Red/Blue blockiness and Chromakey

C

cbrandin

Well-known member
I was wondering... I know DV cameras have trouble with blockiness with pure red and pure blue. I've also learned (here) that blue/green screen work is very hard to get right with the DVX. I'm guessing that's partially because of the blockiness problem. So, I have some questions:

Is blockiness a problem with all pure primary colors - is it just with red and blue, or also with green?

How about subtractive primary colors - like cyan, magenta and yellow?

I'm trying to figure out if chromakeying would work better with a cyan colored screen.

Chris
 
It's not a problem with the DVX, it's a problem with all DV cameras due to the recording format's 4:1:1 color sampling.

Color is encoded in DV by using one four-pixel-wide block to average all the colors together. Every group of four pixels gets set to the same color. The brightness can vary per pixel, but color is mapped as a block of four pixels.

As such, the best way to key would probably be to structure your shot so that you can do a luminance key. You have full brightness information for every pixel, so you have 720x480 of brightness; chroma keying restricts you to 180x480 resolution (hence low-res blocky edges).

Red shows the worst, but pure blue can do it too. If something is lit with a primarily red light or a primarily blue light you're only going to get 1/4 the potential resolution; that's why shooting plays or concerts can be troublesome when the lighting director decides that they need a "red wash" over everything!

As for yellow/cyan/magenta, I don't know anyone who's ever tried that, and I doubt it would provide any better results, but by all means experiment and let us know how it turns out. I'm sure you're going to find that with any pure color the 4:1:1 color sampling is going to play havoc with your keying.

One thing you can do is to blur the chroma channels to get rid of those sharp stair-step edges; most NLE's probably have a "chroma blur" filter by now, I know Vegas does.
 
Yes, or just have Andromeda chip installed, that will give you color sampling of 4:4:4 + uncompressed HD out of the DVX. Even if your DVX had 4:2:2 sampling, you'd still deal with compression to DV tape and end up working with compressed footage which results in artifacts.

Working with pure uncompressed 4:4:4 footage is like taking a candy from a baby! :Drogar-Love(DBG):
 
I have actually never had a problem keying with my DVX. I think great lighting and proper editing tools can help your keys look truly great.

For the record, it should be noted that I have also keyed DVCPro footage and I honestly couldn't tell the difference from DV.
 
You shouldn't see the difference. At the compressed-data level DVCPRO = DV, they're the exact same compression etc. The difference between the two is primarily what type of tape they're recorded on.

(well, plus in PAL DVCPRO uses 4:1:1, but that's another kettle of fish)
 
Barry Green, you are a technology machine.

Thanks for the response, by the way. I had no clue they were the same compression.
 
Actually the main difference, if I remember well, is DVCPRO has SMPTE time code, which means it's frame accurate, DV is not. compression is the same.
 
Green

Green

I've done a lot of chroma keying with DV. Green is probably your best bet, as the green channel is the least compressed. If know your compositing programs, you can reduce blockiness from dv compression by slightly blurring the red and blue channels (either horizontally or vertically, i forget) before keying, and then applying the resulting matte to the original, unblurred footage. This is fairly simple to do in AE or Shake. Also, DVGarage has products that claim to get great mattes from DV, though I have never used them.

http://www.dvgarage.com/prod/prod.php?prod=dvmattefcp

Best of luck

-Jon
www.gingerbreed.com
 
Barry_Green said:
Excellent point. Nothing else is going to key as well as 4:4:4!
Click to expand...
Well .... there's one little trick you can use if you've got full control of the scene.
The "4" component of DV is the luma value, and this full res component translates mostly into the green channel.

So ... if you use a dark subject (dark skin, hair & clothing) in front of a green screen, or a bright subject (blond hair, fair skin, white/yellow clothing ... maybe even a rim light) against a bluescreen, you can get some exceptionally good keys with DV plates thanks to a LOT of sharp contast in the green channel.

Yeah, 4:4:4 is awesome, but there can still be challenges. I've worked with plenty of stuff off the CineAlta F950 that had motion blur, reflected spill and other issues (noisy blue channel) to contend with. Learning to key DV is great .... because it makes the challenges of high end plates more familiar. :)

Have fun.
 
You definitely should, Animal.

I NEVER thought I would be playing with green screens, but now that I know I can do it (and I am very happy with the results) I am obsessed.
 
This is great. I ask a simple question and get this great education in the process. This forum rocks!

Chris
 
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