How Resolution Affects Font Size

How Resolution Affects Font Size

The images on most output devices, such as laser printers and video monitors, are created by coloring a rectangular array of small dots or pixels (picture elements). The number of dots or pixels that can be drawn per unit of length in a horizontal direction is called the horizontal resolution, while the number of pixels that can be drawn per unit of length in a vertical direction is called the vertical resolution. The most commonly used unit of measure for resolution is the number of dots per inch (dpi). Resolution is a device-dependent unit of measure.

To display the resolution of your video monitor, enter this command:

xdpyinfo | grep resol

You should get a response similar to this:

resolution: 93x93 dots per inch

The first number is the horizontal resolution; the second the vertical resolution.

If you draw a single character at a given resolution, the bounding box of the character is the smallest rectangle that enclose that character.

If you display all of the characters in a font in the same place (without advancing), you get a composite image of those characters. If you then draw the smallest rectangle that encloses that composite image, you have the bounding box for the font. The size of a font is usually measured in the vertical direction. That size is usually not smaller than the height of the font bounding box, but it can be greater than that height. It may include additional vertical spacing that is considered part of the font design.

Typographers use small units of measure called points to specify font size. A point is approximately equal to 1/72 of an inch. The exact value is 1/72.27 (0.013837) of an inch, or 0.351 mm.

A point is a device-independent unit of measure. Its size does not depend on the resolution of an output device. A 12-point font should have approximately the same size on different output devices, regardless of the resolution of those devices.

If the resolution of an output device is equal to 72 dots per inch (dpi), the size of a dot or pixel is approximately equal to the size of a point. If the resolution of an output device is greater than 72 dpi, the size of a dot or pixel is smaller than the size of a point, and vice versa. You can use the following formula to calculate a pixel size from a point size:

pixel-size = point-size x device-resolution / 72.27

A bitmap font is usually designed for a particular resolution. Such a font has the point size specified by its designer only when it is used on an output device whose resolution matches the resolution for which that font was designed. This is because a font designer specifies a fixed bitmap for each character. If a pixel is smaller than a point, characters will be smaller than intended, and vice versa.