You can create a bitmap mask to restrict the filtering process to a particular area in the layer.
In the Maps tree, create a bitmap layer.
On the bitmap layer, use the New Shapes tool set to draw shapes over the areas that you want to filter.
On the Layer menu, click Filter.
From the Mask list box in the Filter window, select the bitmap layer that contains the mask.
The Mask list box displays all of the bitmap layers that are in the current area. The bitmap layers are listed whether they are saved or not. For each saved bitmap, both the file and layer names are displayed.
If you want to display or save the results for the mask area only, enable the Output only mask area check box.
The data that is not covered by the mask is set to the No Data value and is not saved in the output file.