[Cellar] How to Document an Unknown DAR

[Dave Rice <[e-mail]>]

Hi all,

Kieran O'Leary pointed this behavior out to me and I believe this is an issue with the documentation on aspect ratios. Many file formats have an method to express an unknown SAR. In ffmpeg this is reported as a SAR of 0:1. A sample may be made with `ffmpeg -f lavfi -i testsrc -t -1 -c:v v210 no_sar.mov`.

When muxig the streams into Matroska, to be lossless, the resulting Matroska file should also express that the SAR is unknown. However with `ffmpeg -i no_sar.mov -c copy is_there_sar.mkv`, the output file sets the PixelWidth and PixelHeight correctly, and does not store the DisplayWidth or DisplayHeight. The problem is that the DisplayWidth and DisplayHeight default to equations based upon PixelWidth and PixelHeight. Thus the resulting file as a DisplayWidth equal to PixelWidth and DisplayHeight equal to PixelHeight. Thus Matroska is not capable of communicating any difference between a 1/1 SAR and an unknown (0/1) SAR. The source file expresses an uncertainty but in the resulting file the same concept is now a certainty.

Dave Rice