重构：ffplay-refactor

#include <iostream>
#include <SDL.h>

using std::cout;
using std::endl;

extern "C"
{
#include <libavformat/avformat.h>
#include <libavcodec/avcodec.h>
#include <libswscale/swscale.h>
#include <libavutil/imgutils.h>
};

//注册两个自定义事件分别用于控制帧率和退出播放
Uint32 REFRESH_EVENT = SDL_RegisterEvents(1);
Uint32 QUIT_EVENT = SDL_RegisterEvents(1);

//控制播放的结束和暂停
int thread_exit = 0;
int thread_pause = 0;

//视频实际的帧率
double FPS;

//启动线程的函数的返回类型和参数类型是规定的
int refresh_video(void *udata)
{
	thread_exit = 0;

	//没有退出时不断刷新画面
	while (!thread_exit)
	{
		//没有按下暂停时，每隔若干时间插入一个刷新事件以切换到下一帧；按下暂停则只有无限延时
		if (!thread_pause)
		{
			SDL_Event ref_event;
			ref_event.type = REFRESH_EVENT;
			SDL_PushEvent(&ref_event);
		}
		SDL_Delay(1000 / round(FPS));
	}

	thread_exit = 0;
	thread_pause = 0;

	//插入退出事件
	SDL_Event quit_event;
	quit_event.type = QUIT_EVENT;
	SDL_PushEvent(&quit_event);

	return 0;
}

int main(int argc, char *argv[])
{	
	const char *src_file = "test.mp4";
	int ret = 0; //各种函数返回的标志

	//读取媒体文件的文件头并将文件格式相关的信息存储在AVFormatContext中
	AVFormatContext *fmt_ctx = NULL;
	ret = avformat_open_input(&fmt_ctx, src_file, NULL, NULL);
	if (ret < 0)
	{
		cout << "Could not open file: " << src_file << endl;
		return -1;
	}

	//将视音频流的信息读取到AVFormatContext中
	ret = avformat_find_stream_info(fmt_ctx, NULL);
	if (ret < 0)
	{
		cout << "Could not find stream information" << endl;
		return -1;
	}

	//输出文件信息用于调试
	cout << "----------------------File Info---------------------" << endl;
	av_dump_format(fmt_ctx, 0, src_file, 0);
	cout << "----------------------------------------------------" << endl;

	//根据类型找到视频流的索引，同时获得相应的解码器
	AVCodec *decoder;
	int video_stream_idx = av_find_best_stream(fmt_ctx, AVMEDIA_TYPE_VIDEO, -1, -1, &decoder, 0);
	if (video_stream_idx < 0)
	{
		cout << "Could not find video stream" << endl;
		return -1;
	}
	if (!decoder)
	{
		cout << "Could not find decoder" << endl;
		return -1;
	}

	//根据索引拿到视频流
	AVStream *video_stream = fmt_ctx->streams[video_stream_idx];

	//计算帧率
	FPS = video_stream->avg_frame_rate.num / video_stream->avg_frame_rate.den;

	//根据解码器创建解码器上下文
	AVCodecContext *dec_ctx = avcodec_alloc_context3(decoder);
	if (!dec_ctx)
	{
		cout << "Could not allocate the " << av_get_media_type_string(AVMEDIA_TYPE_VIDEO) 
			<< "codec context" << endl;
		return -1;
	}

	//把视频流里的参数传到视频解码器上下文中：
	ret = avcodec_parameters_to_context(dec_ctx, video_stream->codecpar);
	if (ret < 0)
	{
		cout << "Failed to copy " << av_get_media_type_string(AVMEDIA_TYPE_VIDEO) 
			<< " codec parameters to decoder context";
		return -1;
	}

	//打开解码器上下文准备进行解码操作
	ret = avcodec_open2(dec_ctx, decoder, NULL);
	if (ret < 0)
	{
		cout << "Could not open codec" << endl;
		return -1;
	}

	//建立上下文，主要是为了后面将源视频转换为YUV420P类型，如有缩放操作也可在此指定
	struct SwsContext *sws_ctx = sws_getContext(dec_ctx->width, dec_ctx->height, dec_ctx->pix_fmt,
		dec_ctx->width, dec_ctx->height, AV_PIX_FMT_YUV420P, SWS_BILINEAR, NULL, NULL, NULL);

	//申请一个保存YUV的frame结构
	AVFrame *frame_YUV = av_frame_alloc();
	ret = av_image_alloc(frame_YUV->data, frame_YUV->linesize, dec_ctx->width, dec_ctx->height, AV_PIX_FMT_YUV420P, 1);
	if (ret < 0)
	{
		cout << "Failed to allocate YUV frame" << endl;
		return -1;
	}

	//创建packet用于获取码流数据
	AVPacket *pkt = av_packet_alloc();
	if (!pkt)
	{
		cout << "Failed to allocate packet" << endl;
		return -1;
	}

