V-Ray for Rhino Manual - Rum

4.Under Properties>Mapping, add a new channel, change the projection type to Planar, and adjust
the mapping widget's size and position as image showed below. If the Tile remains checked, it will
render as the image on the right
How Transparency Mapping works
The diagram below depicts transparency mapping. The idea is using a grayscale image as mask,
black area will not be penetrated and only white area will let light through, other gray area will
then become translucent
The white area get the color assigned in Diffuse1 and end up showing on the surface of the object after
rendered.
For the cup example above, after assign a mask to the Transparency, the red color of the cup is
affected by the white area of the mask and no longer showing red. The second layer of the Diffuse1
color at the diagram below is used to cover the white area of the first layer.
Another method for creating the same result
Totally opposite from the method above, set the Diffuse to white, Transparency as mask, but switch
the black and white area, and let the Diffuse1 color show up at the upper layer. Assign the Diffuse1
color to red and will get the same result as above after rendered
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