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List:       xine-user
Subject:    Re: [xine-user] why does xshm look better then Xv?
From:       Mike Melanson <mike () multimedia ! cx>
Date:       2007-02-07 0:09:29
Message-ID: 45C918B9.3000504 () multimedia ! cx
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Dexter Filmore wrote:
> I just stumbled across this:
> 
> I use ATI fglrx 8.31 I just ran two instances of xine, so one uses the 
> fglrx-Xv-Overlay, one uses Xshm.
> 
> What can I say, Xshm looks better. Edges smoother.

Likely cause: The software algorithm that Xshm is using for rescaling
looks better in your perception than the video card's own hardware
scaling. Now all video hardware is created equally. What type of video
card is it?

> Next funny thing: taking screenshots with xine they look exactly the same, 
> pixel by pixel. 
> 
> Now - if the screenshots look alike, why not the live video?

During playback, the xine-lib engine sends encoded data to a video
decoder and gets a bitmap image out the other end. Then it sends that
bitmap image to the selected video output module which is then
responsible for scaling it. However, if a screenshot is requested, the
engine takes the raw bitmap, without scaling, and provides it to the
frontend for saving (or maybe it saves the file directly; I forget).

A curious artifact of this is sometimes you have strangely sized MPEG
videos, like 480x480 (SVCD res?), that have instructions in the stream
to rescale them to 720x480 during playback. If you ask xine to take a
screenshot, it will save the raw 480x480 image.

-- 
	-Mike Melanson

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