To understand both
the reason why TrueType hinting is required and also how it works,
we need to understand how the TrueType rasterizer functions. The
stages required in displaying a character either on a screen or printer
can be simply defined as follows:
- Scale the outline
stored in the TrueType font to the required size. As the outline
is mathematically defined any scaling or indeed any transformation
is straightforward and precise.
- Overlay the character
outline onto the pixel grid of the output device. The grid
equates to the resolution of the device.
- Switch on any
pixels whose centres are enclosed within the character outline.
This process is known as scan conversion and generates a bitmap.
- Display or print
the image.
Without additional
controls the positioning of the outline on the output grid can
generate visual anomalies. Character stems that are exactly the
same width in the font may appear as different widths on screen
or printout. The reason for this is that two identical stems
can cut through fractions of a pixel. One may enclose one centre
and the other may enclose two centres resulting in one stem actually
appearing twice the thickness of the other! This is shown below.
The black outline
shows the calculated shape and the grey shape shows the rendered
image.
Note that the right hand stem is only one pixel wide, whereas the remainder
of the character is two pixels wide.
TrueType hinting
modifies the character outline prior to scan conversion. At this
stage small adjustments can be made to the outline to ensure
that stems are of equal width, that serifs appear consistently
and that the general aesthetics of the font are maintained. This
process can significantly increase the legibility of TrueType
fonts on low resolution devices such as screens.
The hinted 'H'
with all stems now 2 pixels wide.
The TrueType hinting
commands are very powerful and by far exceed the power of any
other hinting technology used in other fonts such as PostScript.
The hinting process is complex and can be very time consuming,
but can also produce significant improvements over an unhinted
version when used at small sizes on screen.
As a general rule,
all TrueType fonts that contain typefaces should be hinted as
they will be used at small sizes on screen. However, for fonts
containing logos, symbols and signatures we usually
produce these as unhinted fonts. If the logo font is to be used
on screen at small sizes we can provide a quotation for this
work, but we have found that a high quality unhinted logo font
is usually adequate for use at larger sizes or for use with printed
documents.
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