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Overlay filters are used to control which items are displayed in the map window while the filter is in force, and which are not. The filtering can be carried out on the basis of item classes, features, properties or using a combination of filters. See Types of Filter.
When you have an overlay filter in force, items excluded by the filter are not displayed (and so cannot be edited either). If you create items which are excluded by the filter, these will be drawn but not shown.
Filters are named objects, stored in Named Object Libraries (NOLs) which use item properties to include ("pass") some items, but exclude ("fail") others.
Filter objects work by examining the properties of an Item. So one filter could look at the colour of an Item, and only "pass" those that are red. (In the same way as a camera can use coloured glass to filter the images of the photographs it takes.)
Filter objects can be applied to overlays to control the visibility of the items in that overlay. Only items that pass are drawn. Items that fail the filter are not drawn, so you can make them temporarily invisible without having to delete them. In addition, many SIS methods, e.g. Scan, accept a Filter as an argument.
SIS comes supplied with many filters already created. These are grouped as follows:
Expand each group to see the filters available.
These filters are stored in the (standard) library and are read-only.
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