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TrueType Hinting (3 of 5):
Why is Hinting Necessary?

Scaling an unmodified outline's control point co-ordinates to the small sizes of a computer screen can result in severe quality control problems. At low resolutions, with few pixels available to describe the character shapes, features such as stem weights, crossbar widths and serif details can become irregular, inconsistent or even missed completely. These irregularities detract substantially from the legibility and overall attractiveness of a text setting.

These problems are a result of the absolute and finite size of the pixel. Mathematically scaling a character outline presents no problem until the pixel grid of the output device is introduced; at this point it is possible that parts of the outline will pass through only a fraction of a pixel, rather than containing the pixel completely. If the pixel is turned on in such a case, that part of the curve will be wider than the original outline; if it is left off, it will be narrower.

The decision whether or not to turn such a pixel on or to leave it off is a crucial one. On systems and displays utilizing the capabilities of a grayscale rasterizer, the difficulty is lessened somewhat by the possibility of using different levels of gray, but when the text is displayed as a monochrome bitmap, the rasterizer can only make a binary choice (on or off, black or white, 1 or 0). In this case, if the outline has not been grid-fitted by hints, the bitmap will be produced by the results of point co-ordinates being mathematically rounded up or down. The introduction of the 'chance' effects of rounding up and down can significantly impair the quality of type at low resolutions. As figure 2 below shows, rounding can have some unpleasant side effects. If you would like more information about using levels of gray to improve the appearance of text on the screen, see our information about Smooth fonts in Windows 95 (also known as grayscaling or anti-aliasing).

Hinting diagram
Hinting diagram

Figure 2a. Note how heavy the upper serif has become in comparison with the stem weights.

Figure 2b. Here, rounding of points has resulted in uneven vertical stem weights and missing serif details.

Hinting diagram

Figure 2c. This lowercase 'm' has uneven stems and arches.

Next section:  how does hinting help?

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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