When an image is filtered, only pixels found within these shapes are processed. When a table is filtered, only rows found within these shapes are processed. Spatial region filtering for images and tables is accomplished by means of region specifications . A region specification consists of one or more region expressions , which are geometric shapes,combined according to the rules of boolean algebra. When region filters are passed in bracket notation in this manner, the filtering is set up automatically when the file is opened and all processing occurs through the filter.
Programs also can use the filter library API to open filters explicitly.
Each region consists of one or more geometric shapes combined using standard boolean operation. Of course these two examples will both mask the same table rows or pixels. The comma operator will cause funcnts to report two region expressions, each with one shape, in its output. If a pixel is covered by two different regions expressions, it is given the mask value of the first region that contains that pixel. That is, successive regions do not overwrite previous regions in the mask, as was the case with the original PROS regions.