I read that LRP takes three operands. Two operands are the values to interpolate between. The remaining operand is a “blending factor”. Its been years since I’ve looked at a math book. I can’t even think of what that other operand might do. Is it some sort of bias value to shift the result toward one or the other of the two values being interpolated between?

http://oss.sgi.com/projects/ogl-sample/registry/ARB/fragment_program.txt

It’s simple linear interpolation between 2 vectors.

LRP c, t, a, b; c = t*a + (1-t)*b, t is in [0,1] for lerp.

aha! thanks