FFmpeg滤镜代码级分析

前一篇文章《为FFmpeg添加自定义滤镜》详细讲述了FFmpeg的滤镜添加步骤，并给出了代码实例。

本文将以FFmpeg自带的deinterlace滤镜”yadif – yet another deinterlace filter”为例分析

FFmpeg滤镜的代码级实现机制。

总的来说,FFmpeg的滤镜机制和MicroSoft Directshow机制基本相同——不知道谁学了谁的，还是程

序员所见略同：

构建机制上：FFmpeg自己实现了统一的对象模型，DShow是COM的实现;

框架实现上：都使用filter graph来管理滤镜;

连接方式上：FFmpeg使用Pad结构体，DShow使用Pin，两者都是分配器的机制;

下面是以层级来分析代码：

FFmpeg转码层的伪码如下所示，红色粗体是和filter相关的函数:

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1. int transcode(AVFormatContext **output_files,
   
2.               int nb_output_files,
   
3.               AVInputFile *input_files,
   
4.               int nb_input_files,
   
5.               AVStreamMap *stream_maps, 
   
6.               int nb_stream_maps)
   
7. {
   
8.   …
   
9.   解码参数计算;
   
10. 
    
11. 
    
12.   #if CONFIG_AVFILTER
    
13.   if (configure_video_filters(ist, ost)) {
    
14.     fprintf(stderr, "Error opening filters!\n");
    
15.     ret = AVERROR(EINVAL);
    
16.     goto fail;
    
17.   }
    
18.   #endif
    
19. 
    
20. 
    
21.   编码参数计算;
    
22. 
    
23.   output_packet(ist, ist_index, ost_table,
    
24.                 nb_ostreams, &pkt, &errorflag);
    
25. 
    
26.   ……
    
27. 
    
28.   return ret;
    
29. }

函数configure_video_filters是用来初始化filter graph 和filter本身的初始化；

函数output_packet是FFmpeg整个解码，滤镜处理，编码调用的实现。

1. 先来看configure_video_filters的实现：

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1. int configure_video_filters(AVInputStream *ist, 
   
2.                             AVOutputStream *ost)
   
3. {
   
4.    // 类似于directshow一样初始化filter graph等;
   
5.   ost->graph = avfilter_graph_alloc();
   
6.   …
   

    // 初始化filter graph和filter

1.   avfilter_graph_parse(ost->graph, ost->avfilter, 
   
2.                        &inputs, &outputs, NULL)
   
3. 
   
4. 
   
5.   // 检查filter支持的媒体类型
6.   avfilter_graph_config(ost->graph, NULL);
   
7. 
   
8.   。。。
   
9. 
   
10.   return 0;
    
11. }

filter graph和filter的初始化:

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1. int avfilter_graph_parse(AVFilterGraph *graph, 
   
2.                          const char *filters,
   
3.                          AVFilterInOut **open_inputs, 
   
4.                          AVFilterInOut **open_outputs,
   
5.                          void *log_ctx)
   
6. {
   
7. 
   
8.     do {
   
9. 
   
10.         parse_inputs(&filters, &curr_inputs, open_outputs, log_ctx);
    
11. 
    
12.         parse_filter(&filter, &filters, graph, index, log_ctx);
    
13. 
    
14. 
    
15.         if (filter->input_count == 1 && !curr_inputs && !index) {
    
16.             /* First input can be omitted if it is "[in]" */
    
17.             const char *tmp = "[in]";
    
18.             if ((ret = parse_inputs(&tmp, &curr_inputs, open_outputs, 
    
19.                                     log_ctx)) < 0)
    
20.                 goto fail;
    
21.         }
    
22. 
    
23. 
    
24.         link_filter_inouts(filter, &curr_inputs, open_inputs, log_ctx);
    
25. 
    
26. 
    
27.         parse_outputs(&filters, &curr_inputs, open_inputs, 
    
28.                       open_outputs, log_ctx);
    
29. 
    
