View Full Version : Over vs Screen

02 February 2010, 06:56 PM
I have a composite of a car i am working on. I am relatively new to nuke. When trying to blend my specular pass over my diffuse pass, I am getting some weird results that dont show up in combustion. First thing is the alpha looks very aliased. Second thing is the screen looks very extreme as if the gamma is off. When I switch between over and screen there is no difference? Why is this? Am I working in the wrong colorspace? The passes are 8bit color tiffs.


02 February 2010, 07:07 PM
I assume you havent premultiplied the layer you are overing prior to merge. Add a "premult" node to the top layer and it should work fine. Check the Tooltip on the mode selector for a mathy description on merging math.


02 February 2010, 10:01 PM
How is the shader put together in the first place? Usually you need to add the passes unpremultiplied and then multiply them with the alpha ad the end. It saves all of your problems.

Screen is also and invert-multiply-invert so be careful of what you are doing in float space.

02 February 2010, 08:57 AM
Nuke assumes all alphas are in linear space.

If you're rendering out your image and loading it as "sRGB" it'll apply an inverse gamma ONLY to the RGB channel.

In your case if you have 8bit tiffs I'm relatively certain you're defining the image space as sRGB. I'm also assuming that you aren't working in linear space in your 3D app so your 3D app is actually rendering a linear image.

Long story short you'll need to apply a 1/2.2 gamma to your alpha.

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02 February 2010, 08:57 AM
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