d0a10d71_Processing_and_Display_Package_Montage_Parameters_Description
[Parameter] compose [Values] {Over, In, Out, Atop, Xor, Plus, Minus, Add, Subtract, Difference, Bumpmap, Replace, MatteReplace, Mask, Blend, Displace} [Description] 
composite operator
ImageMagick - Image Processing and Display Package
Description