Surface alpha values less than 1

Started by james adamson, March 06, 2020, 06:23:27 am

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james adamson

Hi all.
I have rendered out some frames and my surface alpha (32 bit exr) has varying values around .98. 
Any ideas. I would expect 0 and 1. The sky is 0.

WAS

That's strange. I don't think I've seen that. Do you have any default shaders being used with alphas?

You could try adding a Constant Shader in as your first colour shader see if that helps.

james adamson

No. I do have an image shader going into a default shader in my background, in the default I have diffuse at zero and am only using luminosity but I have made that invisible at render time. So I have pure black in my sky on the alpha and the surface has values around .95 to .98 
As far as I understand the only node capable of outputting its own alpha is the constant. I also only have atmos visible in the sky. No image. At the moment I have to live with it as the render has been going some time and its got a good few days left and I gotta move on. I have just lifted any values from. .95 to 1. Not ideal but it does not seem to be affecting the edges in any noticeable way. It has to be something I have done wrong.
Thanks.
James.

Matt

This is probably because the atmosphere alpha subtracts from the surface alpha. It seems strange that it does this, but there is a good reason. It maintain a purely additive workflow where surface + atmosphere + cloud adds up to 1, and it does this for alpha as well as RGB for consistency. The surface alpha represents the contribution that the surface has to the final pixel, which is diminished when it's partially hidden by atmosphere. This makes compositing simpler in some ways, but more difficult in others.

It used to work differently but it made it harder to composite atmosphere over the terrain, so we switched to this purely additive workflow. But it has this unwanted side effect of making it harder to get a separate matte of the terrain.
Just because milk is white doesn't mean that clouds are made of milk.

Matt

If you are using the alpha to affect your replacement background, it may be better to use the combined alpha of the surface, atmosphere and cloud anyway. This will be 1 on the terrain.
Just because milk is white doesn't mean that clouds are made of milk.

WAS

I wasn't even aware of this. Good info.

james adamson