[FFmpeg-devel] pixel format convertion yuv422p->yuv420p->yuv411p->yuv422p degrade picture

Måns Rullgård mans
Tue Jul 15 18:44:10 CEST 2008

Maksym Veremeyenko wrote:
> Michael Niedermayer ???????(??):
>> On Tue, Jul 15, 2008 at 04:22:33PM +0100, M?ns Rullg?rd wrote:
> [...]
>>> Without thinking a little harder, I don't know whether such a conversion
>>> sequence is possible without such drift.  It will always be lossy, as I'm
>>> sure you already realise.
> I did cyclic test for:
>   +->422->420->422->411->422-->+
>   |                            |
>   +----------------------------+
> no visial degrades found.

The colour drift aside, I'd expect to see gradual, if slight, blurring
of the chroma planes.

>> I suspect that dithering could reduce the drift, for normal interpolation
>> based filters but running 50 converters in a row is a stupid thing to do.
> Such condition possible in:
> Uncompressed (4:2:2) -> DV25 (4:2:0) -> DVCpro25 (4:1:1) -> Uncompr

If it hurts, don't do it.  That looks like it would hurt.  In fact,
4:1:1 hurts as is, even without any additional degrading.

> 50 iterations was only for proving that convertions degrade picture...
> So, as conclusion, that a normal?

The conclusion is that it's probably not worthwhile doing anything about
it.  It would make the code slower with no real benefit in practical use

M?ns Rullg?rd
mans at mansr.com

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