FFmpeg libswscale源码分析3-scale滤镜源码分析

config_props() -->
sws_init_context() -->
ff_get_unscaled_swscale() -->

static int config_props(AVFilterLink *outlink)
{
    AVFilterContext *ctx = outlink->src;
    AVFilterLink *inlink0 = outlink->src->inputs[0];
    AVFilterLink *inlink  = ctx->filter == &ff_vf_scale2ref ?
                            outlink->src->inputs[1] :
                            outlink->src->inputs[0];
    enum AVPixelFormat outfmt = outlink->format;
    const AVPixFmtDescriptor *desc = av_pix_fmt_desc_get(inlink->format);
    ScaleContext *scale = ctx->priv;
    ......
    if (inlink0->w == outlink->w &&
        inlink0->h == outlink->h &&
        !scale->out_color_matrix &&
        scale->in_range == scale->out_range &&
        inlink0->format == outlink->format)
        // 如果当前 scale 滤镜的输入和输出一样，此处不作任何初始化动作
        ;
    else {
        // scale->sws 用于一帧图像，scale->isws[0] 隔行帧顶场，scale->isws[1] 用于隔行帧底场
        struct SwsContext **swscs[3] = {&scale->sws, &scale->isws[0], &scale->isws[1]};
        int i;

        for (i = 0; i < 3; i++) {
            int in_v_chr_pos = scale->in_v_chr_pos, out_v_chr_pos = scale->out_v_chr_pos;
            struct SwsContext **s = swscs[i];
            *s = sws_alloc_context();
            if (!*s)
                return AVERROR(ENOMEM);
            
            // 将 ffmpeg 命令行中传入的参数(命令行未给出的参数取默认值)设置到 SwsContext
            av_opt_set_int(*s, "srcw", inlink0 ->w, 0);
            av_opt_set_int(*s, "srch", inlink0 ->h >> !!i, 0);
            av_opt_set_int(*s, "src_format", inlink0->format, 0);
            av_opt_set_int(*s, "dstw", outlink->w, 0);
            av_opt_set_int(*s, "dsth", outlink->h >> !!i, 0);
            av_opt_set_int(*s, "dst_format", outfmt, 0);
            av_opt_set_int(*s, "sws_flags", scale->flags, 0);
            av_opt_set_int(*s, "param0", scale->param[0], 0);
            av_opt_set_int(*s, "param1", scale->param[1], 0);
            if (scale->in_range != AVCOL_RANGE_UNSPECIFIED)
                av_opt_set_int(*s, "src_range",
                               scale->in_range == AVCOL_RANGE_JPEG, 0);
            if (scale->out_range != AVCOL_RANGE_UNSPECIFIED)
                av_opt_set_int(*s, "dst_range",
                               scale->out_range == AVCOL_RANGE_JPEG, 0);

            if (scale->opts) {
                AVDictionaryEntry *e = NULL;
                while ((e = av_dict_get(scale->opts, "", e, AV_DICT_IGNORE_SUFFIX))) {
                    if ((ret = av_opt_set(*s, e->key, e->value, 0)) < 0)
                        return ret;
                }
            }
            /* Override yuv420p default settings to have the correct (MPEG-2) chroma positions
             * MPEG-2 chroma positions are used by convention
             * XXX: support other 4:2:0 pixel formats */
            if (inlink0->format == AV_PIX_FMT_yuv420p && scale->in_v_chr_pos == -513) {
                in_v_chr_pos = (i == 0) ? 128 : (i == 1) ? 64 : 192;
            }

            if (outlink->format == AV_PIX_FMT_yuv420p && scale->out_v_chr_pos == -513) {
                out_v_chr_pos = (i == 0) ? 128 : (i == 1) ? 64 : 192;
            }

            av_opt_set_int(*s, "src_h_chr_pos", scale->in_h_chr_pos, 0);
            av_opt_set_int(*s, "src_v_chr_pos", in_v_chr_pos, 0);
            av_opt_set_int(*s, "dst_h_chr_pos", scale->out_h_chr_pos, 0);
            av_opt_set_int(*s, "dst_v_chr_pos", out_v_chr_pos, 0);
            
