However, on modern hardware this is rarely an issue.
Microsoft promotes having UPM for TrueType fonts always be a power of 2 for performance reasons. If you intend to output as OpenType PS (.otf), whatever UPM you use, keeping all glyph coordinates within ± 4095 units in X and Y on the grid is “safer.” Although they can theoretically have any UPM within the spec limit, there are various old systems and workflows with lower limits. OpenType PS fonts usually have a UPM size of 1000 font units. The maximum UPM size allowed by font format specs is 16,384 for TrueType and OpenType TT. If a font has 2048 UPM size of 2048, to get a text string of 100 pixels it would need to be scaled by 4.9%. If a font has 1000 UPM, to get a text string with a height of 100 pixels (assuming that we use a raster output device), the rendering system would scale this font by 10%. Generally in western fonts it is 75–80% above and 20–25% below. The position of the baseline in the Em is always at zero in the X direction but how much of the Em is above the baseline and how much is below can vary. This can be more or less than the height of the Em. The font’s effective height is determined by the distance between the highest and lowest points on the outline, which might be the ascender and descender in western (Latin) fonts. It is also independent of any single writing system, allowing a common scale for things as diverse as symbol fonts, Arabic, Chinese and English (this diversity is why we can’t just specify all font size by, say, cap height). Defining a font in this fashion allows a single outline to be scaled to any size requested. For example, when type is scaled to 12 points, it is the Em that is 12 points high, not any particular/specific part of the type or its letters. When type is imaged on screen or in print, the Em is scaled to the desired size. In FontLab, you can set the UPM in File > Font Info > Family Dimensions. Think of these units and the grid as relative coordinates rather than as specific physical distances. The division is usually 1000 or 2048 units, referred to as Units Per eM (UPM) size. In digital type, the Em is simply an imaginary space, subdivided into a grid for design purposes. This may have gotten its name from an average letter “M” that required a block of about that width.
The Em in metal type was originally a square of metal the same height and width as the point size. Making a Font From Color Vector Graphicsįont Sizes and the Coordinate System » The Em and the UPM Size »Įach glyph is designed in a space called an Em.Detecting Element References or Composites.
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