[MPlayer-dev-eng] Fw: Vertical bands in RT21 encoded AVIs

[Reimar Döffinger]

On Mon, May 12, 2014 at 01:13:58PM -0700, Mats Peterson wrote:
> Well, I noticed that even in Windows 3.1 vertical bands will appear, although to a lesser degree. It is more noticeable in these VfW samples than in other RT21 encoded videos. I understand that this is not top priority, since the RT21 codec is obsolete since long. Thanks for the reply anyway.

I don't think there is anything wrong with the decoding.
The way the encoding works is that it copies the previous line and then
applies deltas to it.
The most efficient compression is when you don't save any deltas.
As a result, the encoder will tend to simply copy random noise from the
first line (or whichever line was reasonably similar), thus generating
vertical lines.
Obviously a good encoder would avoid this by e.g. going back and adjusting
the first line it encoded so that overall image quality would be better,
but probably people at that time cared more about encoder speed than
quality.
Though I admit it might be that this first line isn't quite correctly
decoded, we'd need the output of the reference decoder for that to
compare.
As for washed-out colours: That probably is a disagreement on
the YUV to RGB conversion function used.
-vo gl:colorspace=... has a few options (0 to 4) but the right one
to use for this codec might just be missing.
We had similar issues with other codecs that used somewhat modified
YUV conversion functions to make decoding simpler.
Either way, unless this isn't reproducible with ffplay/ffmpeg that
should be reported to the FFmpeg project instead.

> ________________________________
>  From: Carl Eugen Hoyos <cehoyos at ag.or.at>
> To: mplayer-dev-eng at mplayerhq.hu 
> Sent: Monday, May 12, 2014 3:36 PM
> Subject: Re: [MPlayer-dev-eng] Fw: Vertical bands in RT21 encoded AVIs
>  
> 
> Mats Peterson <matsp888 <at> yahoo.com> writes:
> 
> > Furthermore, they are washed out with bad contrast, 
> > blacks are gray rather than black, etc. I have played
> > the videos in Windows 3.1 in DOSBox, and they look 
> > much better there.
> 
> Only the black or also the vertical bands?
> 
> Please understand that this should not be discussed 
> here, use the ffmpeg-user mailing list.
> 
> Carl Eugen
> 
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