The attenuate operator simulates an X-ray of the material. With attenuate, the texel's alpha appears to attenuate light shining through the material along the view direction towards the viewer. The texel alpha channel models material density. The final brightness at each pixel is attenuated by the total texel density along the direction of view.
Attenuation can be implemented with OpenGL by scaling each element by the number of slices, then summing the results. This can be done by combination of the appropriate blend function and blend color:
glBlendFunc(GL_CONSTANT_ALPHA_EXT, GL_ONE) glBlendColorEXT(1.f, 1.f, 1.f, 1.f/number_of_slices)