Delta Hinting
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.
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 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.
