OTF/CFF global/local subroutines

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MaKess
MaKess Posts: 1
in Font Technology

TLDR: in CFF is it legal to call a local subroutine from a global subroutine?

Hi everyone,

I'm currently working on some software that parses/manipulates charstrings that represent the glyphs in CFF (Compact Font Format) fonts.
While working on the part that handles global and local subroutines, I was initially working under the assumption that:

  • charstrings can call local and global subroutines (i.e. use callsubr and callgsubr)
  • local subroutines can call other local and global subroutines (i.e. use callsubr and callgsubr)
  • global subroutines can only call other global subroutines (i.e. use callgsubr)
(which I though is what this documentation described)

however, this seems not to be the case: when I ran some tests on a number of font files, I discovered that for example in "Nimbus Roman Italic" (a standard font installed on Ubuntu Linux) global subroutines exist that contain the callsubr opcode, thereby trying to call local subroutines.

In the context of OTF fonts this can be worked around relatively unambiguously as OTF restricts CFF to contain only one CharSet and therefore only one private dict with local subroutines. But in a general case this seems like a bad idea.

Question:

  • did I misunderstand the restrictions? (i.e. is this in fact legal behavior?)
  • or are those font-files ill-formed?
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