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.
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.
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.
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.
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.
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.