Texture Wireframe Explination

Ignoring Colour Combiner and only using the Alpha Combiner what we need to achive a Wireframe effect is:

(1.0 - 0.0) * Texel0 + Texel0

What this does is overload the Alpha component in the center of billboard cutout style image while allowing the texture to natrually be transparent on the outside.

e.g. 
transparent regeions are 	A = 0 + 0 = 0 
Opaque regeons are 		A = 1 + 1 = 2 ... Overload = 0

What we see now is only the regeons where alpha is neither 0 or 1. e.g.
A = 0.5 + 0.5 = 1

This effect only works for 1bit alpha colour Index (CI) textures (4bit/8bit IA textures will have cutoff regions) and only if Bi-Linear filtering is active.