Process History and Combine Images & Mosaic Layers - Basic Steps to Create & Modify Image Masks

A mask is a single channel grayscale image that defines how two images are blended to form a single composite image. The two images are usually considered as a previous image layer A and a current modified or filtered image layer B. If M is mask then the composite image C is given by

C(p) = (1-M(p)) * A(p) + M(p) * B(p)

where M(p) is the mask value at pixel p normalized to black 0.0 <= M(p) <= 1.0 white.

Image layer B is weighted more then image layer A in the composite C where the mask is lighter and B is weighted less than A where the mask is darker. Simply stated lighter or white mask areas reveal the current image layer B while darker or black mask areas conceal the current image layer B and show more of the previous image layer A.

ImagesPlus has two different image layer tools that are used with filters, mask, blend modes, and opacity. The steps shown below for creating a mask are the same for a Process History layer mask or Combine Images & Mosaic layer mask.

Creating Image Masks

Modified Masks

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