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<title>Developer's Guide: Coordinate Systems
</title>
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<H1><FONT face="Helvetica,Arial"><A NAME="N1"></A>Coordinate Systems
</font></H1>
<P>All coordinates used by the document and graphics objects are given in
the <EM>document coordinate system</EM>. This coordinate system has its
origin in the lower left corner of the page; x increases to the right, y
increases to the top. The unit for these coordinates is the DTP-point
(or PostScript-point), that is, 1/72 inch. Thus Sketch's document
coordinate system is the same as the default coordinate system of a
PostScript interpreter. Coordinates are stored as floats.</P>
<P>The page size is a user settable document property, usually `A4',
`letter' or something similar. Sketch can display the outline of the
page if desired to allow the user to position the drawing on the page.</P>
<P>When displaying the document in a window, the canvas widget (to be more
precise, the graphics device object the canvas uses) converts document
coordinates to <EM>window coordinates</EM>. These are the standard X-Window
coordinates with the origin in the top left corner of the window and x
increasing to the right and y increasing downwards. The unit is 1 pixel.</P>
<P>Sketch defines a few builtin objects to represent and manipulate
coordinates: Points, Rects and Transformations. All of the related
functions, objects and constants are exported by the <CODE>Sketch</CODE>
package.</P>
<H4><FONT face="Helvetica,Arial"><img SRC="Images/bullet.png" WIDTH="9" HEIGHT="9"><A HREF="devguide-5.html">Point Objects
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<H4><FONT face="Helvetica,Arial"><img SRC="Images/bullet.png" WIDTH="9" HEIGHT="9"><A HREF="devguide-6.html">Rect Objects
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<H4><FONT face="Helvetica,Arial"><img SRC="Images/bullet.png" WIDTH="9" HEIGHT="9"><A HREF="devguide-7.html">Transformation Objects
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