Matlab seems to use the classical way to handle alpha channels, so that transparency works in a way 'subtractively', like layers of translucent fabric. If you pile up patches set with FaceAlpha = 0.5, say, you'll eventually end up with a dark zone.
I'd like to generate an additive mix, corresponding to the superposition of several light sources. Piling up enough of these patches would eventually lead to a white zone. If you code in OpenGL you can play with different modes of alpha blending, but I haven't found a similar way to do it in Matlab.
Thanks for any clue!