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Old December 18th, 2005, 12:27 PM   #1
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Downsampling 4:2:0 HDV footage to 4:2:2 SD footage.

Hi there all,

Not sure if this is the correct forum to post this in, so feel free to move it if there's a more appropriate area, btw...

So, I'm well aware of the technique of downsampling 4:2:0 HD(V) fooratge to get a 4:2:2 SD source, and of course it's done by discarding the (even?) scan lines with 0 chroma leaving only the 2-value chroma lines, and then presumably sampling every two horizontal pixels and averaging the result to reduce the horizontal resolution to 1/2 as well.

However, short of writing my own AE plugin, which is possible but a pain in the you-know-where, how can this be done by existing software?

I'm guessing it's possible in AviSynth, but is it possible/easy to do (with or without a plugin) in either Avid or After Effects? AviSynth methods are a bit unweidly for my liking when it comes to working on entire projects, not least in terms of HDD space when we're talking HD resolution!

Any ideas??
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Old December 18th, 2005, 03:15 PM   #2
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Dominic,

4:2:0 does not imply that there are scanlines with no chroma samples. Each chroma sample relates to a 2x2 block of luma samples.

Regardless, a straight downconvert would look good. Any NLE that handles HDV footage should be able to do the downconvert. Which are you using?

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Old December 19th, 2005, 12:49 PM   #3
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Hi Joshua,

Thanks, but still a little confused, to be honest...

My understanding of chroma sampling was that the ratio was in the form of
No of Luma samples : No of chroma samples on even scan lines : No of chroma samples on odd scanlines.

Is this not correct?? Could you please post a detailed description of how the chroma is stored in a 4:2:0 image??

Downconvertion isn't a problem, but I have read many times in many places (including here) that it is possible to downconvert 4:2:0 HD and result in true 4:2:2 SD - needless to say for grading this would be very nice if possible, so I'm trying to work out how to do that (assuming that it is possible!).

Many thanks for your help, I'm pretty uneducated in this part of video knowledge!...
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Old December 19th, 2005, 01:21 PM   #4
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Dominic,

The ratio is Y:Cb:Cr or Y:U:V. Really has nothing to do with scanlines. 4:2:0 is a special case. See the Wikipedia article and Adam Wilt's discussion. Note that there is a slight variation in the 4:2:0 used in PAL DV and HDV, MPEG, etc.

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Old December 20th, 2005, 04:46 PM   #5
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Ah, ok - thanks for clearing that up Joshua!

Think it's still going to take a little getting my head round, but I'm definitely better off on the knowledge front than before. :)
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