Coordinate Systems¶

AutoLISP Reference to Coordinate Systems provided by Autodesk.

To brush up you knowledge about vectors, watch the YouTube tutorials of 3Blue1Brown about Linear Algebra.

WCS¶

World coordinate system - the reference coordinate system. All other coordinate systems are defined relative to the WCS, which never changes. Values measured relative to the WCS are stable across changes to other coordinate systems.

UCS¶

User coordinate system - the working coordinate system defined by the user to make drawing tasks easier. All points passed to AutoCAD commands, including those returned from AutoLISP routines and external functions, are points in the current UCS. As far as I know, all coordinates stored in DXF files are always WCS or OCS never UCS.

User defined coordinate systems are not just helpful for interactive CAD, therefore ezdxf provides a converter class UCS to translate coordinates from UCS into WCS and vice versa, but always remember: store only WCS or OCS coordinates in DXF files, because there is no method to determine which UCS was active or used to create UCS coordinates.

OCS¶

Object coordinate system are coordinates relative to the object itself. The main goal of OCS is to place 2D elements in 3D space and the OCS is defined by the extrusion vector of the entity. As long the extrusion vector is (0, 0, 1) (the WCS z-axis) the OCS is coincident to the WCS, which means the OCS coordinates are equal to the WCS coordinates, most of the time this is true for 2D entities.

OCS entities: ARC, CIRCLE, TEXT, LWPOLYLINE, HATCH, SOLID, TRACE, INSERT, IMAGE

Because ezdxf is just an interface to DXF, it does not automatically convert OCS into WCS, this is the domain of the user/application. These lines convert the center of a 3D circle from OCS to WCS:

ocs = circle.ocs()
wcs_center = ocs.to_wcs(circle.dxf.center)

DCS¶

Display coordinate system - the coordinate system into which objects are transformed before they are displayed. The origin of the DCS is the point stored in the AutoCAD system variable TARGET, and its z-axis is the viewing direction. In other words, a viewport is always a plan view of its DCS. These coordinates can be used to determine where something will be displayed to the AutoCAD user. Ezdxf does not use or support DCS in any way.