OSL
Float Remapping

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Introduction

It is often necessary to remap, or convert, floating point values from one range to another range. For example, if the resultF output of a PxrFractal node is used to control the diffuseGain of a material node the values may require conversion from their original range of 0.0 - 1.0 to a smaller range of, say, 0.2 - 0.8.

This tutorial explains one way of performing a remapping. The diagram shown below represents two ranges of numbers as the bases of two triangles.

If the input_range has a min value of 0.0 and a max value of 1.0 it's range would be 1.0. If the output_range has a min value of 0.2 and a max value of 0.8 it's range would be 0.6.

For a specific input value, represented by the red line, to be remapped to an output value, represented by the blue line, the following must be true.

    a/input_range = b/output_range

Consequently, the value of b would be,

    b = a/input_range * output_range

A remapped value would be,

    remapped = a/input_range * output_range + output_min

For example, if the input value is 0.35 it's remapped output value would be,

    remapped = 0.35/1.0 * 0.6 + 0.2
    remapped = 0.41

Listing 1 uses the arithmetic shown above to implement a float remap shader.

Listing 1

shader
FRemap( float input = 0,
        float input_min = 0,
        float input_max = 1,
        
        float output_min = 0,
        float output_max = 1,
    output float resultF = 0)
{
float input_range = fabs(input_max - input_min);
float output_range = fabs(output_max - output_min);
  
float a = fabs(output_min - input);
  
resultF = a / input_range  * output_range + output_min;
}