View Full Version : Chroma Key 4:2:0

02 February 2005, 11:27 PM
Hello Everybody
(sorry about my english , thanks)

I have un PC XP pro, i turned a film with BlueScreen in Pal system with a DV cam . As we now the DV cam in PAL make a film with compression 4:2:0 signal. So the Chroma signal is very low. Wen i pick up the blue color at After Affect with Keylight , around the object i have many pixel.
My question is how i can blow up the signal 4:2:0 to 4:2:2 , to have a good as possible a the Chroma color.


02 February 2005, 10:12 AM
I doubt you can up-convert it with any accuracy since the information wasn't captured in the first place. Some people reccomend blurring the relevant colour component. Heres a good link about this:
He uses 4:1:1 video and so uses a 4pixel horizontal blur. With 4:2:0 i imagine you would use a 2x2 horizontal and vertical blur.


02 February 2005, 10:18 AM
Try using different types of keying. I know that Combustion lets you choose HLS or RGB among others. Also you probably will never be able to pull a KEY right off due to compression around the edges. You just want an even edge, even if it is a BLUE edge, if it's even all the way around, just use CHOKE to pull it down then feather the edges.

You should be able to do it just fine, but I don't know what quality your bluescreen was to start, maybe you should post an UNALTERED still from the short so we can look at it.

mJR (

02 February 2005, 04:48 PM
Ya, like the fellas have said. You can't really get the data that's been compressed out of 4:2:0 back. You can only make the best of what you got. With that said, I'd suggest in some fashion getting those missing data samples back through the best interpolation techiniques/tools you have. However, I would recommend against the outright blurring of the image, which may have been suggested. My point is... your luminance data is still all there, in the 4 part of the 4:2:0. You don't want to degrade that by blurring it needlessly. Also blurring, in my opinion, might not be the best depending on your software implimentation due to possible vignetting at the borders. What I have done using After Effects in the past with 4:2:2 footage to get interpolated (virtual) 4:4:4 footage is to retain a copy of the original footage. Then take a second copy of the footage, scale it down to half it's size horizontally 360x486 (NTSC) in a half horizontal rez comp, then in a second comp scale it back to full rez. In theory this uses After Effects bicubic scaling transform for interpolation of the chroma samples (the luma too, but we'll drop it for the full rez later). So with your now full rez (virtual 4:4:4) comp you layer the original footage back over it, but applying it in Luminosity mode, so that the luminance data from the original (which was full rez to begin with), gets mixed back in with the interpolated chroma. If I understand the the 4:2:0 scheme, the chroma gets sampled half as much at the luma per horizontal line in the frame. Plus the chroma only gets sampled every other line of each field. So to confuse things a bit, it sounds like the you have to scale both horizontally and vertically to half rez then rescale to full rez in 4:2:0 world. Following all this scaling you pull your matte and apply it.

02 February 2005, 01:11 PM
That's an interesting approach with the scaling. As you say I guess the most important thing is to preserve the luminance.
A few things that are worth trying:
1. Although not directly related to the 4:2:0 sampling, you can de-artifact the DV using the Remove Grain effect (it's very powerful) then pull your key form this footage, then apply back to the original DV footage.
2. Use Channel>Channel Combiner and set from RGB>YUV. Then apply Channel Blur to G and B channel. What you are doing is blurring the chroma values but leaving luminance (Y). You then use another instance of Channel Combiner after this but set it to YUV>RGB, then try keying the footage.
3. Also use Channel Combiner as a starting point for a matte based on luminance. Set luminance to Alpha, invert it and then use Alpha Levels to crush it. Place this layer on top of your Keylight matte.

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