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Python Programming On Win32 by Mark Hammond, Andy Robinson

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Choosing a Coordinate System

There is, however, one significant problem with your printout: the line on your page is probably rather tiny. The coordinates used were pixels. On the screen, you typically get about 100 pixels per inch; if you’d written the previous code for a window, the text would be an inch down from the top left of the window and about as long as the words. On a typical HPDeskJet printer, which has a resolution of 300 dots per inch, the line is just a sixth of an inch long; yet the text is still a sensible size (it will, in fact, be in the default font for your printer, typically 10-point Courier).

For printed reports, you need precise control of text and graphics; numbers need to appear in the columns designed for them. There are several ways to get this. We will use the simplest, and choose a ready-made mapping mode, one of several coordinate systems Windows offers, based on twips. A twip is a twentieth of a point; thus there are 1440 twips per inch. Windows can draw only in integer units, so you need something fairly fine-grained such as twips. (Windows also provides metric and imperial scales, or lets you define your own.) In this coordinate system, the point (0, 0) represents the top left corner of the page, and y increases upwards; so to move down the page, you need negative values of y.

To set up the scale, you need just one line:

dc.SetMapMode(win32con.MM_TWIPS)

To test it, we’ll write a little function to draw a six-inch ruler. If it’s right, you can call ...

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