Re: xawtv and XVideo vs interlaced output

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On Mon, Dec 10, 2001 at 06:18:53AM -0800, Billy Biggs wrote:
> Gerd Knorr (kraxel@xxxxxxxxxxx):
> 
> > Please don't confuse *frame* rate and *field* rate.  The frame rate is
> > 30 fps only.
> 
>   But, you do understand that when watching TV, you don't see both
> fields at the same time.  [ ... ]

Hmm, well, yes and no.  The TV people play tricks there, using the fact
that the human eyes are too slow to actually see what really happens ...

> > Exactly.  I have a pending patch for v4l.c which makes this switchable
> > at runtime (both fields interlaced vs. one field scaled up).
> 
>   But both of these options are unacceptable.  In each case you give up
> too much quality.  A third option, scale both fields but to the same
> location, will have more flicker than optimal but will provide smooth
> motion with no CPU cost.

Scale both fields IMHO isn't acceptable too, because you'll get exactly
the issue described below:  The scanlines don't line up exactly and thus
you get a annonying flickering because the image jumps up and down half
a scanline all the time.  Depending on the refresh rate of your CRT this
might be very visible.

> > By using one field only ...
> 
>   Well that doesn't explain it either.  Since you're not
> double-buffering, the bttv could be DMA'ing a field right on top of the
> existing one.

It doesn't (any more).  Rationale: see above :-)

> > No, the bt848 has no problems to deal with different memory locations
> > for even/odd fields.
> 
>   Sure, but if you're scaling from a stream of 720x240 fields, you'd
> need to have every second field start a half-scanline below the first
> one, which you can't do with memory location tricks.  Instead, you must
> talk to the video card each time to tell it to change the scale
> destination for each field.

Doing this is close to impossible.  The X-Server runs in userland and
thus I can't easily make the irq handler handle this.  Also the X-Server
internal interfaces don't allow to setup overlays with sub-pixel
resolution (which you would need to get the half-scanline issue right).

>   I've written an app to deinterlace to full-frame 59.94hz output.  I'll
> post a version tonight. Motion is much smoother, and quality is similar
> to my TV.  Sports broadcasts and smooth text crawls are now smooth, and
> antialiased text still looks reasonable.  However this does eat alot
> more CPU than xawtv.

With plenty of CPU time at the problem you can fix it of course :-)

  Gerd

-- 
#define	ENOCLUE 125 /* userland programmer induced race condition */





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