To limit the filtering to certain pixels then set a picture defining the mask to this property.
Set this property to nil if the filtering should not be limited to certain pixels.
The mask must be at least as big as the src picture or as big as the size property if the size parameter is used.
The mask can only be a 32 bit picture. If the mask is a 32 bit picture then it can be either black and white or grayscale. If passing a color image as the mask then the Red channel will rule as a Grayscale image for the mask.
If the mask picture has unsupported depth or is smaller than it may be then the src picture and the return value will be left unmodified.