FW: What is the head table checksum for collections?

Dear AHG members,

Today, during  the WebFonts WG call a question was brought up (see below) regarding support for font collections and the meaning of the checkSumAdjustment value for font collections (and how it should be calculated). We introduced quite a few changes in the spec relevant to font collections, this is probably another one that would benefit from clarification. Please respond to this email with your comments and suggested changes. I am copying both email lists on this thread so that the members of two groups can participate in the discussion.

Thank you,
Vladimir


From: Roderick Sheeter [mailto:rsheeter@google.com]
Sent: Wednesday, November 19, 2014 4:39 PM
To: WebFonts WG
Subject: What is the head table checksum for collections?

It is unclear what the head checksum means for collections.

ULONG

checkSumAdjustment

To compute: set it to 0, sum the entire font as ULONG, then store 0xB1B0AFBA - sum.


http://www.microsoft.com/typography/otspec/head.htm


The "entire font" isn't entirely clear for a collection. Does it mean the whole collection or the whole of the individual font? If the "entire font" is the individual font within the collection, what happens if the head table is shared among fonts within the collection?

One option that could make sense would be to define it as over the entire collection, setting it to 0 for *all* head tables in all fonts in the collection, doing the sum, and then updating all of them.

FYI, I asked some font experts at Google and the feedback I got was essentially that this checksum is largely ignored for collections and what value it should have isn't well defined. If we could get it better defined that would be great.

Thoughts?