Cross-platform font handling, and why you don't want to do it

[Quentin Long]

Cross-platform font compatibility is a hellacious rat's nest of mismatches. It may or may not be worse than handling time (Daylight Savings Time? leap years? leap *seconds*? etc, ad nauseum), but it's plenty bad enough on its own merits, or lack thereof.
* You can't be 100% confident that your user will have the same font(s) you do installed on their system.
* If your user has a font *of the same name* as your font installed on their system, you can't be 100% confident that *their* font came from the same font foundry as *your* font. Yes, font names are legally protected, but... Helvetica, anyone?
* If your user has a font *of the same name* as your font installed on their system, you can't be 100% confident that it's the *same* font you have. Again: Font names are legally protected. But what if you have version 15.0 of RandomFontFoundry's WhateverFont on your system, but your user only has version 1.0 of RandomFontFoundry's WhateverFont? Who *knows* what tweaks RandomFontFoundry may have made to WhateverFont in between those two version numbers?
* Can you be 100% confident that version 15.0 of RandomFontFoundry's WhateverFont *for Mac* is going to render *exactly the same* as version 15.0 of RandomFontFoundry's WhateverFont *for Windows* or *for Linux*? No. You cannot. Different type rendering engines on different OSes *ensure* that rendering compatibility will be problematic at best.
* Assuming your user has exactly the same version of exactly the same font on exactly the same OS you do, you can't be 100% confident that your user has installed the font metrics data (kerning, sidebearings, etc)—which means you have no idea if the damn thing will look the same on your user's system as on your system.
If you want to *ensure* that some piece of text looks *exactly the same* cross-platform, the only way to get there is to format the text the way you want it on one platform, take a screenshot, and use that graphic. But even then, can you be 100% confident that your user *hasn't* tweaked some setting on their system which messes with graphics..?

The "take a screenshot" solution is not well-suited for any use-case involving text which varies from time to time, and largely impractical for any use-case involving text which is input by the user. In principle, it should be possible to create a library of glyph-images of all glyphs in a font, and import those glyph-images as necessary—no, mass quantities of "set the imageSource of char X to $GlyphImage" commands ain't gonna fly, cuz letterspacing will look crappy—but that "solution" is tantamount to building your own, redundant, set of text-handling routines. Yuck!
If "take a screenshot" doesn't work for you, I suspect the next-best route to achieving Absolute Cross-Platform Font Fidelity may be this:
 1) Open up your font in your font-editing utility of choice (Fontographer, Glyphs, FontLab, whatever)
2) Export your font N times, once for each of the *other* OSes you need font compatibility with—WhateverFontMac, WhateverFontWindows, WhateverFontLinux, etc
3) Open up your stack within each of the OSes you're working with, always using the appropriate OS-specific version of your font, and see how different your stack looks on the other OSes
4) Tweak the font metrics on the other-OS versions of your font until your stack looks the same on all OSes
5) When it's time to distribute your stack, be sure to bundle *all* the hand-tuned OS-versions of your font with your stack
This procedure is, of course, a royal pain in the arse. Hellacious rat's nest of mismatches, yada yada yada. But regardless of how painful it is? If you need absolute visual fidelity cross-platform, *and* total flexibility in the *content* of text, this procedure may be your least-bad option.
As a perhaps-preferable option:
1) Open up your stack on each of the OSes you're striving for font compatibility on
2) Tweak the appearance-relevant text properties (textFont, textHeight, textShift, etc) on each OS until you get an acceptable approximation of identical appearance
3) Make note of the values of all these text properties for each OS
4) Assuming you have however-many fonts specified by fontName, be sure you bundle all those fonts with your stack when you distribute it
5) Include in your stack's preOpenStack handler a switch which includes "case = WhateverOS" for each OS you want font compatibility for
6) Within each OS's "case", set all the appearance-relevant text properties for that OS
This procedure is not going to achieve anywhere near the degree of cross-platform font fidelity as the "roll your own" solution above. On the plus side, it's *significantly* less nitpicky/picayune/painful. You puts in your money, and you takes your chances...

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