Question

I have an effect file in XNA4, being compiled as Shader Model 3.

This line compiles fine (interpolating between 2 colours):

return lerp(float4(1,0,0,1),float4(0,0,1,1),pf.x);

Changing the green component of the first colour from 0 to 0.5:

return lerp(float4(1,0.5,0,1),float4(0,0,1,1),pf.x);

Results in a failed compilation:

error X6045: When constant registers are read multiple times in a single 
instruction, the _abs modifier must either be present on all of the 
constants, or none of them. 

Googling for the error code returns nothing (edit: other than this question)

Answer 1

According to documentation all of the values which are passed to lerp() should be of the same type and size, so maybe that will work:

return lerp(float4(1,0.5,0,1),float4(0,0,1,1),float4(pf.x,pf.x,pf.x,pf.x));