[Cin] DAR, PAR and SAR, again

[Andrew Randrianasulu]

сб, 20 янв. 2024 г., 19:49 Andrea paz <gamberucci.andrea at gmail.com>:

> I tried reading all the ffmpeg.C code inside
> ../cinelerra-5.1/cinelerra/ffmpeg.C
> I found these references to aspect ratio. I don't understand most of the
> lines.
>

you are welcome anyway! :)



> We start by setting the default parameters (line 1105):
>
> FFVideoStream::FFVideoStream(FFMPEG *ffmpeg, AVStream *strm, int idx, int
> fidx)
>  : FFStream(ffmpeg, strm, fidx),
>    FFVideoConvert(ffmpeg->ff_prefs())
> {
>     this->idx = idx;
>     width = height = 0;
>     transpose = 0;
>     frame_rate = 0;
>     aspect_ratio = 0;
>     length = 0;
>     interlaced = 0;
>     top_field_first = 0;
>     color_space = -1;
>     color_range = -1;
>     fconvert_ctx = 0;
> }
>
> Then after doing many steps, we arrive at the aspect ratio (lines 1954):
>


note, functions can be defined and then reused in non-linear fashion,
unlike linear interpreted languages. Just a remark.



> AVRational FFMPEG::to_sample_aspect_ratio(Asset *asset)
> #(==> SAR=AVRational)
> {
> #if 1
>     double display_aspect = asset->width / (double)asset->height;
> #(==> DAR=Wa/Ha; if SAR=1)
>     double sample_aspect = display_aspect / asset->aspect_ratio;
> #(==> SAR=DAR/(Wa/Ha))
>     int width = 1000000, height = width * sample_aspect + 0.5;
> #(==> W= n°; H=W x SAR + 0.5) ??
>     float w, h;
>     MWindow::create_aspect_ratio(w, h, width, height);
>     return (AVRational){(int)w, (int)h};     #(==> SAR = Int W / Int H) ??
> #else
> // square pixels
>     return (AVRational){1, 1};     #(==> SAR = 1:1)
> #endif
> }
>
> Then a long block dealing with various parameters including sar (lines
> 2548):
>
>
>         AVRational sar = av_guess_sample_aspect_ratio(fmt_ctx, st, NULL);
>         AVRational display_aspect_ratio;
>         if(sar.num) {
>
>             av_reduce(&display_aspect_ratio.num, &display_aspect_ratio.den,
>                   st->codecpar->width  * (int64_t)sar.num,    #(==>
> DAR.num = W x SAR.num)
>                   st->codecpar->height * (int64_t)sar.den,    #(==>
> DAR.den = W x SAR.den)
>                   1024 * 1024);
> /*        report("  Guessed SAR: %d:%d, ", sar.num, sar.den );  #(==>
> SAR is guessed from ffmpeg scan?)
>         report("DAR: %d:%d \n", display_aspect_ratio.num,
> display_aspect_ratio.den); */
>         }
>         if (st->sample_aspect_ratio.num)
>         {
>             av_reduce(&display_aspect_ratio.num, &display_aspect_ratio.den,
>                   st->codecpar->width  *
> (int64_t)st->sample_aspect_ratio.num,
>                   st->codecpar->height *
> (int64_t)st->sample_aspect_ratio.den,
>                   1024 * 1024);
>         report("  container Detected SAR: %d:%d , DAR %d:%d \n",
> st->sample_aspect_ratio.num, st->sample_aspect_ratio.den,
> display_aspect_ratio.num, display_aspect_ratio.den);
>         }
>         if (st->codecpar->sample_aspect_ratio.num)
>         {
>             av_reduce(&display_aspect_ratio.num, &display_aspect_ratio.den,
>                   st->codecpar->width  *
> (int64_t)st->codecpar->sample_aspect_ratio.num,
>                   st->codecpar->height *
> (int64_t)st->codecpar->sample_aspect_ratio.den,
>                   1024 * 1024);
>         report("  codec Detected SAR: %d:%d , DAR %d:%d \n",
> st->codecpar->sample_aspect_ratio.num,
> st->codecpar->sample_aspect_ratio.den, display_aspect_ratio.num,
> display_aspect_ratio.den);
>         }
>
>
> I think the whole block refers to "ffprobe", that is what we read in
> Detail.
>

yeah, I/we added it recently



> In a later block is the line (2777):
>
> vid->aspect_ratio = (double)st->sample_aspect_ratio.num /
> st->sample_aspect_ratio.den;   #(==> SAR=SAR.num/SAR.den) ??
>
> I do not know how to interpret, however.
>

those ratios expressed as A/B, not as arithmetical divide but structure
with two parts.


