This structure contains logical to physical unit mappings for a device context.

pixel | Each logical unit is one device pixel. Positive x is to the right and positive y is down. |

low_metric | Each logical unit is 0.1 millimeter. Positive x is to the right and positive y is up. |

high_metric | Each logical unit is 0.01 millimeter. Positive x is to the right and positive y is up. |

low_english | Each logical unit is 0.01 inch. Positive x is to the right and positive y is up. |

high_english | Each logical unit is 0.001 inch. Positive x is to the right and positive y is up. |

twips | Each logical unit is mapped to one twentieth of a printer's point (1/1440 inch = twip). Positive x is to the right; positive y is up. |

isotropic | Logical units are mapped to arbitrary units with equally scaled axes. One unit along the x-axis is equal to one unit along the y-axis. The functions set_window_extent and set_viewport_extent may be used to specify the units and orientation of the axes. The graphics system ensures that units remain isotropic by adjusting the viewport whenever the window extent is changed. |

anisotropic | Logical units are mapped to arbitrary units with arbitrarily scaled axes. The functions set_window_extent and set_viewport_extent may be used to specify the units and orientation of the axes. |