[MPlayer-dev-eng] vo development and swscaler

[Reimar Döffinger]

On Mon, Mar 30, 2009 at 02:01:11PM -0400, compn wrote:
> On Mon, 30 Mar 2009 19:51:07 +0200, Reimar Döffinger wrote:
> >On Mon, Mar 30, 2009 at 06:06:50PM +0100, Samuel Garcia wrote:
> >> However, if I try to scale down a video, say from 32:64 to 16:32, I get 
> >> wierd results from the colors. For example, a black screen with a red 
> >> line down it, scales down to a smudged white line. It is as if the 
> >> swscaler does not understand the color space correctly.
> >
> >Please be more specific. At least missing information:
> >1) the source colourspace (-v might help finding out)
> >2) what does "a red line down it"?
> >3) how do you know the line is red? In particular since with YV12
> >colourspace it is impossible to have a single line with a specific
> >colour, since the colour is given always for two lines together.
> 
> i think he means his video is of a red line, which, when scaled, turns
> into an off-white line.

Well, if it is a diagonal red line artefacts are a well known and
unavoidable issue if YUV 4:2:0 is involved.
The only question would be if this is
1) because the input e.g. is YV12 with the chroma incorrect (slightly
offset compared to the official/correct place)
2) there is a bug in swscale concerning the chroma plane position (I
think e.g. -vo gl:yuv=2 actually treats chroma slightly wrong)
3) it is just an unavoidable aliasing artefact due to chroma having
lower resolution than luma