30. 
    
31.         filters += strspn(filters, WHITESPACES);
    
32.         chr = *filters++;
    
33. 
    
34.         index++;
    
35.     } while (chr == ',' || chr == ';');
    
36. 
    
37.     if (open_inputs && *open_inputs && 
    
38.         !strcmp((*open_inputs)->name, ""out"") && curr_inputs) {
    
39.         /* Last output can be omitted if it is "[out]" */
    
40.         const char *tmp = "[out]";
    
41.         if ((ret = parse_outputs(&tmp, &curr_inputs, open_inputs, 
    
42.                                               open_outputs, log_ctx)) < 0)
    
43.             goto fail;
    
44. 
    
45.     }
    
46. 
    
47.     return 0;
    
48. }

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1. static int parse_filter(AVFilterContext **filt_ctx, 
2.                         const char **buf, AVFilterGraph *graph,"
3.                         int index, void *log_ctx)
4. {
5.     char *opts = NULL;
6.     char *name = av_get_token(buf, "=,;[\n");
7.     int ret;
8. 
   
9.     if (**buf == '=') {
10.         (*buf)++;
11.         opts = av_get_token(buf, "[],;\n");
12.     }
13. 
    
14.     ret = create_filter(filt_ctx, graph, index, name, opts, log_ctx);
15. 
    
16.     av_free(name);
17.     av_free(opts);
18.     return ret;
19. }

到了真正的Filter graph初始化，这部分和DShow很相似：

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1. int create_filter(AVFilterContext **filt_ctx, 
   
2.                   AVFilterGraph *ctx, int index,
   
3.                   const char *filt_name, const char *args, 
   
4.                   void *log_ctx)
   
5. 
   
6. {
   
7.     AVFilter *filt;
   
8.     char inst_name[30];
   
9.     char tmp_args[256];
   
10.     int ret;
    
11. 
    
12.     // 注册当前filter到静态数组中
    
13.     filt = avfilter_get_by_name(filt_name);
    
14. 
    
15.     // 分配并初始化输入输出Pad
    
16.     ret = avfilter_open(filt_ctx, filt, inst_name);
    
17. 
    
18.     // 将当前filter添加到filter graph中
    
19.     ret = avfilter_graph_add_filter(ctx, *filt_ctx);
    
20. 
    
21.     // 通过函数指针，调用当前filter的初始化函数
    
22.     ret = avfilter_init_filter(*filt_ctx, args, NULL);
    
23.     return 0;
    
24. }

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1. int avfilter_init_filter(AVFilterContext *filter, 
   
2.                          const char *args, 
   
3.                          void *opaque)
   
4. {
   
5.     int ret=0;
   
6. 
   
7.     if (filter->filter->init)
   
8.         ret = filter->filter->init(filter, args, opaque);
   
9.     return ret;
   
10. }

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1. static av_cold int init(AVFilterContext *ctx, 
   
2.                                   const char *args, 
   
3.                                   void *opaque)
   
4. {
   
5.     YADIFContext *yadif = ctx->priv;
   
6.     av_unused int cpu_flags = av_get_cpu_flags();
   
7. 
   
8.     yadif->mode = 0;
   
9.     yadif->parity = -1;
   
10.     yadif->csp = NULL;
    
11. 
    
12.     // 滤镜函数指针初始化赋值
    
13.     yadif->filter_line = filter_line_c;
    
14. 
    
15.     return 0;
    
16. }

2. 接着来看两个filter的Pad连接时的媒体类型的协商机制

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1. int avfilter_graph_config(AVFilterGraph *graphctx, void *log_ctx)
   
2. {
   
3.     int ret;
   

     // 检查filter的权限

1.     if ((ret = ff_avfilter_graph_check_validity(graphctx, log_ctx)))
   
2.         return ret;
   
3.     
4.     // 检查Pad支持的媒体类型
5.     if ((ret = ff_avfilter_graph_config_formats(graphctx, log_ctx)))
   
6.         return ret;
   

     // 

1.     if ((ret = ff_avfilter_graph_config_links(graphctx, log_ctx)))
   
2.         return ret;
   
3. 
   
