Proposal: remove maxstack from CFF2

[Peter Constable]

Vlad:

Sairus and I had some follow-up discussion on this, and I’ve drafted changes to the CFF2 and CFF2 CharString chapters for the next version of the OpenType spec. (There are some other unrelated edits as well.) Please find these attached — and comments welcome.


Peter

From: Levantovsky, Vladimir [mailto:Vladimir.Levantovsky at monotype.com]
Sent: Sunday, April 2, 2017 5:02 PM
To: Peter Constable <petercon at microsoft.com>; Sairus Patel <sppatel at adobe.com>; mpeg-OTspec at yahoogroups.com
Subject: RE: Proposal: remove maxstack from CFF2

Hi Peter, Sairus, all,

Regardless of technicalities on how this proposed change is going to be implemented in the OT1.8 spec, if we don’t have an objection to the actual content changes proposed by Sairus and want to make the corresponding changes in the ISO/IEC 14496-22 Working Draft - we need to submit the proposed changes as an input contribution to WG11. I can ask to consider the late input contribution for this meeting (and, hopefully, have it allowed it to be processed during the meeting this week) but we need to submit it first, and the sooner the better.

Sairus, let me know if you need any help with the submission process.

Thank you,
Vladimir


From: mpeg-OTspec at yahoogroups.com<mailto:mpeg-OTspec at yahoogroups.com> [mailto:mpeg-OTspec at yahoogroups.com] On Behalf Of Peter Constable petercon at microsoft.com<mailto:petercon at microsoft.com> [mpeg-OTspec]
Sent: Thursday, March 30, 2017 9:07 PM
To: Sairus Patel; opentype-list at indx.co.uk<mailto:opentype-list at indx.co.uk>; mpeg-OTspec at yahoogroups.com<mailto:mpeg-OTspec at yahoogroups.com>
Subject: [mpeg-OTspec] RE: Proposal: remove maxstack from CFF2


> 7. In Appendix E, add a bullet to the list:
> OpenType 1.8.2:
This wording is problematic in as much as there is no OpenType 1.8.2. Please let me start this change process by reviewing this and figuring out the appropriate way to incorporate into OpenType.



Peter

From: mpeg-OTspec at yahoogroups.com<mailto:mpeg-OTspec at yahoogroups.com> [mailto:mpeg-OTspec at yahoogroups.com] On Behalf Of Sairus Patel sppatel at adobe.com<mailto:sppatel at adobe.com> [mpeg-OTspec]
Sent: Thursday, March 30, 2017 3:08 PM
To: opentype-list at indx.co.uk<mailto:opentype-list at indx.co.uk>; mpeg-OTspec at yahoogroups.com<mailto:mpeg-OTspec at yahoogroups.com>
Subject: [mpeg-OTspec] Proposal: remove maxstack from CFF2


A little operator, maxstack, crept into the CFF2 spec. We think is not worth it after all. Behdad and others have pointed this out. We’ve thought about it more, and agree. Among the many, many decisions we needed to make for CFF2, this is one we think we should roll back.

We believe we can relatively safely remove it before CFF2 adoption progresses too much further. (Werner is on board re. FreeType’s CFF2 rasterizer as well.) Let us know your thoughts, but we don’t anticipate that this will be contentious.

Here are the details. We propose that CFF2 be modified as follows:

1. In sec 7, Table 9: Top DICT Operator Entries, remove the row for maxstack.

2. In sec 7, replace the following paragraph:

Operators in Top and Font DICTs may be preceded by up to a maximum of 193 operands. The maximum argument stack may be changed by the maxstack operator in the CFF2 Top DICT; this will set the maximum stack value for all CharStrings and Private DICTs. The permitted range of values is 193 to 513 inclusive.

by:

Operators in Top DICT, Font DICTs, Private DICTs, and CharStrings may be preceded by up to a maximum of 513 operands.

3. In sec 12, replace “vsindex, blend, vstore, and maxstack” by “vsindex, blend, and vstore”.

4. Also remove the entry for maxstack in the “Syntax for Font Variations support operators” table further down in sec 12.

5. In Appendix C, Table 17 One-byte CFF2 DICT Operators, remove the row for maxstack, and change the subsequent row to:

25 to 27   |   19 to 1b   |   <reserved>

6. In Appendix D, replace:

Two new Top DICT operators have been added: vstore, and maxstack.

by:

One new Top DICT operator has been added: vstore.

7. In Appendix E, add a bullet to the list:

OpenType 1.8.2:

- Removed all mention of Top DICT operator maxstack (25); it was deemed not to be of enough value.

8. (If appropriate) On OpenType’s Errata page, add an entry similar to 7 above.

Sairus


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