Multi-channel-composition

Multi-channel-composition is the operation of combining a set of channels (usually grayscale images) into a one colored image.

The operation (as implemented by the function Compositors::ComposeMultiChannel) is controlled by the following options:

• tinting by a color
• definition of a black- and white-point
• a gradiation curve

The steps in the operation are:

Let result R-G-B pixel value = 0,0,0
For each channel  
    If tinting is enabled for this channel:
        - get normalized pixel value
        - apply gradation
        - multiply with R-G-B tinting color
        - add R-G-B value to result pixel 

    If tinting is disabled for this channel:
        - get normalized R-G-B value
        - apply gradation to R, G and B
        - add R-G-B value to result pixel 

The operation "Apply gradiation" works in the following way: the normalized pixel value is mapped to an integer (in the range 0..255) by looking up a value:

Above a linear gradation curve is shown. There are three ways commonly used to define the gradation curve:

• linear (a straight line from the point [black point,0] to [white point,255] - as shown above)
• gamma (instead of a straight line we use an exponential with the exponent gamma as a parameter)
• defined by a spline

In the function Compositors::ComposeMultiChannel the gradation curve is given as an array of bytes. Those bytes give the RGB-value at uniformly distributed points between black point and white point. Values between those points are interpolated linearly.

For example, the parameters white_point=0.2, black_point=1.0 and lookupTable={0, 66, 100, 166, 255} will result in this gradation curve:

For generating a look-up-table (which then can be used for Compositors::ComposeMultiChannel) two utility functions are provided: Utils::Create8BitLookUpTableFromSplines and Utils::Create8BitLookUpTableFromGamma.