This panel, brought up with the Geometry Digests button in the File Menu, provides a list of Cell Geometry Digests currently in memory. A Cell Geometry Digest (CGD) is a per-layer/per-cell database of highly compacted representations of cell geometry. Logically, a cell name and layer name are passed to the database, which returns a data block which when expanded yields a representation of the geometry on the given layer in the given cell. The database contains no information about cell instances, and text labels and object properties are excluded.
This is basically a companion to the Cell Hierarchy Digest (CHD), which contains hierarchy information but no geometry information. The two data types together provide complete physical information about the file.
A CGD can be linked to a CHD. After linking, the CHD will retrieve needed geometrical information from the linked CGD, rather than from the original layout file. This can be faster, since the CGD geometry data may be in memory, and are sorted by layer and compacted. Even with all geometry data residing in memory, the combined size of the CHD/CGD structures is still much smaller than the memory required for loading the original layout file into the main database in the normal way. The main database, however, provides the spatial sorting for fast access of objects at a given location, which is absent in the CHD/CGD combination.
Each saved CGD is given a unique but otherwise arbitrary name, which is used to access the CGD. The CGDs presently in memory are listed by name, and can be selected by clicking.
The listing contains a middle column labeled Type, Linked, which will contain Mem, File, or Rem indicating the geometry storage type of the CGD. This will be followed by yes if the CGD is linked to a CHD. An asterisk `*' will follow yes if the CGD will be destroyed when unlinked from its CHD. The right column contains the source file name, if any. The Info button will provide more information about the CGD, including the full path to the source file.
The selected CGD is used as input for operations initiated by the row of buttons arrayed across the top of the panel. These buttons are: