overlay (< 127 darken, > 127 brighten) would be // add AlphaBlendEnable = TRUE; DestBlend = SrcColor; SrcBlend = DestColor; which is the same as SrcColor * DestColor *2
Use glAlphaFunc(GL_LESS, 128); instead of glBendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA); and let glDepthFunc untouched, that'll remove the blending and use on/off alpha at threshold 128 instead.
Most of the games I worked on used photos as a starting point. Those photos would be cut up or painted over to create the textures. Companies would also buy CD libraries of pre-made photo textures and multiple companies would use them. One popular CD library was photos of buildings in Vancouver so there were a bunch of…