created a new filter "Find MicroTexture Regions" that draws a rectangle (prism in 3D) around each microtexture region (from Identify MicroTexture filter) and calculates the area (volume) of that rectangle (prism) that is occupied by microtexture cells.  this has to be used by renaming (filter) ParentIds - GrainIds then Linking (filter) the field map GrainIds to cell array.