代码在：h264/sps_parser.cc

//
// IMPORTANT ONES! Now we're getting to resolution. First we read the pic
// width/height in macroblocks (16x16), which gives us the base resolution,
// and then we continue on until we hit the frame crop offsets, which are used
// to signify resolutions that aren't multiples of 16.
//
// pic_width_in_mbs_minus1: ue(v)
sps.width = 16 * (reader.ReadExponentialGolomb() + 1);

// Figure out the crop units in pixels. That's based on the chroma format's
// sampling, which is indicated by chroma_format_idc.
if (sps.separate_colour_plane_flag || chroma_format_idc == 0) {frame_crop_bottom_offset *= (2 - sps.frame_mbs_only_flag);frame_crop_top_offset *= (2 - sps.frame_mbs_only_flag);
} else if (!sps.separate_colour_plane_flag && chroma_format_idc > 0) {// Width multipliers for formats 1 (4:2:0) and 2 (4:2:2).if (chroma_format_idc == 1 || chroma_format_idc == 2) {frame_crop_left_offset *= 2;frame_crop_right_offset *= 2;}// Height multipliers for format 1 (4:2:0).if (chroma_format_idc == 1) {frame_crop_top_offset *= 2;frame_crop_bottom_offset *= 2;}
}
// Subtract the crop for each dimension.
sps.width -= (frame_crop_left_offset + frame_crop_right_offset);
sps.height -= (frame_crop_top_offset + frame_crop_bottom_offset);

读取图像的宽度和高度，以宏块（16x16）为单位，这给出了基本分辨率。然后，继续读取直到遇到帧裁剪偏移量，这些偏移量用于表示不是16的倍数的分辨率。

在这段代码中，“pic_width_in_mbs_minus1” 是一个无符号指数哥伦布编码（ue(v)），用于计算图像的宽度。最后一行代码则将图像的宽度设置为16乘以读取的值加1。

根据色度格式和色度平面标志来调整帧的裁剪偏移量，然后根据计算得到的偏移量来调整图像的宽度和高度，以实现正确的裁剪。

即支持16x16的块，然后按2的倍数做裁减。