原文地址：http://www.cnblogs.com/wanggang123/p/6707985.html
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。
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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;
}
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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。
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//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 ;
}
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//初始化filter<br>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\n"
);
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\n"
);
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\n"
);
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;
}
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//初始化编码器<br>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;
}
<br>
//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\n"
);
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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源码地址：http://pan.baidu.com/s/1o8Lkozw
视频地址：http://pan.baidu.com/s/1jH4dYN8
