Strata StudioPro Opacity and Stencil Maps
by Nick Woolridge n.woolridge@utoronto.ca
Last updated 30/6/97
This document: http://www.bmc.med.utoronto.ca/strata/
Based on a recent thread on the Stratalist, I have decided to make a few demo images of StudioPro opacity and stencil maps in action, to clear up confusion about them. Along the way, I discovered what I think are a few - ahem - anomalies in the way StudioPro renders these maps. None of these are fatal, but they will, in some cases, require some "working around".
Imagine you have the following map to use as an opacity map. White areas will be opaque, black areas will be transparent, and grey areas will be semi-opaque (not truly translucent).
Setting up a new texture in the following way:
will give you this when rendered on a rounded cube (ground plane on, single global light):
[Note the use of the same map in the diffuse and ambient channels above. This is essential for true transparency. If your object is reflective or glossy, you will need the map in the specular and reflectivity channels as well.]
Notice the truly opaque sections of the image, the semi-opaque circles and the cast shadows which are affected by the opacity map. Cool!
Anomaly: But wait, upon closer examination, the cast shadows are not exactly coherent; for instance, the upper right corner of the cube is opaque on the top surface, yet the shadow corresponding to it (near the far right margin of the image) shows three semi-opaque dots in conjunction. They never occur in this configuration on the cube.
In sum, there seems to be a discontinuity between the mapping of the texture on the object and the mapping of the texture in the shadow, if you catch my drift.
Update: Some people have taken issue with my interpretation of the above image, which has led me to re-examine the image and model ... and I'm more convinced than ever that there is a rendering anomaly. In fact, there seems to be difference between the way the texture is mapped on the inside and outside of the cube. I detail that on this page.