to clarify things (maybe only for me )
perhaps ppl r confusing GL_ADD_SIGNED with blending, blending is something else completely.

heres what GL_ADD_SIGNED does (it doesnt use blending thus whatevers on the screen is ignored)
take the current colour (can be the lighting fragment value) add the texture texel value + then minus 0.5, clamp the result to 0->1

fragment col, texture col - 0.5    0.25           1              = 0.75    0.25           0.5            = 0.25    0.25           0.0            = 0.0 

NOTE im assuming the first input is a fragment colour(eg lighting or glColor3f()) + the second is a texture but with the combine extension u can have a lot more flexibilty of what the 2 inputs are eg both can be textures

i believe subtractive blending is part of opengl1.2 (perhaps part of the imaging subset i forget)



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