Overlay Program Characteristics
Input files should be GDSII.
All files to be merged should share the same units and resolution. For example, micron at 0.001 um resolution.
Input Cell to Process
For each file to be merged, the user can select a top level cell in the hierarchy. While most layout files have a single top level cell, it is possible that there might be more than one.
Scanning Input Files
Each file to be merged can be scanned to determine the top level cell name in the file as well as the layers that contain data. Scanning can take anywhere from a few tenth's of a second, for small files, to many minutes for larger files. Since scanning is I/O intensive, large files that are mounted on remote disks may scan much slower than the same file when read from a local drive.
Output GDSII File
The program creates a new top level cell for the output file. By default this cell is called TOP. It will not collide with any cell names originating from the merging files since those names are appended with _0, _1, _2 ...
Cell Name Collision Avoidance
In order to avoid accidental and unwanted re-definition of cell data, each and every cell originating from an input file has its name modified by appending _N, where N is the order in which the files are specified for merging. Details ...
Memory Usage vs. Disk I/O
The Overlay program does not attempt to load any of the input files or output file to memory. This enables it to handle GDSII files that are much larger than what would fit into memory. This does mean that there is considerable reading and writing of data; when dealing with large files the time required to perform the merging will be tied to both the total size of the data and the disk read and write speed.
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