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：
cpp
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：
cpp
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：
cpp
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。