[MPEG-OTSPEC] exponentiation (was RE: "font directory" / "offset table" / "table directory"

[Norbert Lindenberg]

You could simply add “where ** is the exponentiation operator”.

Developers working on font-related tools tend to be familiar with Python, which uses ** (or pow()) for exponentiation, while ^ is bitwise XOR.

Best regards,
Norbert
Lindenberg Software LLC



> On Aug 30, 2020, at 21:58, Peter Constable <pgcon6 at msn.com> wrote:
> 
> https://github.com/MicrosoftDocs/typography-issues/issues/77
>  
> The notation “**” is not understood by some readers. It was reported by one as an error. The spec needs to be unambiguously interpretable by all.
>  
>  
> Peter
>  
> From: Behdad Esfahbod <behdad at behdad.org> 
> Sent: Sunday, August 30, 2020 7:45 PM
> To: Peter Constable <pgcon6 at msn.com>
> Cc: MPEG OT Spec list (mpeg-otspec at lists.aau.at) <mpeg-otspec at lists.aau.at>
> Subject: Re: [MPEG-OTSPEC] exponentiation (was RE: "font directory" / "offset table" / "table directory"
>  
> On Sun, Aug 30, 2020, 6:53 PM Peter Constable <pgcon6 at msn.com> wrote:
> This describes symbols used in various languages:
> 
> https://en.wikipedia.org/wiki/Exponentiation#In_programming_languages
> 
> There isn't a perfect answer. I think the best answer is whatever would be most intuitive / less likely to be mis-interpreted.
>  
> Which happens to be "**". What's the problem again?
>  
>  
> 
> -----Original Message-----
> From: Peter Constable <pgcon6 at msn.com> 
> Sent: Sunday, August 30, 2020 5:43 PM
> To: Peter Constable <pgcon6 at msn.com>; Laurence Penney <lorp at lorp.org>; MPEG OT Spec list (mpeg-otspec at lists.aau.at) <mpeg-otspec at lists.aau.at>
> Subject: exponentiation (was RE: [MPEG-OTSPEC] "font directory" / "offset table" / "table directory"
> 
> Separate topic:
> 
> In the description of cmap format 4, "**" is used as an exponentiation operator in the description of the searchRange field, but it appears some younger readers don't recognize that notation from older programming languages. So, I different notation should be used, preferably one that would be readily understood by readers who might or might not be familiar different language. (This will apply also to description of a similar field in the table directory (aka offset table).)
> 
> One possibility is to use pow(). E.g., 
> 
> 2 * pow(2, floor(log2(segCount)))
> 
> 
> Another is to use "^". E.g., 
> 
> 2 * (2 ^ floor(log2(segCount)))
> 
> (I'd keep the extra parens to avoid any uncertainty regarding operator precedence.)
> 
> 
> There may be other possibilities.
> 
> Recommendations?
> 
> 
> Peter
> 
> -----Original Message-----
> From: mpeg-otspec <mpeg-otspec-bounces at lists.aau.at> On Behalf Of Peter Constable
> Sent: Sunday, August 30, 2020 5:32 PM
> To: Laurence Penney <lorp at lorp.org>; MPEG OT Spec list (mpeg-otspec at lists.aau.at) <mpeg-otspec at lists.aau.at>
> Subject: Re: [MPEG-OTSPEC] "font directory" / "offset table" / "table directory"
> 
> I had considered "^", but that is bitwise XOR in C, Python and other languages du jour. I'm open to input, though.
> 
> (This is yet another topic.)
> 
> -----Original Message-----
> From: mpeg-otspec <mpeg-otspec-bounces at lists.aau.at> On Behalf Of Laurence Penney
> Sent: Sunday, August 30, 2020 1:39 PM
> To: MPEG OT Spec list (mpeg-otspec at lists.aau.at) <mpeg-otspec at lists.aau.at>
> Subject: Re: [MPEG-OTSPEC] "font directory" / "offset table" / "table directory"
> 
> > On 30 Aug 2020, at 20:43, Peter Constable <pgcon6 at msn.com> wrote:
> > 
> > Separating the fingerprint from the rest of the structure would end up being messy in describing the organization of TTCs … it feels cleaner to say the offsets are to the table directory of each font resource, and that the font resource beings with sfntVersion.
> 
> Yes, that makes sense.
> 
> >> To avoid confusion it may be helpful to provide pseudocode for calculating them.
> > 
> > There are similar fields in cmap format 4, and they are described with formulae. E.g., "2 × (2**floor(log2(segCount)))". Now, one of the issues opened on that chapter is that someone wasn't familiar with the Fortran "**" operator and so misunderstood the intent. It seems like something less language specific is needed. Taking that in mind, something similar could be done here. E.g., "pow(floor(log2(numTables)), 2) × 16". Or include both prose description and formula.
> 
> Good point, "**" is not clear to whippersnappers.
> 
> BTW the two definitions can be made identical apart from the factors 16 (for table directory) and 2 (for cmap). Please also consider an asterisk (*) over the typographic multiply sign, unless the whole thing uses math typography, and also consider the ascii circumflex (^) over pow().
> 
> - Laurence
> 
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