EM square

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Torben Holm's picture
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Joined: 8 May 2004 - 3:20pm
EM square
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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
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Joined: 21 Dec 2002 - 11:00am
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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
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Joined: 3 Sep 2002 - 11:00am
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<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).

Regards,

T</font>