Question on DSIG and head flag 11

Hello,

I’d like to ask the Working Group about the correct interpretation of two related conformance requirements: namely, to drop the DSIG table from the input font data, and to set the flag 11 from the ‘head’ table.

It is not clear to me whether these requirements apply to TrueType-flavoured fonts only (i.e. containing ‘glyf’ and ‘loca’ tables), or also to CFF-flavoured fonts.

It is my understanding that a CFF-flavoured font does not undergo any "modifying transform" when it is compressed to WOFF2. Brotli (de-)compression is lossless and produce data which is bitwise identical. The only thing that could modify the overall checksum of the decompressed font, compared to the original input font, is a reordering of the table data, since this will change the offset values in the table directory. However, the spec does not say that an encoder should reorder tables, other than requiring loca to follow glyf (plus having them paired in font collections).  

So it appears a bit strange to me that — at least in the reference implementation —, for CFF fonts the only “modifying transform” that might “invalidate the DSIG” is… the very act of dropping of the DSIG!

Not that I like DSIG tables -- and I know that web browsers don’t care about it. But if one wants to get rid of it in WOFF2 fonts, I think the spec should be more clear as to the reasons for dropping it, and whether this equally involves TT- and CFF-flavoured fonts.

The examples put forward in the spec all have to do with TT-flavoured fonts:

> due to certain possible encoding variations (such as e.g. various levels of optimization of outline point coordinates in the 'glyf' table, or difference in offset calculations of the 'loca' table) […]

Finally, about the bit 11 of the ‘head’ flags, I guess that for the same reasons above this only applies to TT-flavoured fonts.

I’d be nice if the spec could be more clear about these two requirements.
Thanks a lot for you work,

--  
Cosimo Lupo, Font Design, Dalton Maag Ltd
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Received on Tuesday, 31 March 2015 12:20:28 UTC