GEOS SDK TechDocs|
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5 Coordinate Space | 
5.2 Coordinate Transformations The standard coordinate space is what the application uses to describe where things are to be drawn. It is device independent, based on real world units: typographer's points, about 1/72nd of an inch. The origin of the coordinate system is normally the upper left corner of the document, but this can change with context and with changes made by the kernel or even your geode. The coordinate space normally extends from -16384 to 16383 horizontally and vertically. These constants may be referenced as MIN_COORD and MAX_COORD. Note that if you're going to be printing your document, you should restrict yourself to coordinates between -4096 to +4096 to account for scaling to draw on high resolution printers. It is possible to define a 32-bit coordinate space, which gives more room but costs speed and memory. For details about 32-bit spaces, see Larger Document Spaces.
Whenever you draw something, you must specify the coordinates where that thing will be drawn. The coordinates you pass specify where in the coordinate plane that thing will be drawn; the plane, in turn, may be translated, scaled, and/or rotated from the standard window coordinate system (see Coordinate Transformations).
The system also maintains a device coordinate system, with a device pixel defined as the unit of the grid. This is the type of coordinate system most programmers are used to, but it is certainly not device independent. The graphics system will do all the worrying about device coordinates so your program doesn't have to. (Note, however, that
GrDrawImage()
,
GrDrawHugeImage()
, and
GrBrushPolyline()
are more device-dependant than most routines; see the Drawing Shapes chapter, bitmap section and polyline section for information on these routines).
Standard GEOS coordinates depart from the device model, taking an approach closer to a pure mathematical Cartesian plane. Programmers used to working with device-based coordinates are especially encouraged to read Device Coordinates to learn about some of the differences.
GEOS SDK TechDocs|
| 5 Coordinate Space | 5.2 Coordinate Transformations