... The task is designed to make a mask image (ie an image of boolean values) by applying a selection expression to the pixel values of an input image. ... Hence for example if one wished to mark as TRUE all pixels of the input which exceeded 7.3 in value, one would call immask with expression =`VAL 7.3'. ... All attributes (keywords), including WCS keywords, but excluding keywords related to any data subspace (DSS) in the input image, are copied to the output image. ...