Give me the Blues

Let’s continue with the automatic hinting. One characteristic property of (Latin) fonts are the so called Blue Zones. Think of the Blue Zones as the lines in your notebook from the 1st class in school when you learned writing. Translated to fonts, a blue zone is a horizontal stripe within which all points must be aligned to get evenly rendered text. The following diagram illustrates the six blue zones that are calculated in Freetype and Classpath for Latin fonts:

The blue zones are (from top to bottom) SMALL_F_TOP, CAPITAL_TOP, SMALL_TOP, SMALL_BOTTOM and CAPITAL_BOTTOM (collapsed into one zone) and SMALL_MINOR.
The above diagram can be a little misleading, as it seems to imply that these zones are lines. Which is not true. A blue zone is a thin stripe that envelopes the maxima of certain test glyphs. For instance, in order to find out the CAPITAL_TOP blue zone, we analyse the glyphs for the letters THEZOCQS. The maxima of their top points span up the blue zone as illustrated in the next figure:

Later in the hinting process we align all points within such a blue zone at a fixed height. This smooths out the vertical hoppledihopp that is visible in non-hinted rendered TrueType fonts.

Stay tuned to find out how the stem widths are comuted and what else there is done in the autohinter.

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