View Full Version : Luma key extract for compositing rim light.

09 September 2009, 07:36 PM

I have I guess a rather strange way of generating rims/kickers for my CG lighting work. Over the last month or so I've been experimenting with rendering out my rim light pass separately, tinting it in post, extracting a matte by using a luma extract (pre-multiplied, although I see no difference without pre-multiplication) and then using this extracted matte to paste the rim pass back onto my existing composite.
I do this because I find it easier to achieve a nice, punchy colored look to my rims and kickers. I can get a hot hot hot blue, for example, that if I tried to do in the render or use a screen/add node, would blow out to white.
All would be as fine as champagne and oysters if it wasn't for the composited rim pass exhibiting the dreaded black matte line. I can normally roll with this as I like to employ a fair bit of edge blur which sort of deals with it. However, this feels REALLY hacky and bad practise. Anyone help me out?


Lighting & Compositing
Taylor Drew Productions.

09 September 2009, 07:42 PM
I'm working with purely 3D renders (that I'm lighting and rendering). Outputing 16bit TIFFs.
It's for kids TV too and I'm the only lighter/compositer in the company so I'm stretched thin. But that's also kind of how I like it! :)

09 September 2009, 05:31 PM
Any chance you can post sample images of the final image using your Lighting method?

10 October 2009, 03:36 PM
With a Luma matte, I wouldn't expect to see a difference. Alpha matte would be a different story.
What application are you using? In AE, first precompose the matte and drop it into a new compostition (Precompose it), then I would just apply a very slight blur then use a simple choker to trim the matte back until the line disappears. You may not need the pre-blur, but if you are working with NTSC or PAL resolutions, it can help make the lines a little cleaner.

If you are using a node based compositer, I don't know if this works the same way...I don't have any experience with the really expensive toys. But this usually works for me.

10 October 2009, 07:45 PM
Thanks CCRider, but, you guessed it: I'm using a node based compositor! I do have access to afterfx but I'm reluctant to switch out of my current workflow as apart from this luma extract problem, everything is going swell.
I will definately look into choking the matte a bit to see if that helps.
Still looking for an elegant, streamlined node based solution though.


10 October 2009, 08:09 PM
Can't fault you for that...but there is bound to be a way to get the same results with a node system but I can't tell you how to get experience is limited to AE... Plenty of folks here with node experience though so someone here should be able to help with that.

I do lots of motion graphix as well so AE works great for me.

Good luck man!


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