When masking a bitmap by reducing the polygons it's displayed on, what is the name of doing this?
I often see this done for physics, sort of, to have an edge around an object for detection etc, but a long while ago remember seeing a similar approach being used with polygons for visual masking, but have completely forgotten what it was called or how to find it.
ADDITIONAL INFO:
In this example, the polygons are used to "mask" some of the image:
http://fancyratstudios.com/2010/02/programming/progresstimer-for-cocos2d/
What's the name of using polygons to do masking in this manner?
According to the definition you gave, the technique you're referring to could be Gouraud shading, whose definition is also reported in the Video-Based Rendering book on page 50:
Hole-filling: for polygons that are not depicted in any image, determine appropriate vertex colors; during rendering, Gouraud shading is used to mask the missing texture information.