	//创建frame用于获取解码的数据
	AVFrame *frame = av_frame_alloc();
	if (!frame)
	{
		cout << "Failed to allocate video frame" << endl;
		return -1;
	}
	
	//初始化SDL
	if (SDL_Init(SDL_INIT_VIDEO | SDL_INIT_AUDIO | SDL_INIT_TIMER) != 0)
	{
		SDL_Log("Unable to initialize SDL: %s\n", SDL_GetError());
		return -1;
	}

	int win_width = dec_ctx->width;
	int win_height = dec_ctx->height;
	//创建与视频大小相匹配的窗口
	SDL_Window *win = SDL_CreateWindow("SimplePlayer", SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED,
		win_width, win_height, SDL_WINDOW_OPENGL | SDL_WINDOW_RESIZABLE);
	if (!win)
	{
		SDL_Log("Unable to create window: %s\n", SDL_GetError());
		return -1;
	}
	
	//根据窗口创建渲染器
	SDL_Renderer *renderer = SDL_CreateRenderer(win, -1, SDL_RENDERER_ACCELERATED);
	if (!renderer)
	{
		SDL_Log("Unable to create renderer: %s\n", SDL_GetError());
		return -1;
	}
	
	//基于渲染器创建一个IYUV像素格式且变化频繁的纹理
	SDL_Texture *texture = SDL_CreateTexture(renderer, SDL_PIXELFORMAT_IYUV, SDL_TEXTUREACCESS_STREAMING, 
		dec_ctx->width, dec_ctx->height);
	if (!texture)
	{
		SDL_Log("Unable to create texture: %s\n", SDL_GetError());
		return -1;
	}

	//启动另一个线程
	SDL_Thread *refresh_thread = SDL_CreateThread(refresh_video, NULL, NULL);

	SDL_Event event;
	SDL_Rect rect;
	//循环读取数据包解码为帧，同时包含其他操作
	while (1)
	{	
		//若事件队列非空，则从取出一个事件
		SDL_WaitEvent(&event);
		if (event.type == REFRESH_EVENT)
		{
			//若正确读取一个数据包
			if (av_read_frame(fmt_ctx, pkt) >= 0)
			{
				//可能拿到音频流、字幕流的数据,因此要判断
				if (pkt->stream_index != video_stream_idx)
				{
					//该packet出现问题时需要释放内存才能继续下一个packet的读取
					av_packet_unref(pkt);
					continue;
				}

				//发送数据包
				ret = avcodec_send_packet(dec_ctx, pkt);
				if (ret < 0)
				{
					cout << "Failed to send video stream packet" << endl;
					av_packet_unref(pkt);
					continue;
				}

				//接收解码帧
				ret = avcodec_receive_frame(dec_ctx, frame);
				if (ret < 0)
				{
					cout << "Failed to receive frame " << dec_ctx->frame_number << endl;
					continue;
				}
				//cout << "Successfully decoding the frame " << dec_ctx->frame_number << endl;

				//解码帧转换为指定格式的YUV
				sws_scale(sws_ctx, frame->data, frame->linesize, 0, dec_ctx->height,
					frame_YUV->data, frame_YUV->linesize);

				//用得到的YUV更新纹理
				SDL_UpdateTexture(texture, NULL, frame_YUV->data[0], frame_YUV->linesize[0]);

				//清空渲染目标
				SDL_RenderClear(renderer);

				//该矩形区域用于调整显示窗口的大小
				rect.x = 0;
				rect.y = 0;
				rect.w = win_width;
				rect.h = win_height;

				//复制纹理到渲染器
				SDL_RenderCopy(renderer, texture, NULL, &rect);

				//渲染输出画面
				SDL_RenderPresent(renderer);
			}
			else
			{
				thread_exit = 1; //读取错误则准备退出
			}
		}
		else if (event.type == SDL_KEYDOWN)
		{
			//空格键暂停/重新播放
			if (event.key.keysym.sym == SDLK_SPACE)
			{
				thread_pause = !thread_pause;
			}				
		}
		else if (event.type == SDL_WINDOWEVENT)
		{
			SDL_GetWindowSize(win, &win_width, &win_height); //调整窗口大小
		}
		else if (event.type == SDL_QUIT)
		{
			thread_exit = 1; //设置标志，准备退出
		}
		else if (event.type == QUIT_EVENT)
		{
			break; //退出循环，真正退出
		}

	}

	
	//释放内存
	av_frame_free(&frame);
	av_frame_free(&frame_YUV);
	av_packet_free(&pkt);
	sws_freeContext(sws_ctx);
	avcodec_free_context(&dec_ctx);
	avformat_close_input(&fmt_ctx);

	//退出SDL  
	SDL_Quit();

	return 0;
}