Font hinting
Encyclopedia
Font rasterization
Font rasterization is the process of converting text from a vector description to a raster or bitmap description. This often involves some anti-aliasing on screen text to make it smoother and easier to read...
grid. At low screen resolutions, hinting is critical for producing a clear, legible text. It can be accompanied by antialiasing and (on liquid crystal display
Liquid crystal display
A liquid crystal display is a flat panel display, electronic visual display, or video display that uses the light modulating properties of liquid crystals . LCs do not emit light directly....
s) subpixel rendering
Subpixel rendering
Subpixel rendering is a way to increase the apparent resolution of a computer's liquid crystal display or Organic Light Emitting Diode display by rendering pixels to take into account the screen type's physical properties...
for further clarity.
Overview
For the purpose of on-screen text display, font hinting designates which primary pixels are interpolatedInterpolation
In the mathematical field of numerical analysis, interpolation is a method of constructing new data points within the range of a discrete set of known data points....
to more clearly render a font.
Hints are usually created in a font editor
Font editor
A font editor is a class of application software specifically designed to create or modify computer font files.Font editors differ greatly depending if they are designed to edit bitmap fonts or outline fonts. Modern font editors mostly deal with the outline fonts, because bitmap fonts are an older...
during the typeface design process and embedded in the font. A font can be hinted either automatically (through processed algorithms based on the character outlines) or set manually. Most font editors are able to do automatic hinting, and this approach is suitable for many fonts. However, commercial fonts of the highest quality are often manually hinted to provide the sharpest appearance on computer displays. Verdana
Verdana
Verdana is a humanist sans-serif typeface designed by Matthew Carter for Microsoft Corporation, with hand-hinting done by Thomas Rickner, then at Monotype. Demand for such a typeface was recognized by Virginia Howlett of Microsoft's typography group...
is one example of a font that contains a large amount of hinting data, much of which was accomplished manually by type engineer Tom Rickner.
Implementations
One popular and recognizable form of hinting is found in the TrueTypeTrueType
TrueType is an outline font standard originally developed by Apple Computer in the late 1980s as a competitor to Adobe's Type 1 fonts used in PostScript...
font format, released in 1991 by Apple Computer
Apple Computer
Apple Inc. is an American multinational corporation that designs and markets consumer electronics, computer software, and personal computers. The company's best-known hardware products include the Macintosh line of computers, the iPod, the iPhone and the iPad...
. Hinting in TrueType invokes tables of font data used to render fonts properly on screen. One aspect of TrueType hinting is grid-fitting, which modifies the height and width of font characters to line up to the set pixel grid of screen display. The open-source FreeType
FreeType
FreeType is a software library written in C that implements a font rasterization engine. It is used to render text on to bitmaps and provides support for other font-related operations.-Details:...
2 font rendering engine uses an auto-hinter when such hinting data is not present or its use is restricted by a software patent
Software patent
Software patent does not have a universally accepted definition. One definition suggested by the Foundation for a Free Information Infrastructure is that a software patent is a "patent on any performance of a computer realised by means of a computer program".In 2005, the European Patent Office...
. As of 2011, the FreeType website states that the relevant font hinting patents have now all expired, and TrueType is now enabled in FreeType by default.
Guidelines
According to the TrueType Reference Guide, font instructors (those performing font hinting) must balance the following two constraints when hinting a font:- At small sizes, chance effects should not be allowed to magnify small differences in the original outline design of a glyph.
- At large sizes, the subtlety of the original design should emerge.
The Guide suggests that, for screen viewing, fonts should be readable at 9 points per em
Em (typography)
An em is a unit of measurement in the field of typography, equal to the currently specified point size.The name of em is related to M. Originally the unit was derived from the width of the capital "M" in the given typeface....
at 72 dpi
DPI
DPI may stand for* Dots per inch, a measure of printing resolution* Dhaka Polytechnic Institute, an educational institution of Bangladesh* Death Pact International, one of the first open source musical concept projects...
. Particular attention should be paid to the cap height
Cap height
In typography, cap height refers to the height of a capital letter above the baseline for a particular typeface. It specifically refers to the height of capital letters that are flat—such as H or I—as opposed to round letters such as O, or pointed letters like A, both of which may display...
, x-height
X-height
In typography, the x-height or corpus size refers to the distance between the baseline and the mean line in a typeface. Typically, this is the height of the letter x in the font , as well as the u, v, w, and z...
, and baseline
Baseline (typography)
In European and West Asian typography and penmanship, the baseline is the line upon which most letters "sit" and below which descenders extend.In the example to the right, the letter 'p' has a descender; the other letters sit on the baseline....
, so that the font retains its normal character while not producing exaggerated effects at small sizes.
External links
- The raster tragedy at low resolution or: why correct math looks wrong on screen and how to fix it. Microsoft Corporation. March 1998. A guided and illustrated demonstration by Beat Stamm, Lead Developer for Visual TrueType at Microsoft Typography, on to hint TrueType font glyphs for the screen.
- The Raster Tragedy at Low-Resolution Revisited: Opportunities and Challenges beyond “Delta-Hinting”. Beat Stamm. March 2011. A revised and extended version of the original article covering anti-aliasing including sub-pixel rendering, opportunities made possible by anti-aliasing, challenges in the rasterizer and elsewhere, and a discussion of font hinting in the context of these opportunities and challenges.
- FreeType
- Tutorial on the DejaVu font wiki
- Texts Rasterization Exposures Article from the Anti-Grain Geometry Project.