Re: [ft-devel] max_instructions in ttfautohint

[Cosimo Lupo]

Thank you Werner.

I just forwarded the question to the OpenType mailing list.

I wonder, does FreeType actually use that value when allocating memory for 
TrueType instructions?
I guess it doesn’t..

And yes, FVal is very old — although the warning is in accordance with the TT 
and OT specs on the matter.

Let’s see what people from MS have to say.  
Thanks again,

Cosimo  

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On Monday, 8 June 2015 at 11:37, Werner LEMBERG wrote:

>  
> > 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 count 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 should be?
>  
> Honestly, I don't know. I simply go the safer route by specifying a
> larger value. I suggest to ask this question on the OpenType mailing
> list, where all the companies' font experts are chatting. However, my
> gut feeling is that it doesn't matter today, and we can simply ignore
> the warning.
>  
> Regarding FontValidator: The binary available from the MS typography
> site is sooo old (at least it was the last time I looked). Maybe you
> could also urge MS to make an updated version available, which is
> certainly already used internally at MS.
>  
>  
> Werner