4.     return 0;
   
5. }

检查Pad支持的媒体类型 :

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1. int ff_avfilter_graph_config_formats(AVFilterGraph *graph, 
   
2.                                                           AVClass *log_ctx)
   
3. {
   
4.     int ret;
   
5. 
   
6.     /* find supported formats from sub-filters, and merge along links */
   
7.     if ((ret = query_formats(graph, log_ctx)) < 0)
   
8.         return ret;
   
9. 
   
10.     /* Once everything is merged, it's possible that we'll still have
    
11.      * multiple valid media format choices. We pick the first one. */
    
12.     pick_formats(graph);
    
13. 
    
14.     return 0;
    
15. }

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1. static int query_formats(AVFilterGraph *graph, 
   
2.                          AVClass *log_ctx)
   
3. {
   
4.     int i, j, ret;
   
5.     int scaler_count = 0;
   
6.     char inst_name[30];
   
7. 
   
8.     /* ask all the sub-filters for their supported media formats */
   
9.     for (i = 0; i < graph->filter_count; i++) {
   
10.         if (graph->filters[i]->filter->query_formats)
    
11.             graph->filters[i]->filter->query_formats(graph->filters[i]);
    
12.         else
    
13.             avfilter_default_query_formats(graph->filters[i]);
    
14.     }
    
15. 
    
16.     /* go through and merge as many format lists as possible */
    
17.     for (i = 0; i < graph->filter_count; i++) {
    
18. 
    
19.        … 
    
20.     
    
21.     }
    
22. 
    
23.     return 0;
    
24. }

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1. /* 检查连接用Pad支持的媒体类型 */
   
2. static int query_formats(AVFilterContext *ctx)
   
3. {
   
4.     static const enum PixelFormat pix_fmts[] = {
   
5.         PIX_FMT_YUV420P,
   
6.         PIX_FMT_YUV422P,
   
7.         PIX_FMT_YUV444P,
   
8.         PIX_FMT_YUV410P,
   
9.         PIX_FMT_YUV411P,
   
10.         PIX_FMT_GRAY8,
    
11.         PIX_FMT_YUVJ420P,
    
12.         PIX_FMT_YUVJ422P,
    
13.         PIX_FMT_YUVJ444P,
    
14.         AV_NE( PIX_FMT_GRAY16BE, PIX_FMT_GRAY16LE ),
    
15.         PIX_FMT_YUV440P,
    
16.         PIX_FMT_YUVJ440P,
    
17.         AV_NE( PIX_FMT_YUV420P16BE, PIX_FMT_YUV420P16LE ),
    
18.         AV_NE( PIX_FMT_YUV422P16BE, PIX_FMT_YUV422P16LE ),
    
19.         AV_NE( PIX_FMT_YUV444P16BE, PIX_FMT_YUV444P16LE ),
    
20.         PIX_FMT_NONE
    
21.     };
    
22. 
    
23.     avfilter_set_common_pixel_formats(ctx, 
    
24.                                       avfilter_make_format_list(pix_fmts));
    
25. 
    
26.     return 0;
    
27. }

3. 最后来看滤镜的连接和对每帧图像处理时的调用

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1. int output_packet(AVInputStream *ist, int ist_index,
   
2.                   AVOutputStream **ost_table,
   
3.                   int nb_ostreams,
   
4.                   const AVPacket *pkt, 
   
5.                   int *errorflag)
   
6. {
   
7. 
   
8.   /* decode the packet if needed */
   
9.   if (ist->decoding_needed) {
   
10. 
    
11.     解码;
    
12. 
    
