一、引言

在上一节《音视频入门基础：H.264专题（11）——计算视频分辨率的公式》中，讲述了通过SPS中的属性计算H.264编码的视频的分辨率的公式。本文讲解FFmpeg源码中计算视频分辨率的实现。

二、FFmpeg源码中计算视频分辨率的实现

从文章《音视频入门基础：H.264专题（10）——FFmpeg源码中，存放SPS属性的结构体和解码SPS的函数分析》中，我们可以知道，FFmpeg源码中通过ff_h264_decode_seq_parameter_set函数解码SPS，从而拿到SPS中的属性。

在ff_h264_decode_seq_parameter_set函数中有如下代码，通过下面的这部分代码拿到计算视频分辨率所需的属性：

int ff_h264_decode_seq_parameter_set(GetBitContext *gb, AVCodecContext *avctx,
                                     H264ParamSets *ps, int ignore_truncation)
{
    //...
    
    sps->gaps_in_frame_num_allowed_flag = get_bits1(gb);
    sps->mb_width                       = get_ue_golomb(gb) + 1;
    sps->mb_height                      = get_ue_golomb(gb) + 1;

    sps->frame_mbs_only_flag = get_bits1(gb);

    if (sps->mb_height >= INT_MAX / 2U) {
        av_log(avctx, AV_LOG_ERROR, "height overflow\n");
        goto fail;
    }
    sps->mb_height *= 2 - sps->frame_mbs_only_flag;

    //...

    sps->crop = get_bits1(gb);
    if (sps->crop) {
        unsigned int crop_left   = get_ue_golomb(gb);
        unsigned int crop_right  = get_ue_golomb(gb);
        unsigned int crop_top    = get_ue_golomb(gb);
        unsigned int crop_bottom = get_ue_golomb(gb);
        int width  = 16 * sps->mb_width;
        int height = 16 * sps->mb_height;

        if (avctx->flags2 & AV_CODEC_FLAG2_IGNORE_CROP) {
            av_log(avctx, AV_LOG_DEBUG, "discarding sps cropping, original "
                                           "values are l:%d r:%d t:%d b:%d\n",
                   crop_left, crop_right, crop_top, crop_bottom);

            sps->crop_left   =
            sps->crop_right  =
            sps->crop_top    =
            sps->crop_bottom = 0;
        } else {
            int vsub   = (sps->chroma_format_idc == 1) ? 1 : 0;
            int hsub   = (sps->chroma_format_idc == 1 ||
                          sps->chroma_format_idc == 2) ? 1 : 0;
            int step_x = 1 << hsub;
            int step_y = (2 - sps->frame_mbs_only_flag) << vsub;

            if (crop_left  > (unsigned)INT_MAX / 4 / step_x ||
                crop_right > (unsigned)INT_MAX / 4 / step_x ||
                crop_top   > (unsigned)INT_MAX / 4 / step_y ||
                crop_bottom> (unsigned)INT_MAX / 4 / step_y ||
                (crop_left + crop_right ) * step_x >= width ||
                (crop_top  + crop_bottom) * step_y >= height
            ) {
                av_log(avctx, AV_LOG_ERROR, "crop values invalid %d %d %d %d / %d %d\n",     
                      crop_left, crop_right, crop_top, crop_bottom, width, height);
                goto fail;
            }

            sps->crop_left   = crop_left   * step_x;
            sps->crop_right  = crop_right  * step_x;
            sps->crop_top    = crop_top    * step_y;
            sps->crop_bottom = crop_bottom * step_y;
        }
    } else {
        sps->crop_left   =
        sps->crop_right  =
        sps->crop_top    =
        sps->crop_bottom =
        sps->crop        = 0;
    }

    //...
}

然后在FFmpeg源码的源文件libavcodec/h264_parser.c的parse_nal_units函数中，有如下代码：

static inline int parse_nal_units(AVCodecParserContext *s,
                                  AVCodecContext *avctx,
                                  const uint8_t * const buf, int buf_size)
{
    //...
    
    for (;;) {
        switch (nal.type) {
        case H264_NAL_SPS:
            ff_h264_decode_seq_parameter_set(&nal.gb, avctx, &p->ps, 0);
            break;
         
        //...

        case H264_NAL_IDR_SLICE:
        
        //...

        s->coded_width  = 16 * sps->mb_width;
        s->coded_height = 16 * sps->mb_height;
        s->width        = s->coded_width  - (sps->crop_right + sps->crop_left);
        s->height       = s->coded_height - (sps->crop_top   + sps->crop_bottom);
        if (s->width <= 0 || s->height <= 0) {
            s->width  = s->coded_width;
            s->height = s->coded_height;
        }
        //... 
        }
        //...
    }
}

可以看到parse_nal_units函数中最终是通过下面的语句拿到视频分辨率的：

s->width = s->coded_width - (sps->crop_right + sps->crop_left);
s->height = s->coded_height - (sps->crop_top + sps->crop_bottom);

可以看到FFmpeg源码中计算视频分辨率的实现跟文章《音视频入门基础：H.264专题（11）——计算视频分辨率的公式》中描述的公式是一致的。