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

[Behdad Esfahbod]

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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