ffmpeg学习十二：滤镜（实现视频缩放，裁剪，水印等）

这篇文章对使用滤镜进行视频缩放，裁剪水印等做简单介绍。

一.滤镜

滤镜可以实现多路视频的叠加，水印，缩放，裁剪等功能，ffmpeg提供了丰富的滤镜，可以使用ffmpeg -filters来查看：
Filters:
T.. = Timeline support
.S. = Slice threading
..C = Command support
A = Audio input/output
V = Video input/output
N = Dynamic number and/or type of input/output
| = Source or sink filter
T.. adelay A->A Delay one or more audio channels.
… aecho A->A Add echoing to the audio.
… aeval A->A Filter audio signal according to a specified expression.
T.. afade A->A Fade in/out input audio.
… aformat A->A Convert the input audio to one of the specified formats.
… ainterleave N->A Temporally interleave audio inputs.
… allpass A->A Apply a two-pole all-pass filter.
… amerge N->A Merge two or more audio streams into a single multi-channel stream.
… amix N->A Audio mixing.
… anull A->A Pass the source unchanged to the output.
T.. apad A->A Pad audio with silence.
… aperms A->A Set permissions for the output audio frame.
… aphaser A->A Add a phasing effect to the audio.
… aresample A->A Resample audio data.
… aselect A->N Select audio frames to pass in output.
… asendcmd A->A Send commands to filters.
… asetnsamples A->A Set the number of samples for each output audio frames.
… asetpts A->A Set PTS for the output audio frame.
… asetrate A->A Change the sample rate without altering the data.
… asettb A->A Set timebase for the audio output link.
… ashowinfo A->A Show textual information for each audio frame.
… asplit A->N Pass on the audio input to N audio outputs.
…..
这里只是列出其中一小部分，可见ffmpeg提供了非常丰富的滤镜。

滤镜的几个基本概念

Filter:代表单个filter
FilterPad:代表一个filter的输入或输出端口，每个filter都可以有多个输入和多个输出，只有输出pad的filter称为source,只有输入pad的filter称为sink
FilterLink：若一个filter的输出pad和另一个filter的输入pad名字相同，即认为两个filter之间建立了link
FilterChain:代表一串相互连接的filters，除了source和sink外，要求每个filter的输入输出pad都有对应的输出和输入pad
**FilterGraph:**FilterChain的集合
经典示例：

图中每一个节点就是一个Filter，每一个方括号所代表的就是FilterPad，可以看到split的输出pad中有一个叫tmp的，而crop的输入pad中也有一个tmp，由此在二者之间建立了link，当然input和output代表的就是source和sink，此外，图中有三条FilterChain，第一条由input和split组成，第二条由crop和vflip组成，第三条由overlay和output组成，整张图即是一个拥有三个FilterChain的FilterGraph。

使用libavfilter为视频添加滤镜

ffmpeg官网给出的filtering_video.c介绍了滤镜的用法，这个例子将一个视频文件解码成原始的一帧数据，然后再将这一帧数据使用滤镜惊醒缩放，缩小到78x24大小后，将像素中的点转换成字符，然后显示在终端中，效果如下：

这个程序结构非常清晰的介绍了滤镜的用法，但这种将视频中的像素转换为字符，然后显示在终端的做法并不能很直观的感受滤镜的作用，因此，我对这个程序做了简单的修改，将滤镜处理后的视频保存在文件中而不是显示在终端。下面为我修改过后，完整的程序，只有一个.c文件：

