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A printer driver provides values in its GDIINFO structure specifying the
aspect ratio, logical pixels-per-inch, and mapping modes for the printer.
The following sections detail these modes.
 
Aspect Ratio
 
The aspect ratio defines the relative dimensions of the printer's pixels.
The ratio consists of three values: an x-, y-, and xy-aspect. These
represent the relative width, height, and diagonal length (or hypotenuse) of
a pixel. GDI uses the aspect ratio to determine how to draw squares and
circles as well as drawing lines at an angle.
 
The aspect values have the following relationship:
 
(dpAspectXY * dpAspectXY) == (dpAspectX * dpAspectX) +
(dpAspectY * dpAspectY)
 
Because the dimensions are given as relative values, they may be scaled as
needed to obtain accurate integer values. Keep the values under 1000 for
numerical stability in GDI calculations. For example, a device with a 1:1
aspect ratio (such as a 300 dpi laser printer) can use 100 for the
dpAspectX member and the dpAspectY member and 141 (100 * 1.41421...) for
dpAspectXY.
 
Logical Pixels-Per-Inch
 
Printer drivers should always use real inches. A 300 dpi laser printer puts
300 in both members.
 
Mapping Modes
 
The printer driver provides viewport and window extents for the standard
mapping modes: MM_LOENGLISH, MM_HIENGLISH, MM_LOMETRIC, MM_HIMETRIC, and
MM_TWIPS.
 
Place the device resolution in pixels-per-inch in the viewport extents and
the number of logical units-per-inch in the window extent. The y-coordinate
of the viewport is negated to reflect the fact that the x-axis is along the
top of the paper in the default mapping mode (MM_TEXT, which specifies
device coordinates) with y increasing while going down the page; whereas in
the other mapping modes, the x-axis is along the bottom edge of the page.
 
For example, on a 300 dpi laser printer, the MM_TWIPS mapping mode will
require that the dpTwpWin member be set to (1440,1440) and the dpTwpVpt
member be set to (300, -300).
 
 
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