[ft-devel] max_instructions in ttfautohint

[Cosimo Lupo]

Hi,

I've got a question about the maxp table's 'maxSizeOfInstructions' field.

When ttfautohint recalculates the maxp values, it also includes the size of the 
fpgm and prep instructions in the maxSizeOfInstructions value.

However, the Apple's TrueType Reference Manual says that only "instructions 
associated with a particular glyph" should be included in the computation.

Similarly, in Microsoft's OpenType spec its says the value is the "maximum byte 
cound for glyph instructions”.

When analysing a font produced by ttfautohint using Microsoft FontValidator, 
the latter raises the following warning (W1900):  
> maxp: The value doesn't match the calculated value
> The maxSizeOfInstructions value should be based on the largest set of 
> instructions (in the glyf table) for a single simple or composite glyph. The 
> maxStackElements value should similarly be based on the largest value for a 
> single simple or composite glyph. The length and content of the fpgm and prep 
> tables, used font-wide, are not relevent when specifying these values.
It is curious because even fonts produced with VTT, another Microsoft tool, 
seems to go against the spec by also including fpgm and prep when recalculating 
the maxp's maxSizeOfInstructions.

Therefore I'm wondering what the correct value shoud be?

Btw, there's also an open pull request on Behdad's fonttools about this issue:
https://github.com/behdad/fonttools/pull/274

Thank you,
Cheers

--
Cosimo Lupo