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TrueType Hinting (2 of 5):
What is Hinting?

At its most basic level hinting (or, more accurately, instructing) a font is a method of defining exactly which pixels are turned on in order to create the best possible character bitmap shape at small sizes and low resolutions. Since it is a glyph's outline that determines which pixels will constitute a character bitmap at a given size, it is often necessary to modify the outline to create a good bitmap image; in effect modifying the outline until the desired combination of pixels is turned on. A hint is a mathematical instruction added to the font to distort a character's outline at particular sizes. Technically, hints result in operations which modify a contours' scaled control point co-ordinates before the outline is scan converted. In TrueType a combination of these hints, and the resulting distortions, affords a very fine degree of control over the bitmap shape produced.

Modifying an outline in this manner results in what is known as a grid-fit. On the basis of the instructions contained in the individual font file, the TrueType rasterizer adjusts the glyph outlines to fit the bitmap grid appropriate for whichever size the text is to be displayed at. This outline adjustment is carried out on a case-by-case basis and is illustrated in figure 1 below.

diagram
diagram

Figure 1a. An outline that hasn't been grid-fitted. Note how poorly the outline corresponds to the pixel pattern, and above all how awkward the bitmap of the M is.

Figure 1b. The same outline grid-fitted. Now the outline has been adjusted to fit snugly around each pixel, ensuring that the correct pixels are turned on.

Grid-fitting explicitly ensures that certain features of the glyphs are regularized, and allows us to overcome many of the problems traditionally associated with displaying text at low resolutions. Because the outlines are only distorted at a specified number of small sizes, the contours of the letterforms at higher resolutions remain unchanged, and undistorted.

Although many font formats and applications offer some hinting facilities, these hints typically consist of a few global parameters that are only capable of specifying distances that should be kept the same. TrueType enables the designer to stipulate exactly how the glyphs and their spacing will appear at low resolutions in isolation as well as within a text setting.

Next section:  why is hinting necessary?

introduction | what is hinting? | why is it necessary? | how does it help? | TrueType hinting vs. other methods


Last updated 30 June 1997.

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