> At some point (lines 3660, but especially 3710) there are formulas
> that also involve PAR (like par = width x sar):
>
>
> int FFMPEG::ff_total_video_layers()
> {
>     return vstrm_index.size();
> }
>
> int FFMPEG::ff_total_vstreams()
> {
>     return ffvideo.size();
> }
>
> int FFMPEG::ff_video_width(int stream)
> {
>     FFVideoStream *vst = ffvideo[stream];
>     return !vst->transpose ? vst->width : vst->height;
> }
>
> int FFMPEG::ff_video_height(int stream)
> {
>     FFVideoStream *vst = ffvideo[stream];
>     return !vst->transpose ? vst->height : vst->width;
> }
>
> int FFMPEG::ff_set_video_width(int stream, int width)
> {
>     FFVideoStream *vst = ffvideo[stream];
>     int *vw = !vst->transpose ? &vst->width : &vst->height, w = *vw;
>     *vw = width;
>     return w;
> }
>
> int FFMPEG::ff_set_video_height(int stream, int height)
> {
>     FFVideoStream *vst = ffvideo[stream];
>     int *vh = !vst->transpose ? &vst->height : &vst->width, h = *vh;
>     *vh = height;
>     return h;
> }
>
> int FFMPEG::ff_coded_width(int stream)
> {
>     return ffvideo[stream]->avctx->coded_width;
> }
>
> int FFMPEG::ff_coded_height(int stream)
> {
>     return ffvideo[stream]->avctx->coded_height;
> }
>
> float FFMPEG::ff_aspect_ratio(int stream)
> {
>     //return ffvideo[stream]->aspect_ratio;
>     AVFormatContext *fmt_ctx = ffvideo[stream]->fmt_ctx;
>     AVStream *strm = ffvideo[stream]->st;
>     AVCodecParameters *par = ffvideo[stream]->st->codecpar;
>     AVRational dar;
>     AVRational sar = av_guess_sample_aspect_ratio(fmt_ctx, strm, NULL);
>         if (sar.num && ffvideo[stream]->get_rotation_angle() == 0) {
>             av_reduce(&dar.num, &dar.den,
>                       par->width  * sar.num,
>                       par->height * sar.den,
>                       1024*1024);
>                       return av_q2d(dar);
>                       }
>         return ffvideo[stream]->aspect_ratio;
> }
>
>
> Certainly, although the previous par were intended as "parameter,"
> this time it is definitely PAR as aspect ratio;


Structure named AvCodecParameters - I copied it from ffmpeg if I remember
correctly. So, par here really should be parameter.


but I don't know
> whether as "Pixel aspect ratio" or "Picture aspect ratio."
> So it would seem that ffmpeg uses it (or is it a variation of CinGG?).
>
> On line 3872 it is found:
>
>     AVCodecParameters *avpar = st->codecpar;
>     int sa_num = avpar->sample_aspect_ratio.num;
>     if( !sa_num ) sa_num = 1;
>     int sa_den = avpar->sample_aspect_ratio.den;
>     if( !sa_den ) sa_num = 1;
>
> I don't quite understand if it sets the sar parameters: if we specify
> them it takes the value given to it, otherwise it takes 1 by default.
>


I think your understanding is correct. Just this might be not something
user enters but libav* functions fills in during decoding.

>
> I couldn't find any other references to aspect ratio in ffmpeg.C.
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.cinelerra-gg.org/pipermail/cin/attachments/20240120/27767ce3/attachment-0001.htm>