Ffmpeg 视频教程 向视频中添加文字

    Ffmpeg支持添加文字功能，具体如何将文字叠加到视频中的每一张图片，FFmpeg调用了文字库FreeSerif.ttf。当我们

用到ffmpeg 添加文字功能时 我们需要先下载改文字库，下载地址是http://www.fonts2u.com/free-serif.font，这算是

前期准备工作。准备工作完成以后，我来介绍下Ffmpeg实现视频文件添加文字功能的基本流程，流程图如下图所示：

　　　　　　图1. Ffmpeg 向视频添加文字流程

Ffmpeg 以Filter的方式添加文字，下面给出具体的代码。

1. 打开视频文件。

int OpenInput(char *fileName)
{
	context = avformat_alloc_context();
	context->interrupt_callback.callback = interrupt_cb;
	AVDictionary *format_opts =  nullptr;

	int ret = avformat_open_input(&context, fileName, nullptr, &format_opts);
	if(ret < 0)
	{
		return  ret;
	}
	ret = avformat_find_stream_info(context,nullptr);
	av_dump_format(context, 0, fileName, 0);
	if(ret >= 0) 
	{
		std::cout <<"open input stream successfully" << endl;
	}
	return ret;
}

 2. 初始化 初始化包括输出上下文以及Filter，解码器，编码器四个部分。

//初始化输出上下文
int OpenOutput(char *fileName)
{
	int ret = 0;
	ret  = avformat_alloc_output_context2(&outputContext, nullptr, "mpegts", fileName);
	if(ret < 0)
	{
		goto Error;
	}
	ret = avio_open2(&outputContext->pb, fileName, AVIO_FLAG_READ_WRITE,nullptr, nullptr);	
	if(ret < 0)
	{
		goto Error;
	}

	for(int i = 0; i < context->nb_streams; i++)
	{
		AVStream * stream = avformat_new_stream(outputContext, outPutEncContext->codec);
		stream->codec = outPutEncContext;
		if(ret < 0)
		{
			goto Error;
		}
	}
	av_dump_format(outputContext, 0, fileName, 1);
	ret = avformat_write_header(outputContext, nullptr);
	if(ret < 0)
	{
		goto Error;
	}
	if(ret >= 0)
		cout <<"open output stream successfully" << endl;
	return ret ;
Error:
	if(outputContext)
	{
		avformat_close_input(&outputContext);
	}
	return ret ;
}

 //初始化filter
int InitFilter(AVCodecContext * codecContext)
{
	char args[512];
	int ret = 0;	

	AVFilter *buffersrc  = avfilter_get_by_name("buffer");
	AVFilter *buffersink = avfilter_get_by_name("buffersink");
	AVFilterInOut *outputs = avfilter_inout_alloc();
	AVFilterInOut *inputs  = avfilter_inout_alloc();
	string  filters_descr ="drawtext=fontfile=D\\:FreeSerif.ttf:fontsize=100:text=hello world:x=100:y=100";
	enum AVPixelFormat pix_fmts[] = { AV_PIX_FMT_YUV420P, AV_PIX_FMT_YUV420P};

	filter_graph = avfilter_graph_alloc();
	if (!outputs || !inputs || !filter_graph) {
		ret = AVERROR(ENOMEM);
		goto end;
	}

	/* buffer video source: the decoded frames from the decoder will be inserted here. */
	sprintf_s(args, sizeof(args),
		"video_size=%dx%d:pix_fmt=%d:time_base=%d/%d:pixel_aspect=%d/%d",
		codecContext->width, codecContext->height, codecContext->pix_fmt,
		codecContext->time_base.num, codecContext->time_base.den,
		codecContext->sample_aspect_ratio.num, codecContext->sample_aspect_ratio.den);

	ret = avfilter_graph_create_filter(&buffersrc_ctx, buffersrc, "in",
		args, NULL, filter_graph);
	if (ret < 0) {
		av_log(NULL, AV_LOG_ERROR, "Cannot create buffer source
");
		goto end;
	}

	/* buffer video sink: to terminate the filter chain. */
	ret = avfilter_graph_create_filter(&buffersink_ctx, buffersink, "out",
		NULL, NULL, filter_graph);
	if (ret < 0) {
		av_log(NULL, AV_LOG_ERROR, "Cannot create buffer sink
");
		goto end;
	}

	ret = av_opt_set_int_list(buffersink_ctx, "pix_fmts", pix_fmts,
		AV_PIX_FMT_YUV420P, AV_OPT_SEARCH_CHILDREN);
	if (ret < 0) {
		av_log(NULL, AV_LOG_ERROR, "Cannot set output pixel format
");
		goto end;
	}

	/* 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.c_str(),
		&inputs, &outputs, NULL)) < 0)
		goto end;

	if ((ret = avfilter_graph_config(filter_graph, NULL)) < 0)
		goto end;
	return ret;
end:
	avfilter_inout_free(&inputs);
	avfilter_inout_free(&outputs);
	return ret;
}

//初始化编码器
int InitEncoderCodec( int iWidth, int iHeight)
{
	AVCodec *  pH264Codec = avcodec_find_encoder(AV_CODEC_ID_H264);
	if(NULL == pH264Codec)
	{
		printf("%s", "avcodec_find_encoder failed");
		return  -1;
	}
	outPutEncContext = avcodec_alloc_context3(pH264Codec);
	outPutEncContext->gop_size = 30;
	outPutEncContext->has_b_frames = 0;
	outPutEncContext->max_b_frames = 0;
	outPutEncContext->codec_id = pH264Codec->id;
	outPutEncContext->time_base.num =context->streams[0]->codec->time_base.num;
	outPutEncContext->time_base.den = context->streams[0]->codec->time_base.den;
	outPutEncContext->pix_fmt            = *pH264Codec->pix_fmts;
	outPutEncContext->width              =  iWidth;
	outPutEncContext->height             = iHeight;

	outPutEncContext->me_subpel_quality = 0;
	outPutEncContext->refs = 1;
	outPutEncContext->scenechange_threshold = 0;
	outPutEncContext->trellis = 0;
	AVDictionary *options = nullptr;
	outPutEncContext->flags |= AV_CODEC_FLAG_GLOBAL_HEADER;

	int ret = avcodec_open2(outPutEncContext, pH264Codec, &options);
	if (ret < 0)
	{
		printf("%s", "open codec failed");
		return  ret;
	}
	return 1;
}

//初始化解码器
int InitDecodeCodec(AVCodecID codecId)
{
	auto codec = avcodec_find_decoder(codecId);
	if(!codec)
	{
		return -1;
	}
	decoderContext = context->streams[0]->codec;
	if (!decoderContext) {
		fprintf(stderr, "Could not allocate video codec context
");
		exit(1);
	}

	if (codec->capabilities & AV_CODEC_CAP_TRUNCATED)
		decoderContext->flags |= AV_CODEC_FLAG_TRUNCATED; 
	int ret = avcodec_open2(decoderContext, codec, NULL);
	return ret;

}

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