【ffplay】视频的宽高比详解 -PAR、DAR 和 SAR

---

前言

在做播放器渲染的时候会遇到渲染图像拉伸的情况，一般遇到这种图像显示比例不正确，多半是没有正确处理视频的纵横比。

简单介绍以下视频的三种纵横比，PAR、DAR 和 SAR，以及实际开发当中需要注意的情况。

Pixel Aspect Ratio 像素纵横比

像素是显示图像的最小单位，就是一个一个的小格子。一般来说像素的宽高比都是 1:1 ；如果不是1:1， 则该像素可以理解为长方形像素。常用的PAR比率有(1:1，10:11, 40:33, 16:11, 12:11 )

现代设备的像素纵横比都是 1:1的。（个人猜测，如果有错误地方欢迎指正）

Display Aspect Ratio 显示纵横比

DAR 显示纵横比，可以理解为实际显示到屏幕上的宽高比例。
拖拽播放窗口要按照这个比例缩放，否则图像会拉伸。一般来说 16：9 、 4：3 比较常见。

Sample Aspect Ratios 采样纵横比

SAR 采样纵横比，实际上告诉你视频需要按照采样纵横比拉伸。这样渲染出来的视频才是实际的要显示的效果。

网上很多博客说 SAR 表示横向的像素点数和纵向的像素点数的比值，即为我们通常提到的分辨率宽高比。其实这也是错误的。不知道谁开的头。但凡看看 ffplay源码也不至于这样人云亦云。

采样纵横比的意义是用来计算实际要渲染的显示纵横比。

实际的显示纵横比 DAR 计算公式如下：

实际显示纵横比(DAR) = SAR(采样纵横比) * 原视频的width / 原视频的height

比如一个视频的 width = 1280 height = 720 sar = 81:256
实际的渲染比 DAR = 81/ 256 * 1280 / 720 = 9: 16

如果不考虑采样纵横比，渲染出来的图像是16：9的，会出现图像拉伸情况（竖版视频以横板显示）。实际上是 9：16的渲染比例。

工程实战

实际开发中，我们只关心采样纵横比就可以。渲染图像要根据上面的公式计算出实际的显示纵横比，按照实际的显示纵横比去渲染，才不会出现图像拉伸的情况。

FFmpeg 获取采样纵横比的api av_guess_sample_aspect_ratio

/**
 * Guess the sample aspect ratio of a frame, based on both the stream and the
 * frame aspect ratio.
 *
 * Since the frame aspect ratio is set by the codec but the stream aspect ratio
 * is set by the demuxer, these two may not be equal. This function tries to
 * return the value that you should use if you would like to display the frame.
 *
 * Basic logic is to use the stream aspect ratio if it is set to something sane
 * otherwise use the frame aspect ratio. This way a container setting, which is
 * usually easy to modify can override the coded value in the frames.
 *
 * @param format the format context which the stream is part of
 * @param stream the stream which the frame is part of
 * @param frame the frame with the aspect ratio to be determined
 * @return the guessed (valid) sample_aspect_ratio, 0/1 if no idea
 */
AVRational av_guess_sample_aspect_ratio(AVFormatContext *format, AVStream *stream, AVFrame *frame);

// 优先考虑 stream->sample_aspect_ratio 其次考虑 frame->sample_aspect_ratio
AVRational av_guess_sample_aspect_ratio(AVFormatContext *format, AVStream *stream, AVFrame *frame)
{
    AVRational undef = {0, 1};
    AVRational stream_sample_aspect_ratio = stream ? stream->sample_aspect_ratio : undef;
    AVRational codec_sample_aspect_ratio  = stream && stream->codecpar ? stream->codecpar->sample_aspect_ratio : undef;
    AVRational frame_sample_aspect_ratio  = frame  ? frame->sample_aspect_ratio  : codec_sample_aspect_ratio;

    av_reduce(&stream_sample_aspect_ratio.num, &stream_sample_aspect_ratio.den,
               stream_sample_aspect_ratio.num,  stream_sample_aspect_ratio.den, INT_MAX);
    if (stream_sample_aspect_ratio.num <= 0 || stream_sample_aspect_ratio.den <= 0)
        stream_sample_aspect_ratio = undef;

    av_reduce(&frame_sample_aspect_ratio.num, &frame_sample_aspect_ratio.den,
               frame_sample_aspect_ratio.num,  frame_sample_aspect_ratio.den, INT_MAX);
    if (frame_sample_aspect_ratio.num <= 0 || frame_sample_aspect_ratio.den <= 0)
        frame_sample_aspect_ratio = undef;

    if (stream_sample_aspect_ratio.num)
        return stream_sample_aspect_ratio;
    else
        return frame_sample_aspect_ratio;
}

ffplay SDL渲染窗口宽高的计算

ffplay 在渲染视频的时候是考虑了视频的采样纵横比，这样才可以正确的渲染出采样纵横比不为1 的视频，这样的播放器兼容性更高一些。

下面贴出来关键的处理代码。

void read_thread()
{
	// 根据视频的SAR 采样纵横比 计算 sdl播放窗口的默认宽高
	if (st_index[AVMEDIA_TYPE_VIDEO] >= 0) {
        AVStream *st = ic->streams[st_index[AVMEDIA_TYPE_VIDEO]];
        AVCodecParameters *codecpar = st->codecpar;
        AVRational sar = av_guess_sample_aspect_ratio(ic, st, NULL);
        if (codecpar->width)
            set_default_window_size(codecpar->width, codecpar->height, sar);
    }
}

// 计算 sdl渲染窗口的 width and height 
static void calculate_display_rect(SDL_Rect *rect,
                                   int scr_xleft, int scr_ytop, int scr_width, int scr_height,
                                   int pic_width, int pic_height, AVRational pic_sar)
{
	// 视频的SAR 采样纵横比
    AVRational aspect_ratio = pic_sar;
    int64_t width, height, x, y;

    if (av_cmp_q(aspect_ratio, av_make_q(0, 1)) <= 0)
        aspect_ratio = av_make_q(1, 1);

	// aspect_ratio 输出的宽高比 = 采样纵横比aspect_ratio * 视频width / 视频height
    aspect_ratio = av_mul_q(aspect_ratio, av_make_q(pic_width, pic_height));

    /* XXX: we suppose the screen has a 1.0 pixel ratio */
    height = scr_height;
    width = av_rescale(height, aspect_ratio.num, aspect_ratio.den) & ~1;
    if (width > scr_width) {
        width = scr_width;
        height = av_rescale(width, aspect_ratio.den, aspect_ratio.num) & ~1;
    }
    x = (scr_width - width) / 2;
    y = (scr_height - height) / 2;
    rect->x = scr_xleft + x;
    rect->y = scr_ytop  + y;
    rect->w = FFMAX((int)width,  1);
    rect->h = FFMAX((int)height, 1);
}

---

技术参考

0. Advanced Aspect Ratios - PAR, DAR and SAR：https://www.animemusicvideos.org/guides/avtech3/theory-videoaspectratios.html