Keying in this Crazy thing called Halo

[tekmaster]

Hey all

Im really having a hard time pulling a good key in Halo. Is there a good way to do it, or is the keyer in Halo just not that powerful?

[edward]

Are you using ChromaKey? There are problems with that:

1. It uses HSV which we all know is 70s colour technology.

2. It's using colour for keying which we know will only work with specific types of images.

If you're simply doing blue/green screen keying of people (ie. lots of fleshtones in the foreground), I implemented a COP VOPnet a while back based on ultimatte equations from Jim Blinn and Alvy Ray Smith's paper, "Blue Screen Matting" (ftp://ftp.alvyray.com/Acrobat/BluSig96.pdf) which actually worked pretty well for the test images I tried.

[tekmaster]

Do you think you can tell me how you did the COP VOP network?

Because, aside from the stock keyer, Halo seems to be a pretty powerful compositor.

[edward]

Just look at the paper. There's some equations. Then in VOPs, create a VEX Compositing Filter. Just make your network according to the equations. If you want me to, I could try to see if my old file is still around somewhere when I go to work tomorrow.

[edward]

Ok, here's the hip file. I had lost the original test images I was playing with and just grabbed another one off the net.

If you look at the VOP network, the Parameter VOPs are named according to the paper. From a user's perspective, the process went something like this:

- Start off with the default parameters.

- Look at the A plane of the bluescreenkey cop

- Adjust each parameter in turn except for the first and last 3 until you get as good a looking matte as you can. Typically, adjust it so that you get the areas you want as white as you can and the areas you don't want as dark as you can.

- Once you're done that, adjust the Lower/Upper Alpha clamp parameters to isolate the range of the alpha you want to clamp at. Typically, you just need to raise the Lower Alpha clamp value high enough so that it completely remove the blue screen.

- Look at your result and then try tweaking the parameters again. Typically, I haven't found the Global Adjust parameter useful.

- The last parameter can be used to add back in some blue if the matte removed too much of it.

Oh and I just realized that I can't upload files here. Check in the main forum.

[edward]

Might as well put a link to the actual thread with the file:

http://odforce.net/forum/index.php?showtopic=2354