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The screen metrics, specified by the GDIINFO structure, define such items as
width and height in millimeters, screen resolution, aspect ratio, and
mapping modes. GDI uses screen metrics to generate coordinate data that is
appropriate for the display hardware.
 
Logical Pixels Per Inch
 
A display driver sets the dpLogPixelsX and dpLogPixelsY members to specify
the number of pixels per logical inch along horizontal and vertical lines on
the screen. A display driver uses logical inches (about 40 percent larger
than physical inches) for readability reasons.
 
The GDI font mapper uses these values to determine which screen fonts to use
with the display. The display driver should make sure the dpLogPixelsX and
dpLogPixelsY members match an existing font. If these members do not match
one of the default screen fonts, an appropriate font must be provided with
the display driver.
 
Screen Resolution and Size
 
A display driver sets the dpHorzRes and dpVertRes members to specify the
width and height of the screen in pixels, and sets the dpHorzSize and
dpVertSize members to specify the width and height of the screen in
millimeters. These values must have the following relationships:
 
dpHorzSize = (dpHorzRes/dpLogPixelsX) * 25.4
 
dpVertSize = (dpVertRes/dpLogPixelsY) * 25.4
 
In these equations, 25.4 represents the number of millimeters per inch.
 
Aspect Ratios
 
The aspect ratio defines the relative dimensions of the display's pixels.
The ratio consists of three values: an x-, y-, and an 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 ** 2 == (dpAspectX ** 2) + (dpAspectY ** 2)
 
Since the dimensions are given as relative values, they may be scaled as
needed to get accurate integer values. They should be kept under 1000 for
numerical stability in GDI calculations. For example, a device with a 1:1
aspect ratio (such as a VGA) can use 100 for dpAspectX and dpAspectY and 141
(100 * 1.41421...) for dpAspectXY.
 
Styled-Line Length
 
The styled-line length (dpStyleLen) specifies the length of the smallest
line segment the display driver uses to build the dots and dashes of a
styled line. GDI uses this number when it draws into bitmaps and on
displays. To ensure consistency between displays and printers, the styled
line segment length is always two times the value of the dpAspectXY member.
 
Standard Mapping Modes
 
Some Windows application programs rely on standard mapping modes to produce
printer output with spacing that is proportional to the screen. By using the
standard mapping modes, an application can show a border or graphic picture
that is proportionately the same size on the printer as it is on the
screen.
 
All standard mapping-mode ratios must be the same because it might be
preferable for an application to use the metric system rather than the
inches/feet (English) system for its calculations. For example, Windows
Write allows the user to choose whether to express the border widths in
millimeters or inches. Therefore, it is up to the display driver to provide
the correct numbers.
 
Standard mapping modes are expressed as two coordinate pairs: the width and
height (in logical units) of a "window" and the width and height (in
physical units, that is, pixels) of a "viewport" that maps onto that window.
The driver for a VGA adapter, for example, might set the coordinates pairs
for the low-resolution metric mapping mode (tenths-of-millimeters) to
(254,254) and (96,96). These coordinates map an 1-inch by 1-inch window
(25.4 millimeters equals 1 inch) to a 96-pixel by 96-pixel viewport. These
coordinate pairs define a set of equations that specify how coordinates in
logical space are transformed to coordinates in device space.
 
The standard mapping-mode members in the GDIINFO structure can be set as
follows:
 
dpMLoWin.x = dpHorzSize*10;    dpMLoWin.y = dpVertSize *10;
dpMLoVpt.x = dpHorzRes;        dpMLoVpt.y = -dpVertRes;
 
dpMHiWin.x = dpHorzSize*100;   dpMHiWin.y = dpVertSize *100;
dpMHiVpt.x = dpHorzRes;        dpMHiVpt.y = -dpVertRes;
 
dpELoWin.x = dpHorzSize*1000;  dpELoWin.y = dpVertSize *1000;
dpELoVpt.x = dpHorzRes * 254;  dpELoVpt.y = -dpVertRes * 254;
 
dpEHiWin.x = dpHorzSize*10000; dpEHiWin.y = dpVertSize *10000;
dpEHiVpt.x = dpHorzRes * 254;  dpEHiVpt.y = -dpVertRes * 254;
 
dpTwpWin.x = dpHorzSize*14400; dpTwpWin.y = dpVertSize *14400;
dpTwpVpt.x = dpHorzRes * 254;  dpTwpVpt.y = -dpVertRes * 254;
 
 
A twip is 1/20th of a printer's point (1/72 of an inch).
 
Windows performs 16-bit, signed calculations on these values, so the numbers
must not be greater than 32,768. However, if the screen is larger than just
a few inches wide, the values will exceed this limit when calculating the
English mapping modes and may even exceed it on the metric mapping modes.
Fortunately, the values can be scaled down by dividing by some fixed
amount.
 
 
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