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Building OpenType Fonts
Many OpenType fonts available today support features in the GSUB table (substitutions), which handles tasks like substituting "fi" with an "fi" ligature, or replacing lowercase characters with their small-caps equivalent. However, features in the GPOS table (positioning), are generally not supported, except for pair positioning (horizontal axis only), which is used for kerning. GPOS features are needed in any language system which requires combining diacriticals to be properly positioned in relation to the base character that they modify.
Furthermore, it is nearly impossible to find tools that are suitable for adding such features to OpenType fonts. (There are more options for building a font from scratch, as there is no need to decompile the font into a source format). Here is a summary of the font editing and building tools that we have evaluated so far:
| Product | General Decompile | General Compile | GPOS features Decompile | GPOS features Compile |
|---|---|---|---|---|
| FontLab 4.6 for Windows [1] | yes | yes | yes[2] | no |
| Adobe Font Development Kit (FDK) | no | yes | no | no |
| Microsoft Visual OpenType Layout Tool (VOLT) | no | yes | no | yes |
| DTL DataMaster [3] | yes | yes | ?? | no |
| TTX | yes | yes | yes | yes |
[1] FontLab appears to use the Adobe FDK for its compiler
[2] appears to decompile at least some features properly,
leaving them commented out (since it cannot compile them)
[3]DataMaster may also use the Adobe FDK, as it uses the
same source format and appears to have the same limitations
We are told that it will likely be several months at least before the Adobe FDK is enhanced to handle the missing GPOS features.
From the above table, it appears that TTX is the only tool that can be used to maintain font files for which source code is not available. See Using TTX for Font Development for more information.
This article was written by Victor Mote, April 20, 2004.