I find it strange that the Clamp does not do what I am trying to achieve since the clamp only let a value pass if it’s between the min and max value. I am not sure what you mean by manual texturing filtering. I dont know if that needs programming, I am not a programmer 
I am sure there is a math way of achieving of only getting one intensity out of the red channel.
Maybe the problem is in my image.
Here are my red color values in photoshop (black and white layer in a group with only red channel activated):
1 = 255,255,255 (RGB)
0.5 = 128,128,128 (RGB)
0.25 = 64,64,64 (RGB)