XH-A1, colorspace, Color and chromakey

tverhaar

tverhaar

Active member
Hi Folks -

I am wondering if some XH-A1 users can comment on the issues surrounding chromakeying in the 4:2:0 colorspace of the XH-A1? I have pretty much been planning to buy an XH-A1 but with the release of Apple's FCS2 and the terrific capabilities of the Color app, I have been re-thinking a little. Is it really much easier/better to work in the 4:2:2 colorspace of the "pro" format cameras (DVCpro, XDCAM, etc) or is that not a major issue?

Any thoughts, comments or even further insightful questions to this thread would be most welcome.

Thanks.
 
4:2:2 helps for greenscreen, but you can get a good key from 4:2:0.

Check Barry's color space article in the articles section for more info.
 
Remember that good lighting plays just as an important role (if not more important) than color space in greenscreening. An unevenly lit scene in 4:2:2 will yield worst result than a well lit one shot at 4:2:0.
 
On a technical note: The luma sampling is actually fairly high with HDV, and if I'm not mistaken roughly 70% of the luma info is carried in the Y (green) channel, so in many ways greenscreen with HDV is like pulling a luma key--if it's lit properly and the green signal is well defined it should be farily easy to pull a quality key. It works even better if you convert your footage to a less compressed or lossless intermediate codec and process it from there. (Sheer, ProRes, CineForm etc.)
 
Doesn't quite work that way though. If you were to do a luma key, you'd get great results, but a chroma key is inherently limited by the color sampling of the recording format.

Put a blue object on a green screen, and a luma key could give you a perfect edge, but a chroma key is going to be blocky because of the 4:2:0 of the recording format.

Green usually does perform better than blue, but not anywhere near as good as the luma resolution would indicate; color res of 4:2:0 is 1/4 the res of the luma.

If you light everything perfectly, and use 24F mode (definitely not 60i mode!) you can probably pull a decent key from an A1. In those same circumstances 4:2:2 is going to be better, unquestionably. But 4:2:0 progressive isn't bad. 4:2:0 interlaced is a nightmare, but 4:2:0 progressive is okay. The XHA1 uses 4:2:0 progressive when recording 24F and 30F.

If you took a straight feed off the camera's analog outputs and captured with a capture card into a 4:2:2 format, you'd probably substantially improve the quality of any resulting keys from that footage.
 
Good points, Barry. All I know is that from my experiences greenscreen backgrounds have worked much better than blue when pulling keys from HDV recordings--and you're correct--the frame modes do work better because of progressive 4:2:0. And as I've indicated, it's better to transcode the material into a better 4:2:2 intermediate codec.

If you took a straight feed off the camera's analog outputs and captured with a capture card into a 4:2:2 format, you'd probably substantially improve the quality of any resulting keys from that footage.
Click to expand...

I've done this before and taken both the analog and HDV recordings (simultaneous)and pulled keys from both. There was a very negligible difference overall when using better software (AE Keylight, etc.) The actual 4:2:2 added color resolution can help when using a more quick and dirty keyer. (FCP built-in)
 
green screen in 60i

green screen in 60i

Hey kind of a newbie here.....Barry, why is it bad to shoot green screen in 60i mode? I just did a green screen shoot like that with an XH-A1 and I'm having a bit of trouble getting a good clean key
 
For a couple of reasons. First, interlaced video is just harder to get a key from in the first place, because of the very nature of interlacing and what it does to the edges on movement.

Second, the way color sampling works in interlaced 4:2:0 is simply horrid as compared to how clean and well it works in progressive 4:2:0. 4:2:0 comes in two flavors, either sensible (as in progressive modes) or preposterous (as in interlaced). The progressive form is a lot cleaner and easier to work with, the interlaced mode spreads the chroma across as many as four fields, making it much more difficult to key with.
 
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