#define _XOPEN_SOURCE 600 /* for usleep */
#include 

#include 
#include 
#include 
#include 
#include 
#include 
#include 

/*缩放滤镜，以下命令将视频缩小一般,iw,ih指的是输入的视频宽和高，
*此外也可以直接使用数字知名缩放的大小，比如:scale=200:100
*/
//const char *filter_descr = "scale=iw/2:ih/2";
/*裁剪滤镜，一下命令将视频的左上角的四分之一裁剪下来*/
//const char *filter_descr = "crop=iw/2:ih/2:0:0";
/*添加字符串水印*/
const char *filter_descr = "drawtext=fontfile=FreeSans.ttf:fontcolor=green:fontsize=30:text='Hello'";


static AVFormatContext *fmt_ctx;
static AVCodecContext *dec_ctx;
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(&fmt_ctx, filename, NULL, NULL)) < 0) {
        av_log(NULL, AV_LOG_ERROR, "Cannot open input file\n");
        return ret;
    }

    if ((ret = avformat_find_stream_info(fmt_ctx, NULL)) < 0) {
        av_log(NULL, AV_LOG_ERROR, "Cannot find stream information\n");
        return ret;
    }

    /* select the video stream */
    ret = av_find_best_stream(fmt_ctx, AVMEDIA_TYPE_VIDEO, -1, -1, &dec, 0);
    if (ret < 0) {
        av_log(NULL, AV_LOG_ERROR, "Cannot find a video stream in the input file\n");
        return ret;
    }
    video_stream_index = ret;
    dec_ctx = fmt_ctx->streams[video_stream_index]->codec;
    av_opt_set_int(dec_ctx, "refcounted_frames", 1, 0);

    /* init the video decoder */
    if ((ret = avcodec_open2(dec_ctx, dec, NULL)) < 0) {
        av_log(NULL, AV_LOG_ERROR, "Cannot open video decoder\n");
        return ret;
    }

    return 0;
}

static int init_filters(const char *filters_descr)
{
    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();
    AVRational time_base = fmt_ctx->streams[video_stream_index]->time_base;
    enum AVPixelFormat pix_fmts[] = { AV_PIX_FMT_YUV420P, AV_PIX_FMT_NONE };

    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. */
    snprintf(args, sizeof(args),
            "video_size=%dx%d:pix_fmt=%d:time_base=%d/%d:pixel_aspect=%d/%d",
            dec_ctx->width, dec_ctx->height, dec_ctx->pix_fmt,
            time_base.num, time_base.den,
            dec_ctx->sample_aspect_ratio.num, dec_ctx->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_NONE, AV_OPT_SEARCH_CHILDREN);
    if (ret < 0) {
        av_log(NULL, AV_LOG_ERROR, "Cannot set output pixel format\n");
        goto end;
    }

    /*
     * Set the endpoints for the filter graph. The filter_graph will
     * be linked to the graph described by filters_descr.
     */

    /*
     * The buffer source output must be connected to the input pad of
     * the first filter described by filters_descr; since the first
     * filter input label is not specified, it is set to "in" by
     * default.
     */
    outputs->name       = av_strdup("in");
    outputs->filter_ctx = buffersrc_ctx;
    outputs->pad_idx    = 0;
    outputs->next       = NULL;

    /*
     * The buffer sink input must be connected to the output pad of
     * the last filter described by filters_descr; since the last
     * filter output label is not specified, it is set to "out" by
     * default.
     */
    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)
        goto end;

    if ((ret = avfilter_graph_config(filter_graph, NULL)) < 0)
        goto end;

end:
    avfilter_inout_free(&inputs);
    avfilter_inout_free(&outputs);

    return ret;
}

FILE * file_fd;

static void write_frame(const AVFrame *frame)
{
    if(frame->format==AV_PIX_FMT_YUV420P){
        printf("format is yuv420p\n");
    }else{
        printf("format is not yuv420p\n");
    }

    printf("frame widht=%d,frame height=%d\n",\
        frame->width,frame->height);
    fwrite(frame->data[0],1,frame->width*frame->height,file_fd);
    fwrite(frame->data[1],1,frame->width/2*frame->height/2,file_fd);
    fwrite(frame->data[2],1,frame->width/2*frame->height/2,file_fd);
}

int main(int argc, char **argv)
{
    int ret;
    AVPacket packet;
    AVFrame *frame = av_frame_alloc();
    AVFrame *filt_frame = av_frame_alloc();
    int got_frame;
    file_fd = fopen("hello.yuv","wb+");
    if (!frame || !filt_frame) {
        perror("Could not allocate frame");
        exit(1);
    }
    if (argc != 2) {
        fprintf(stderr, "Usage: %s file\n", argv[0]);
        exit(1);
    }

    av_register_all();
    avfilter_register_all();

    if ((ret = open_input_file(argv[1])) < 0)
        goto end;
    if ((ret = init_filters(filter_descr)) < 0)
        goto end;

