Data Compression
^^^^^^^^^^^^^^^^^^^^^^^

In order to reduce the size of the layout file (GDS file) and to use hierarchical design as much as possible to improve development efficiency when viewing the layout and verifying the design, **PhotoCAD** will compress the data for the same layout pattern when outputting the GDS file.

For example, if you create a device layout cell in a script that defines a and b cells, and if a and b output identical layout graphics, then data compression will automatically be performed when outputting the GDS file, and the data compression will save the latest created cell in the GDS file.

Refer to the source code of ``gpdk`` > ``components`` > ``bend`` > ``bend_bezier.py`` and make certain modifications to define multiple instances ( components ) by code when creating devices using the ``BendBezier`` class::

        if __name__ == "__main__":
            from gpdk.util.path import local_output_file

            gds_file = local_output_file(__file__).with_suffix(".gds")
            library = fp.Library()

            TECH = get_technology()
            # =======================================================================
            # fmt: off

            library += BendBezier(name="q1", start=(0, 0), controls=[(30, 30)], end=(60, 0), waveguide_type=TECH.WG.FWG.C.WIRE)
            library += BendBezier(name="c1", start=(0, 0), controls=[(-10, 30), (70, 30)], end=(60, 0), waveguide_type=TECH.WG.FWG.C.WIRE, transform=fp.translate(0, 40))
            library += BendBezier(name="q2", start=(0, 0), controls=[(30, 30)], end=(60, 0), waveguide_type=TECH.WG.FWG.C.WIRE)
            library += BendBezier(name="c2", start=(0, 0), controls=[(-10, 30), (70, 30)], end=(60, 0), waveguide_type=TECH.WG.FWG.C.WIRE, transform=fp.translate(0, 40))


            # fmt: on
            # =============================================================
            fp.export_gds(library, file=gds_file)
            # fp.plot(library)

As the code shows, 4 instances are defined: ``q1``, ``c1``, ``q2``, ``c2``. where ``q1`` and ``q2`` instances have different names but other parameters are exactly the same, so do ``c1`` and ``c2``.

Run the script and check in the layout tool, you can see ``c2`` and ``q2`` in the cell list, however you can't see ``q1`` and ``c1``, thinking that ``c1`` and ``c2`` generate exactly the same graphics, and ``c2`` is the latest defined instance, similarly ``q2`` and ``q1`` generate exactly the same graphics, and ``q2`` is the latest defined instance.

As a result, when exporting the GDS file, use ``c2`` instead of ``c1`` and ``q2`` instead of ``q1`` by data compression.

 .. image:: ../images/datacompression.png