            // 调用初始化函数 sws_init_context()
            if ((ret = sws_init_context(*s, NULL, NULL)) < 0)
                return ret;
            if (!scale->interlaced)    // 未启用隔行标志，则不处理 scale->isws[0] 和 scale->isws[1]
                break;
        }
    }

    ......

    return 0;

fail:
    return ret;
}

static int filter_frame(AVFilterLink *link, AVFrame *in)
{
    ScaleContext *scale = link->dst->priv;
    AVFilterLink *outlink = link->dst->outputs[0];
    AVFrame *out;
    const AVPixFmtDescriptor *desc = av_pix_fmt_desc_get(link->format);
    
    ......
    
    // 1. 色度子采样因子
    //    log2_chroma_w 指由一行亮度样本数(luma width)右移多少位得到一行色度样本数(chroma width)
    //    log2_chroma_h 指由亮度样本行数(luma height)右移多少位得到色度样本行数(chroma height)
    //    以 YUV410P像素格式为例，
    //    水平方向子采样因子为 1/4，则 scale->hsub = desc->log2_chroma_w = 2
    //    垂直方向子采样因子为 1/2，则 scale->vsub = desc->log2_chroma_h = 1
    scale->hsub = desc->log2_chroma_w;    // 水平方向
    scale->vsub = desc->log2_chroma_h;

    ......
    
    // 2. 拷贝帧属性
    av_frame_copy_props(out, in);

    ......
    // 3. 调用 scale_slice() 函数执行转换，分三种情况：
    if(scale->interlaced>0 || (scale->interlaced<0 && in->interlaced_frame)){
        // 3.1 scale->interlaced 的值由 scale 滤镜的 interl 参数确定，有三个值：
        //     1: 使能隔行缩放方式
        //     0：禁用隔行缩放方式
        //     -1: 根据源帧中的隔行/逐行标志决定是使用隔行缩放还是逐行缩放
        //     此处 if 第一个分支，即进行隔行缩放
        scale_slice(link, out, in, scale->isws[0], 0, (link->h+1)/2, 2, 0);
        scale_slice(link, out, in, scale->isws[1], 0,  link->h   /2, 2, 1);
    }else if (scale->nb_slices) {
        // 3.2 此处 if 的第二个分支，是逐行缩放，一个图像帧有多个 slice 的情况
        int i, slice_h, slice_start, slice_end = 0;
        const int nb_slices = FFMIN(scale->nb_slices, link->h);
        for (i = 0; i < nb_slices; i++) {
            slice_start = slice_end;
            slice_end   = (link->h * (i+1)) / nb_slices;
            slice_h     = slice_end - slice_start;
            scale_slice(link, out, in, scale->sws, slice_start, slice_h, 1, 0);
        }
    }else{
        // 3.3 此处 if 第三个分支，是逐行缩放，一个图像帧只有一个 slice 的情况
        scale_slice(link, out, in, scale->sws, 0, link->h, 1, 0);
    }


    return ff_filter_frame(outlink, out);
}

static int scale_slice(AVFilterLink *link, AVFrame *out_buf, AVFrame *cur_pic, struct SwsContext *sws, int y, int h, int mul, int field)
{
    ScaleContext *scale = link->dst->priv;
    const uint8_t *in[4];
    uint8_t *out[4];
    int in_stride[4],out_stride[4];
    int i;

    for(i=0; i<4; i++){
        int vsub= ((i+1)&2) ? scale->vsub : 0;
         in_stride[i] = cur_pic->linesize[i] * mul;
        out_stride[i] = out_buf->linesize[i] * mul;
         in[i] = cur_pic->data[i] + ((y>>vsub)+field) * cur_pic->linesize[i];
        out[i] = out_buf->data[i] +            field  * out_buf->linesize[i];
    }
    if(scale->input_is_pal)
         in[1] = cur_pic->data[1];
    if(scale->output_is_pal)
        out[1] = out_buf->data[1];

    return sws_scale(sws, in, in_stride, y/mul, h,
                         out,out_stride);
}