Device vs. Design Units

An application can retrieve font metrics for a physical font only after the font has been selected into a device context. When a font is selected into a device context, it is scaled for the device. The font metrics specific to the device are known as device units.

Portable metrics in fonts are known as design units. To apply to a given device, design units must be converted to device units. Use the following formula to convert design units to device units.

DeviceUnits = (DesignUnits/unitsPerEm) * (PointSize/72) * DeviceResolution

The variables in this formula have the following meanings.

Variable	Description
DeviceUnits	Specifies the DesignUnits font metric converted to device units. This value is in the same units as the value given for DeviceResolution.
DesignUnits	Specifies the font metric to be converted to device units. This value can be any font metric, including the width of a character or the ascender value for an entire font.
unitsPerEm	Specifies the em square size for the font.
PointSize	Specifies size of the font, in points. (One point equals 1/72 of an inch.)
DeviceResolution	Specifies number of device units (pixels) per inch. Typical values might be 300 for a laser printer or 96 for a VGA screen.

This formula should not be used to convert device units back to design units. Device units are always rounded to the nearest pixel. The propagated round-off error can become very large, especially when an application is working with screen sizes.

To request design units, create a logical font whose height is specified as unitsPerEm. Applications can retrieve the value for unitsPerEm by calling the EnumFontFamilies function and checking the ntmSizeEM member of the NEWTEXTMETRIC structure.

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