FFMPEG中有一个类库：libavfilter。该类库提供了各种视音频过滤器。之前一直没有怎么使用过这个类库，最近看了一下它的使用说明，发现还是很强大的，有很多现成的filter供使用，完成视频的处理很方便。在此将它的一个例子基础上完成了一个水印叠加器，并且移植到了VC2010下，方便开发人员学习研究它的使用方法。
该例子完成了一个水印叠加的功能。可以将一张透明背景的PNG图片作为水印叠加到一个视频文件上。需要注意的是，其叠加工作是在解码后的YUV像素数据的基础上完成的。程序支持使用SDL显示叠加后的YUV数据。也可以将叠加后的YUV输出成文件。
流程图（2014.9.29更新）
下面附一张使用FFmpeg的libavfilter的流程图。可以看出使用libavfilter还是需要做不少的初始化工作的。但是使用的时候还是比较简单的，，就两个重要的函数：av_buffersrc_add_frame()和av_buffersink_get_buffer_ref()。
PS：这张图中只列出了和libavfilter有关的函数和结构体。代码中其它函数可以参考：100行代码实现最简单的基于FFMPEG+SDL的视频播放器（SDL1.x）
代码
下面直接贴上代码：
/** * 最简单的基于FFmpeg的AVFilter例子（叠加水印） * Simplest FFmpeg AVfilter Example (Watermark) * * 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 * * * 本程序使用FFmpeg的AVfilter实现了视频的水印叠加功能。 * 可以将一张PNG图片作为水印叠加到视频上。 * 是最简单的FFmpeg的AVFilter方面的教程。 * 适合FFmpeg的初学者。 * * This software uses FFmpeg’s AVFilter to add watermark in a video file. * It can add a PNG format picture as watermark to a video file. * It’s the simplest example based on FFmpeg’s AVFilter. * Suitable for beginner of FFmpeg * */#include <stdio.h>#define __STDC_CONSTANT_MACROS#ifdef _WIN32#define snprintf _snprintf//Windowsextern "C"{#include "libavcodec/avcodec.h"#include "libavformat/avformat.h"#include "libavfilter/avfiltergraph.h"#include "libavfilter/avcodec.h"#include "libavfilter/buffersink.h"#include "libavfilter/buffersrc.h"#include "libavutil/avutil.h"#include "libswscale/swscale.h"#include "SDL/SDL.h"};#else//Linux…#ifdef __cplusplusextern "C"{#endif#include <libavcodec/avcodec.h>#include <libavformat/avformat.h>#include <libavfilter/avfiltergraph.h>#include <libavfilter/avcodec.h>#include <libavfilter/buffersink.h>#include <libavfilter/buffersrc.h>#include <libavutil/avutil.h>#include <libswscale/swscale.h>#include <SDL/SDL.h>#ifdef __cplusplus};#endif#endif//Enable SDL?#define ENABLE_SDL 1//Output YUV data?#define ENABLE_YUVFILE 0const char *filter_descr = "movie=my_logo.png[wm];[in][wm]overlay=5:5[out]";static AVFormatContext *pFormatCtx;static AVCodecContext *pCodecCtx;AVFilterContext *buffersink_ctx;AVFilterContext *buffersrc_ctx;AVFilterGraph *filter_graph;static int video_stream_index = -1;static int64_t last_pts = AV_NOPTS_VALUE;static int open_input_file(const char *filename){int ret;AVCodec *dec;if ((ret = avformat_open_input(&pFormatCtx, filename, NULL, NULL)) < 0) {printf( "Cannot open input file\n");return ret;}if ((ret = avformat_find_stream_info(pFormatCtx, NULL)) < 0) {printf( "Cannot find stream information\n");return ret;}/* select the video stream */ret = av_find_best_stream(pFormatCtx, AVMEDIA_TYPE_VIDEO, -1, -1, &dec, 0);if (ret < 0) {printf( "Cannot find a video stream in the input file\n");return ret;}video_stream_index = ret;pCodecCtx = pFormatCtx->streams[video_stream_index]->codec;/* init the video decoder */if ((ret = avcodec_open2(pCodecCtx, dec, NULL)) < 0) {printf( "Cannot open video decoder\n");return ret;}return 0;}static int init_filters(const char *filters_descr){char args[512];int ret;AVFilter *buffersrc = avfilter_get_by_name("buffer");AVFilter *buffersink = avfilter_get_by_name("ffbuffersink");AVFilterInOut *outputs = avfilter_inout_alloc();AVFilterInOut *inputs = avfilter_inout_alloc();enum PixelFormat pix_fmts[] = { PIX_FMT_YUV420P, PIX_FMT_NONE };AVBufferSinkParams *buffersink_params;filter_graph = avfilter_graph_alloc();/* buffer video source: the decoded frames from the decoder will be inserted here. */snprintf(args, sizeof(args),"video_size=%dx%d:pix_fmt=%d:time_base=%d/%d:pixel_aspect=%d/%d",pCodecCtx->width, pCodecCtx->height, pCodecCtx->pix_fmt,pCodecCtx->time_base.num, pCodecCtx->time_base.den,pCodecCtx->sample_aspect_ratio.num, pCodecCtx->sample_aspect_ratio.den);ret = avfilter_graph_create_filter(&buffersrc_ctx, buffersrc, "in",args, NULL, filter_graph);if (ret < 0) {printf("Cannot create buffer source\n");return ret;}/* buffer video sink: to terminate the filter chain. */buffersink_params = av_buffersink_params_alloc();buffersink_params->pixel_fmts = pix_fmts;ret = avfilter_graph_create_filter(&buffersink_ctx, buffersink, "out",NULL, buffersink_params, filter_graph);av_free(buffersink_params);if (ret < 0) {printf("Cannot create buffer sink\n");return ret;}/* Endpoints for the filter graph. */outputs->name= av_strdup("in");outputs->filter_ctx = buffersrc_ctx;outputs->pad_idx = 0;outputs->next= NULL;inputs->name= av_strdup("out");inputs->filter_ctx = buffersink_ctx;inputs->pad_idx = 0;inputs->next= NULL;if ((ret = avfilter_graph_parse_ptr(filter_graph, filters_descr,&inputs, &outputs, NULL)) < 0)return ret;if ((ret = avfilter_graph_config(filter_graph, NULL)) < 0)return ret;return 0;}int main(int argc, char* argv[]){int ret;AVPacket packet;AVFrame frame;int got_frame;avcodec_register_all();av_register_all();avfilter_register_all();if ((ret = open_input_file("cuc_ieschool.flv")) < 0)goto end;if ((ret = init_filters(filter_descr)) < 0)goto end;#if ENABLE_YUVFILEFILE *fp_yuv=fopen("test.yuv","wb+");#endif#if ENABLE_SDLSDL_Surface *screen; SDL_Overlay *bmp; SDL_Rect rect;if(SDL_Init(SDL_INIT_VIDEO | SDL_INIT_AUDIO | SDL_INIT_TIMER)) {printf( "Could not initialize SDL – %s\n", SDL_GetError()); return -1;} screen = SDL_SetVideoMode(pCodecCtx->width, pCodecCtx->height, 0, 0);if(!screen) {printf("SDL: could not set video mode – exiting\n");return -1;}bmp = SDL_CreateYUVOverlay(pCodecCtx->width, pCodecCtx->height,SDL_YV12_OVERLAY, screen); SDL_WM_SetCaption("Simplest FFmpeg Video Filter",NULL);#endif/* read all packets */while (1) {AVFilterBufferRef *picref;if ((ret = av_read_frame(pFormatCtx, &packet)) < 0)break;if (packet.stream_index == video_stream_index) {avcodec_get_frame_defaults(&frame);got_frame = 0;ret = avcodec_decode_video2(pCodecCtx, &frame, &got_frame, &packet);if (ret < 0) {printf( "Error decoding video\n");break;}if (got_frame) {frame.pts = av_frame_get_best_effort_timestamp(&frame);/* push the decoded frame into the filtergraph */if (av_buffersrc_add_frame(buffersrc_ctx, &frame) < 0) {printf( "Error while feeding the filtergraph\n");break;}/* pull filtered pictures from the filtergraph */while (1) {ret = av_buffersink_get_buffer_ref(buffersink_ctx, &picref, 0);if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF)break;if (ret < 0)goto end;if (picref) {#if ENABLE_YUVFILEint y_size=picref->video->w*picref->video->h;fwrite(picref->data[0],1,y_size,fp_yuv);//Yfwrite(picref->data[1],1,y_size/4,fp_yuv); //Ufwrite(picref->data[2],1,y_size/4,fp_yuv); //V#endif#if ENABLE_SDLSDL_LockYUVOverlay(bmp);int y_size=picref->video->w*picref->video->h;memcpy(bmp->pixels[0],picref->data[0],y_size); //Ymemcpy(bmp->pixels[2],picref->data[1],y_size/4); //Umemcpy(bmp->pixels[1],picref->data[2],y_size/4); //Vbmp->pitches[0]=picref->linesize[0];bmp->pitches[2]=picref->linesize[1];bmp->pitches[1]=picref->linesize[2];SDL_UnlockYUVOverlay(bmp);rect.x = 0;rect.y = 0;rect.w = picref->video->w;rect.h = picref->video->h;SDL_DisplayYUVOverlay(bmp, &rect);//Delay 40msSDL_Delay(40);#endifavfilter_unref_bufferp(&picref);}}}}av_free_packet(&packet);}#if ENABLE_YUVFILEfclose(fp_yuv);#endifend:avfilter_graph_free(&filter_graph);if (pCodecCtx)avcodec_close(pCodecCtx);avformat_close_input(&pFormatCtx);if (ret < 0 && ret != AVERROR_EOF) {char buf[1024];av_strerror(ret, buf, sizeof(buf));printf("Error occurred: %s\n", buf);return -1;}return 0;}结果
程序的运行效果如图所示。
需要叠加的水印为一张PNG（透明）图片（在这里是my_logo.png）。
少一点预设的期待，那份对人的关怀会更自在
