feat(ColorMaps): Add grayscale sigmoid colormap based on DICOM standard

[jadh4v]

Context

Add a non-linear sigmoid grayscale colormap as an alternative to linear grayscale. This mapping is described in the DICOM standard in section C.11.2.1.3.1 SIGMOID Function and equation C.11-1. We use a normalized window_width of [-1, 1] and window_center = 0. The colormap is generated using a 16-point sampling of this function.

See: https://dicom.nema.org/medical/dicom/current/output/chtml/part03/sect_c.11.2.html

There are two ways to access the sigmoid function:

1. Developer can use the newly added sigmoid presets that already are generated on the sigmoid curve, or
2. Use the ScalarMappingMode property on vtkColorTransferFunction to adapt any color map to sigmoid mapping. the vtkColorTransferFunction provides two ScalarMappingModes. LINEAR continues to be the default value and behavior, and SIGMOID allows to switch the scalar to color mapping to the sigmoid function. Additionally, you can control the growth rate factor using setSigmoidGrowthRate() which is usually a negative number and equal to -4 according to the DICOM standard.

Results

A comparison of linear vs sigmoid grayscale using the ImageViewer example.

npm run example ImageViewer

linear	sigmoid

The sampled 16-point sigmoid function plotted through matplotlib:

Changes

• Documentation and TypeScript definitions were updated to match those changes

PR and Code Checklist

• semantic-release commit messages
• Run npm run reformat to have correctly formatted code

Testing

• This change adds or fixes unit tests
• Tested environment:
  • vtk.js:
  • OS:
  • Browser:

[jadh4v]

cc: @finetjul please also review