Half-height picture by dropping a field

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sdm
Bright Spark User
Posts: 194
Joined: Mon Feb 19, 2007 4:53 pm

Half-height picture by dropping a field

Post by sdm »

As long as we're asking for ponies ...

I'd like to be able to take a 1920x1080 source, drop half the vertical resolution to become 1920x540 by dropping even or odd lines, then scale the image to 960x540.

I believe this would remove any need for de-interlacing, and leave a comb-free frame. My only concern with dropping fields is that we may not be left with smooth motion (because half the points-in-time are gone).

If its a good idea, it'd be cool to see it some day. If its a bad idea for the reason I mentioned or other reasons, I'd be interested to hear.

Thanks,
--sdm.

rhester
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Posts: 2888
Joined: Tue Apr 18, 2006 10:24 pm

Re: Half-height picture by dropping a field

Post by rhester »

This is available in AVISynth, and the problem is that the result is exactly what you state - half-resolution. It's extraordinarily wasteful in terms of available spatial resolution in frames with low motion, hence the prevalent use of motion-estimation for deinterlacing. You also have the nasty 'stairstep' effect on straight lines, which can sometimes be corrected for via solutions like EEDI...but in short, no, it will generally not produce acceptable results. If it were so simple, that would be all anyone ever uses. =)

Rodney

sdm
Bright Spark User
Posts: 194
Joined: Mon Feb 19, 2007 4:53 pm

Re: Half-height picture by dropping a field

Post by sdm »

Thanks Rodney.
You also have the nasty 'stairstep' effect on straight lines
I just want to clarify something if I wasn't clear - I want the resulting image to be 1/2 the vertical resolution, and 1/2 the horizontal resolution. So the picture is 1/4 the original size (in area). The way I picture it, the horizontal scaling removes the stairstep effect.
Unless, I suppose, because the two lines were always meant to be separated by a line (from the other field), a colour difference produce and unwanted effect.
Is that it?

--sdm.

sdm
Bright Spark User
Posts: 194
Joined: Mon Feb 19, 2007 4:53 pm

Re: Half-height picture by dropping a field

Post by sdm »

It's extraordinarily wasteful in terms of available spatial resolution in frames with low motion
Yes, I guess thats true, especially if the thrown away lines contain important detail (albeit from a different time). This is a bad idea.

--sdm.

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JohnAStebbins
HandBrake Team
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Joined: Sat Feb 09, 2008 7:21 pm

Re: Half-height picture by dropping a field

Post by JohnAStebbins »

The stairstep effect would not be mitigated by the horizontal scaling. In fact, your producing opportunity for more stairsteps. Picture a 1 pixel wide line that's on a slight incline. Before decimation, each step up is 1/1080 of the picture height. After decimation each step is 1/540 of the picture height. The step is twice as large and thus twice a visible. The horizontal scaling has no affect on the size of this step.

jbrjake
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Joined: Wed Dec 13, 2006 1:38 am

Re: Half-height picture by dropping a field

Post by jbrjake »

rhester wrote:You also have the nasty 'stairstep' effect on straight lines, which can sometimes be corrected for via solutions like EEDI...
I'm looking at EEDI2 -- I snagged a copy of the source code a few weeks ago when I was gathering pieces for the decomb algorithm from tritical's website. I think it *might* be possible to port it over pretty easily. I haven't taken the time to fully figure out its logic yet. But I believe it should be possible to do something like yadifmod, patching in the EEDI2 output to replace yadif_check's simple edge-directed interpolation.

sdm
Bright Spark User
Posts: 194
Joined: Mon Feb 19, 2007 4:53 pm

Re: Half-height picture by dropping a field

Post by sdm »

I thought I could prove you wrong John and Rodney...

Instead, proved myself wrong:
Image
Top is supposed to be my original,
Middle is after dropping a field,
Bottom is after horizontal scale.

Forgive me.
-sdm.

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