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

[Peter Constable]

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<mailto:pgcon6 at msn.com>> wrote:
This describes symbols used in various languages:

https://en.wikipedia.org/wiki/Exponentiation#In_programming_languages<https://eur03.safelinks.protection.outlook.com/?url=https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FExponentiation%23In_programming_languages&data=02%7C01%7C%7C6cc6c9ffb7154a51002508d84d57e03f%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C637344387099775763&sdata=lCnbwtaqMjRC7LHyKu0CsjUehHIJJYRKs4VbRvv%2BOm4%3D&reserved=0>

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<mailto:pgcon6 at msn.com>>
Sent: Sunday, August 30, 2020 5:43 PM
To: Peter Constable <pgcon6 at msn.com<mailto:pgcon6 at msn.com>>; Laurence Penney <lorp at lorp.org<mailto:lorp at lorp.org>>; MPEG OT Spec list (mpeg-otspec at lists.aau.at<mailto:mpeg-otspec at lists.aau.at>) <mpeg-otspec at lists.aau.at<mailto: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<mailto: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<mailto:lorp at lorp.org>>; MPEG OT Spec list (mpeg-otspec at lists.aau.at<mailto:mpeg-otspec at lists.aau.at>) <mpeg-otspec at lists.aau.at<mailto: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<mailto: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<mailto:mpeg-otspec at lists.aau.at>) <mpeg-otspec at lists.aau.at<mailto: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<mailto: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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