[mythtv-users] Deinterlacing Methods

Daniel Kristjansson danielk at cuymedia.net
Mon Jan 15 02:54:10 UTC 2007

On Sun, 2007-01-14 at 21:44 +0100, Wim Fokkens wrote:
> Hi Daniel
> > You will never get a perfect picture with a progressive display..
> I have to disagree with that.

I'm sorry but that is impossible.

> And the picture quality is absolutely perfect and easily beats the picture
> quality of my STB connected with RGB scart. I think the main reason for this
> easy win is the fact that my radeon card can do very good deinterlacing in
> hardware. And PowerDVD codec makes good use of this.

Very good deinterlacing is possible. And with material that was
originally not interlaced, like movies, you can reconstruct
the original progressive frame. But no physically constructable
device can display it perfectly. You would need to flash the
backlight on for an infinitesimally short time once per frame,
this would require infinite power. You could get close enough,
say by projecting a light which flickers on for a few
milliseconds through film 24 times per second. But LCD's and
plasma displays are far from this ideal. Plasma phosphors glow
for milliseconds, and worse the image is updated row by row.
LCD's backlights blink at something like 40,000 hz they
effectively give off light constantly. This gives us a nice
flicker free display for static images but it also means you
see the row by row update and the image is constant for a long
time so your eye can not reconstruct it into a moving image as
smooth as the moving image reconstructed by your mind when you
see images that flash onto the screen whole for a short period
of time. You can construct a device which shows progressive
material better using three DLPs or three LCDs and a shuttered
HID lamp, but these devices are very expensive; and you need
to watch them in a darkened room.

> I also have a MythTV pc with the following specs:
> I am trying very hard to get the same picture
> quality as my MyTheater system. But I don't even come close.
> A can get to the same level as James Buckley did.
> I also tried XVMC but judging from the picture it is only doing Hardware
> Motion Compensation but no hardware deinterlacing. 
> Why is this? Is there no support for hardware deinterlacing in the Nvidia
> driver?

There is some support. The XvMC API allows us to display the
odd field, even field, or both fields of an image. This is
enough for us to do either bob or one field deinterlacing.
But it is not enough for us to something like 3-2 pulldown,
i.e. perfect reconstruction of material that was originally
not interlaced. We also can't do anything like Lancos, kernel,
or linear blend with XvMC. nVidia has another library which
they call PureVideo, it is much better and is probably what
is being used in your MyTheater system but it is not available
on Linux unless you pay a good deal of money for it.

Mark Kendall has started on an OpenGL video renderer which
should allow us to implement some really slick deinterlacers
in hardware. But this isn't very far along yet and we will
not be writing an XvMC replacement in OpenGL fragment 
programs anytime soon; we may eventually do this since it
doesn't look like nVidia will be adding MPEG-4 AVC support
to XvMC...

-- Daniel

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