13. #if CONFIG_AVFILTER
    
14.     if(ist->st->codec->codec_type == AVMEDIA_TYPE_VIDEO)
    
15.     if (start_time == 0 || ist->pts >= start_time) {
    
16.       for(i=0;i<nb_ostreams;i++) {
    
17.         ost = ost_table[i];
    
18.         if (ost->input_video_filter && ost->source_index == ist_index) {
    
19.           if (!picture.sample_aspect_ratio.num)
    
20.           picture.sample_aspect_ratio = ist->st->sample_aspect_ratio;
    
21.           picture.pts = ist->pts;
    
22. 
    
23.           av_vsrc_buffer_add_frame(ost->input_video_filter, &picture, AV_VSRC_BUF_FLAG_OVERWRITE);
    
24. 
    
25.         }
    
26.       }
    
27.     }
    
28. #endif
    
29. 
    
30. #if CONFIG_AVFILTER
    
31.     frame_available = ist->st->codec->codec_type
    
32.                       != AVMEDIA_TYPE_VIDEO ||
    
33.                       !ost->output_video_filter ||
    
34.                       avfilter_poll_frame(ost->output_video_filter->inputs[0]);
    
35.     while (frame_available) {
    
36.       if (ist->st->codec->codec_type == AVMEDIA_TYPE_VIDEO &&
    
37.           ost->output_video_filter) {
    
38.         AVRational ist_pts_tb = ost->output_video_filter->inputs[0]->time_base;
    
39.         if (av_vsink_buffer_get_video_buffer_ref(ost->output_video_filter,
    
40.             &ost->picref, 0) < 0)
    
41.           goto cont;
    
42.         if (ost->picref) {
    
43.           AVRational tempVar = {1, AV_TIME_BASE};
    
44.           avfilter_fill_frame_from_video_buffer_ref(&picture, ost->picref);
    
45.           //ist->pts = av_rescale_q(ost->picref->pts, ist_pts_tb, AV_TIME_BASE_Q);
    
46.           ist->pts = av_rescale_q(ost->picref->pts, ist_pts_tb, tempVar);//modify by chenrui
    
47.         }
    
48.       }
    
49. #endif
    
50. 
    
51.       编码;
    
52. 
    
53.       return 0;
    
54.       fail_decode:
    
55.       return -1;
    
56.    }
    
57.   }
    
58. }

先来看滤镜的连接：

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1. int av_vsrc_buffer_add_frame(AVFilterContext *buffer_src,
2.                              const AVFrame *frame, int flags)
3. {
4.     // 从帧列表中得到当前帧
5.     AVFilterBufferRef *picref =
6.         avfilter_get_video_buffer_ref_from_frame(frame, 
7.                                                  AV_PERM_WRITE);
8.     if (!picref)
9.         return AVERROR(ENOMEM);
10.     ret = av_vsrc_buffer_add_video_buffer_ref(buffer_src, 
11.                                               picref, flags);
12. 
    
13. 
    
14.     picref->buf->data[0] = NULL;
15.     avfilter_unref_buffer(picref);
16. 
    
17.     return ret;
18. }
19. 
    
20. 
    
21. int av_vsrc_buffer_add_video_buffer_ref(AVFilterContext *buffer_filter,
22.                                         AVFilterBufferRef *picref, int flags)"
23. {
24. 
    
25.     智能插入相应的filter; 
26. 
    
27.     c->picref = avfilter_get_video_buffer(outlink, AV_PERM_WRITE,
28.                                           picref->video->w, picref->video->h);
29. 
    
30.     av_image_copy(c->picref->data, c->picref->linesize,
31.                   picref->data, picref->linesize,
32.                   picref->format, picref->video->w, picref->video->h);
33.     avfilter_copy_buffer_ref_props(c->picref, picref);
34. 
    
35.     return 0;
36. }
37. 
    
38. 
    
39. AVFilterBufferRef *avfilter_get_video_buffer(AVFilterLink *link,
40.                                       int perms, int w, int h)
41. {
42.     AVFilterBufferRef *ret = NULL;
43. 
    
44.     if (link->dstpad->get_video_buffer)
45.         ret = link->dstpad->get_video_buffer(link, perms, w, h);
46. 
    
47.     if (!ret)
48.         ret = avfilter_default_get_video_buffer(link, perms, w, h);
49. 
    
50.     if (ret)
51.         ret->type = AVMEDIA_TYPE_VIDEO;
52. 
    
53.     return ret;
54. }
55. 
    
56. AVFilterBufferRef *get_video_buffer(AVFilterLink *link, int perms, 
57.                                     int w, int h)
58. {
59.     AVFilterBufferRef *picref;
60.     int width = FFALIGN(w, 32);
61.     int height= FFALIGN(h+2, 32);
62.     int i;
63. 
    
64.     picref = avfilter_default_get_video_buffer(link, perms, width, height);
65. 
    
66.     picref->video->w = w;
67.     picref->video->h = h;
68. 
    
69.     for (i = 0; i < 3; i++)
70.         picref->data[i] += picref->linesize[i];
71. 
    
72.     return picref;
73. }

最后来看图像处理函数的实现：

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1. int avfilter_poll_frame(AVFilterLink *link)
   
2. {
   
3.     int i, min = INT_MAX;
   
4. 
   
5.     if (link->srcpad->poll_frame)
   
6.         return link->srcpad->poll_frame(link);
   
7.     for (i = 0; i < link->src->input_count; i++) {
   
8.         int val;
   
9.         if (!link->src->inputs[i])
   
10.             return -1;
    
11.         val = avfilter_poll_frame(link->src->inputs[i]);
    
12.         min = FFMIN(min, val);
    
13.     }
    
14. 
    
15.     return min;
    
16. }
    
17. 
    
18. 
    
19. int poll_frame(AVFilterLink *link)
    
20. {
    
21.     YADIFContext *yadif = link->src->priv;
    
22.     int ret, val;
    
23. 
    
24.     if (yadif->frame_pending)
    
25.         return 1;
    
26. 
    
27.     val = avfilter_poll_frame(link->src->inputs[0]);
    
28. 
    
29.     if (val==1 && !yadif->next) { 
    
30.       //FIXME change API to not requre this red tape
    
31.       if ((ret = avfilter_request_frame(link->src->inputs[0])) < 0)
    
32.         return ret;
    
33.       val = avfilter_poll_frame(link->src->inputs[0]);
    
34.     }
    
35.     assert(yadif->next || !val);
    
36. 
    
37.     return val * ((yadif->mode&1)+1);
    
38. }
    
39. 
    
40. int avfilter_request_frame(AVFilterLink *link)
    
41. {
    
42. 
    
43. 
    
44.     if (link->srcpad->request_frame)
    
45.         return link->srcpad->request_frame(link);
    
46.     else if (link->src->inputs[0])
    
47.         return avfilter_request_frame(link->src->inputs[0]);
    
48.     else return -1;
    
49. }
    
50. 
    
51. 
    
52. 
    
53. static int request_frame(AVFilterLink *link)
    
54. {
    
55.     BufferSourceContext *c = link->src->priv;
    
56. 
    
57.     if (!c->picref) {
    
58.         av_log(link->src, AV_LOG_WARNING,
    
59.                "request_frame() called with no available frame!\n");
    
60.         return AVERROR(EINVAL);
    
61.     }
    

     // 图像处理函数的真正实现

1.     avfilter_start_frame(link, avfilter_ref_buffer(c->picref, ~0));
   
2.     avfilter_draw_slice(link, 0, link->h, 1);
   
3.     avfilter_end_frame(link);
   
4. 
   
5.     avfilter_unref_buffer(c->picref);
   
6.     c->picref = NULL;
   
7. 
   
8.     return 0;
   
9. }

完结。