Why does the surface opacity accept vectors? Opacity is only a single channel that is usually controlled by a greyscale or float value.


ps.I would assume that alpha and opacity co-relate and would be one and the same. I apreciate the fact you can control them seperately however if they are the same would they not both use vectors going along with current scheme of things? Why the difference? :shock:

opacity is a vector because it is the amount of light to be passed through each of the RGB channels . so (0,.5,1) would be opaque in the blue channel (no light passing through), let half the amount of green light pass through, and let all red light pass through.

alpha is a float like you normally would understand transparency as.

a lot of this stuff can be learned through the several vop demos here:

http://sidefx.vislab.usyd.edu.au/houdini_video/by_topic/rendering/index.html [sidefx.vislab.usyd.edu.au]

they are really good and you can learn a ton of this kind of stuff in them…

Wouldn't the colour of the diffuse channel control the colour of the rays going through the surface? Or is this done to give independant control regardless of the colour of the object?

ie. it would let a green piece of glass cast a red transparent shadow.


ps. Thanks deecue! watched em all twice at least already! ^_^

im not completely sure of all the technical definitions, but my understanding is that the diffuse is controling the color of the object based on the way light scatters/reflects in all directions (and most importantly back in to the camera).. this in combination with positions of lights and your camera, direction of your normals, and your choice of lighting model will create that color that you see at render time. (remember from color theory, we don't see colors, we see surfaces absorbing and reflecting light which is interpretted as color in our brain).

so diffuse is controling the amount of light absorbed and reflected/scattered on the surface. and opacity allows you to control the amount of transmission of light through the surface in all directions.

So if i have a sphere that has a specific diffuse and an opacity less than (1,1,1). And i have a box behind that sphere with a specific diffuse and an opacity = (1,1,1).. Light will be absorbed and reflected off the sphere and back in to the camera as well as passed through. It then is aborbed and reflected back by the box. Then that light is transmitted back through the sphere and in to the camera thus giving the effect as if the sphere is transparent and we can see the box behind it. So by lowering the sphere's opacity, it has let the light pass through it's surface and reflect off another and come back through it and in to the camera…