Why is Surface Opacity or Volume Opacity in shaders a vector?

[magneto]

Hi,

 

Shouldn't this be a float type? I see for example the default uniform volume shader uses float for opacity but has to output to vector Of. Is this normal? Don't know how to visualize a vector opacity in practice.

 

 

Thanks

[rayman]

Isn`t this supposed to mean that opacity has color?

[magneto]

In this case how is a color opacity interpreted? By grayscale or luminance?

 

Also the same can easily be achieved by using the surface color and then multiply with float opacity, right? So I thought having colors at both places is confusing. But I am not an expert Shading TD like Symek

[Georgie]

https://www.sidefx.com/docs/houdini13.0/vex/contexts/shading_contexts

 

Click the Opacity vs Alpha link. Explains it pretty good. 

 

Got to admit it was a really cool question. I never actually thought about that so first thing I did was fire up Houdini and play with a couple of clay shaders on a box and a sphere. Results were really weird and could figure out why 'till I found that info. 

 

Cheers and thanks for the good question.

 

 

Of vector

The final opacity for the surface. A value of {1,1,0} will be opaque in red/green, but let through blue light from behind. Seeopacity vs. alpha.

 

 

Edited December 22, 2014 by Georgie

[fathom]

figure that color represented as RGB is really just 3 floats.  RGB opacity just means 3 different opacity settings for each component being calculated.  if you think of the RGB components being computed separately, it might make more sense conceptually.