Raster fonts are designed to be attractive and readable at a particular aspect ratio. (The aspect ratio is the ratio of the width and height of a pixel.) The digitized aspect of a font is the ideal x-aspect and y-aspect of that font. Windows provides an aspect-ratio filter to select fonts designed for a particular aspect ratio from all of the available fonts. The GetAspectRatioFilter function retrieves the setting for the current aspect-ratio filter. An application can use the SetMapperFlags function to change the algorithm the font mapper uses when it maps physical fonts to logical fonts.
The aspect ratio of the screen is not as critical for scalable fonts as it is for raster fonts. The dimensions of the em square for a TrueType font are used when scaling the font to a specified point size. (An em square is a square whose width is approximately equal to the width of the uppercase M.) Because the height of the em square is given in pixels, it can be thought of as the point size in device units. For example, a font could be referred to as a 50-ppem (pixels per em square) font. The pixel size determines the physical point size. For example, a 75-ppem font on a 300-DPI device is an 18-point font, while on a 150-DPI device it would be a 36-point font. The number of pixels required for the desired point size is computed by using the resolution of the output device and the em square size, according to the following formula:
ppem = (PointSize/72) * DeviceResolution
According to this formula, a 12-point font on a 72-DPI screen is at 12 ppem, while on a 300-DPI device it is at 50 ppem.
TrueType fonts can be scaled linearly, nonlinearly, or optically, depending on their design. Linear scaling means that the character width is scaled and rounded to the appropriate ppem. Nonlinear scaling means that hinted character widths can be larger or smaller than the scaled widths. Optical scaling is a superset of nonlinear scaling; it includes the preservation of the color and contrast of a font across point sizes. Optical scaling can involve changing the proportions of the stroke widths to preserve their perceived width and color.
The TrueType fonts shipped with Windows 3.1 scale nonlinearly. Windows applications can also support linearly and optically scaled TrueType fonts.