    /* read all packets */
    while (1) {
        if ((ret = av_read_frame(fmt_ctx, &packet)) < 0)
            break;

        if (packet.stream_index == video_stream_index) {
            got_frame = 0;
            ret = avcodec_decode_video2(dec_ctx, frame, &got_frame, &packet);
            if (ret < 0) {
                av_log(NULL, AV_LOG_ERROR, "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_flags(buffersrc_ctx, frame, AV_BUFFERSRC_FLAG_KEEP_REF) < 0) {
                    av_log(NULL, AV_LOG_ERROR, "Error while feeding the filtergraph\n");
                    break;
                }

                /* pull filtered frames from the filtergraph */
                while (1) {
                    ret = av_buffersink_get_frame(buffersink_ctx, filt_frame);
                    if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF)
                        break;
                    if (ret < 0)
                        goto end;
                    write_frame(filt_frame);
                    av_frame_unref(filt_frame);
                }
                av_frame_unref(frame);
            }
        }
        av_free_packet(&packet);
    }
end:
    avfilter_graph_free(&filter_graph);
    avcodec_close(dec_ctx);
    avformat_close_input(&fmt_ctx);
    av_frame_free(&frame);
    av_frame_free(&filt_frame);

    if (ret < 0 && ret != AVERROR_EOF) {
        fprintf(stderr, "Error occurred: %s\n", av_err2str(ret));
        exit(1);
    }
    fclose(file_fd);
    exit(0);
}

文件编译参考前面的文章，便宜时会报错：No such filter: ‘drawtext’。这是因为我们没有是能这个滤镜。
解决办法：程序找不到 drawtext这个filter，一般是因为使用默认编译选项是该filter并未被编译进库里面，所以重新再编译ffmpeg，并且在执行”./configure ……”时加上“–enable-libfreetype”。这样就ok了。
完整的配置如下，我们之前已经使能了aac,h.264编解码器。

./configure --enable-libx264 --enable-gpl --enable-decoder=h264 --enable-encoder=libx264 --enable-shared --enable-static --disable-yasm -enable-nonfree  --enable-libfdk-aac --enable-shared --enable-libfreetype --prefix=tmp

配置结束后执行make && make install安装即可。
编译后，执行实例如下：
./out.bin rain.mp4
然后再当前文件下会生成hello.yuv的视频文件，这是原始数据格式的视频，可以使用ffplay来播放：
ffplay -s 1280x640 hello.yuv
改程序会打印出视频的长和宽，这里的1280x640请使用打印出来的长和宽来代替。
三.缩放，裁剪，添加字符串水印
滤镜的用法基本就是如上程序给出的步骤，我们可以通过指定不同的描述字符串来实现不同的滤镜功能。下面的字符串将视频缩放为200x100大小，着我在程序里的注释中已经给出了。
原视频：

缩小为一半：
const char *filter_descr = “scale=iw/2:ih/2”;
缩小后现象不明显，就不贴图了。
如下命令将视频裁剪出中间的1/4。
const char *filter_descr = “crop=iw/2:ih/2:iw/4:ih/4”;
效果如下：

下面字符串给视频添加hello的字符串水印：
const char *filter_descr = “drawtext=fontfile=FreeSans.ttf:fontcolor=green:fontsize=30:text=’Hello’”;
效果如下：

更复杂的滤镜的使用，期待和大家一起学习。