EM square

itemone's picture

I'm currently designing a typeface using an EM square of 2048 points (the standard used in TT-fonts). Will my typeface suffer any problems if I were to change this to an EM square of 2000 points, as this fits better with my requirements.

John Hudson's picture

An sfnt font (TTF, OTF) can have an em square of any size. There is a gain in performance speed if the em square is a power of 2, hence 2048 being the most common standard, but this is not a requirement of the specification. Note, however, that some customers, e.g. Microsoft, make a power of 2 a procurement requirement because of the performance gain. From what I understand, this performance gain is pretty small, but MS consider it significant enough to be desirable.

Thomas Phinney's picture

<font class="dontLookLikeCrap">This is one of those cases where an ancient issue has become almost religious belief.

Adobe's testing has never shown any performance difference based on an em square being a power of 2. I've never seen Microsoft point to any testing that demonstrated it such a performance difference, despite being asked.

The original expectation (and likely reality) of a performance difference for TrueType outlines was specific to the 68K Motorola series of processors, and byte-shifting operation optimization within those processors. This rationale does not transfer to PowerPC or Intel processors.

Also, I've never seen a claimed performance difference for PostScript outlines, though it would seem to be just as probable (or improbable today).



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