About fonts > TrueType
TrueType Hinting (5 of 5):
Hinting vs. Other Methods
TrueType affords the designer or engineer more flexibility and control over the
final bitmap appearance than any other font format in use today. Through
the use of the extensive range of commands in the TrueType instruction set, the
designer is able to move any point on a glyph's outline as little or as much as
necessary to turn on or off any pixel on the bitmap grid. This provides as much
control as a simple bitmap editor. Commands can be used not only to improve
legibility of fonts at small sizes, but also to fundamentally alter the
appearance of any bitmap at any size - enabling the production, for example, of
a font where different sizes will produce a different shape. Because TrueType is
a programming language (at the assembly level), the format offers font designers
and engineers an amazing degree of versatility. The sizes affected by the
hinting commands can also be determined by setting low and high size thresholds,
A practical example of this flexibility was demonstrated recently in a Microsoft
font designed specifically for display on television under the NTSC signal.
During preliminary work it became clear that at small sizes it would be
impossible to render certain character features satisfactorily - such as the
diagonal bar of the uppercase N. The designer, conscious of making every glyph
as individual as possible, established a threshold below which he programmed
certain characters to change shape. Illustrated below are the outlines and
resulting bitmaps from 21 and 22 ppem.
Figure 9. The outlines and resulting bitmaps at sizes of 21 ppem and 22 ppem. Despite the differences in shape, these bitmaps are produced by the same font. By adjusting the outline sufficiently, the designer has managed to ensure that each bitmap remains individual and identifiable.
Since TrueType enables the designer or programmer to alter a letterform at every
single size, it follows that optical scaling can be built into the TrueType
font, allowing the subtle adjustment of characters at different sizes to ensure
their correct appearance.
The designing of bitmap letterforms has always involved a degree of optical
scaling, with each and every size having to be rasterized - and often designed -
separately. Unfortunately, these modifications have always involved, even been
determined by, an element of compromise; this pixel or that pixel?
In a TrueType font, the ability to create optically correct letterforms is
extended up to very large sizes (up to 2048 ppem) and by the capabilities of
TrueType Open, a new extension to the TrueType specification (EDITOR: This eventually
evolved into the OpenType initiative).
Non-linear scaling is often (understandably) confused with optical scaling.
Whereas the concept of optical scaling involves altering the shapes of
characters in order to ensure a correct appearance at particular sizes,
non-linear scaling in TrueType simply enables the widening or narrowing of the
widths of a glyph and its side-bearings at sizes where, were the widths to scale
in a linear fashion (for example, increasing in a regular pattern from small to
large sizes) we might encounter spacing or weight problems. In a situation
where, for example, the lowercase 'i' would normally scale to occupy an advance
width of four pixels, we might elect to force it to fit within three pixels,
placing one pixel of white space on either side of a one pixel black stem. In
such cases, we are clearly forcing the font to scale in a non-linear fashion.
In effect, non-linear scaling means that TrueType hinting is not limited merely
to controlling the shape of each character. The type designer is able to adjust
the inter character spacing by varying the amount of white space to either side
of a letterform. This facility is critical in helping maintain an even,
consistent color across lines of text, and is a highlight of TrueType hinting
not available in other font formats.
Diagonal control is another feature of TrueType hinting that helps lead to
better visual quality in a font. Keeping diagonal strkes as symmetrical as
possible helps to avoid unnecessary jaggedness or 'stepping'. The use of
separate vectors for measuring distances and moving points on a glyph's outline
enables a very fine degree of control over point positioning in a diagonal
stroke element. In the example shown below it can be seen that the distance a
point must be moved to ensure a correct bitmap stroke is measured in one
direction, while it is actually moved in a slightly different direction. This
feature allows us to measure the true width of an element such as a diagonal
stroke, and to maintain it, rather than basing our judgments inaccurately on a
straight horizontal measure. Attaining stroke weight consistency with this
feature can also aid the color, spacing and symmetry of a font at small sizes
and low resolutions.
Figure 10a. TrueType enables the measurement of a distance such as a diagonal stroke weight along the projection vector, while the movement of the point to be shifted can take place completely independently...
...enabling the exact positioning of the outline in order to create a perfect pixel pattern.
Controlling diagonals in this manner enables TrueType fonts to represent italic
and oblique letterforms with much greater fidelity than other font formats. The
illustration below shows how much more even TrueType diagonal control can make
Figure 10b. Diagonal control in an italic font. ATM Times Italic at 11 ppem (8 points).
TrueType Times New Roman Italic at 11 ppem (8 points).
introduction | what is hinting? | why is it necessary? | how does it help? | TrueType hinting vs. other methods
Intelligence in the hints
With TrueType, the intelligence is in the hints rather than in the rasterizer.
That is, all alterations to the original outline description are performed
through instructions contained in the font rather than by the rasterizer
acting on its own. This approach has three important implications.
First, it means that the bulk of the calculations takes place during font
production rather than at runtime. In this sense, TrueType is similar to a
compiled language, while outline font technologies that perform most of their
work during execution are more like interpreters.
Second, having the intelligence in the hints means that font vendors can
precisely control the final appearance of the fonts - because they apply the
hints. In contrast, with approaches that rely on the rasterizer to apply hints
or perform other outline adjustments, font vendors have less control over the
final appearance of the fonts.
Finally, having the intelligence in the hints allows tool vendors to improve
their hinting technology without requiring users to buy a new rasterizer or
additional printer ROM. This means that end users can upgrade their fonts for
just the price of the fonts themselves, without incurring any other hardware or
Last updated 30 June 1997.
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