# Reference Points

The PixelMap tool works by creating two reference points based on the histogram data of the input image (or lets you create a series of them, if you like). The first point is identified as (input1, output1) and the second point is identified as (input2, output2). Output1, also known as the first mapped value, can be used to map the invisible pixels by default, but you can override this behavior. VisionPro will be adding Point 0 and Point 3 to reflect the full range mapping of input image to full range of output image.

Each reference point defines the mapping of a single input pixel value to a single output pixel value. The PixelMap tool creates a linear mapping of pixel values between each pair of reference points. The result set of linear mappings form a mapping function.

All input pixel values less than the lowest reference point are mapped to the output value defined by the reference point. All input pixel values greater than the highest reference point are mapped to the output value defined by the reference point. If you specify a single mapping point, then all input pixel values are mapped that single output pixel value. If no mapping points are defined, then the pixels are mapped using a single linear mapping.

The following figure shows how reference points are used to define a mapping function.

Note: You can define a reference point's input and output values as absolute (pixel) values, or you can define them as relative values (the fraction, in the range 0.0 through 1.0, of the range of possible pixel values at which to place the reference point). Specifying relative values allows you to use a given mapping function with images of different bit depths without needing to modify it.