This gets interesting. Did a bit more digging and found:
First, in PPT 2003 and 2007, if I set text in MS Arial Unicode, then try to do
Replace Fonts, it only offers me (what I think are) Unicode fonts in the
Replace With dropdown.
It also restricts the Replace With choices in some other circumstances.
Next, doublebyte is not the same as Unicode, at least not in the usual way
"doublebyte" is used.
This is probably way oversimplified, but in Unicode, every character is a two
byte number that tells the system where the font stores the info needed to draw
the character.
Before Unicode, languages like Chinese/Japanese/Korean (CJK) used a different
type of encoding to point to the right character info in a font.
You might have a string of single bytes; these would point to the same
characters as our usual Western fonts. Then there might be a couple of "flag"
or "shift" bytes, an ESC character followed by another byte, for example.
This'd tell the system to shift to doublebyte mode; from then until another
shift sequence came by, the system would read the first byte of each character
pair as a pointer to the character TABLE, and the next byte as the pointer to
the character within that table.
Kinda like:
Letter 001
Letter 008
Letter 042
YO! SHIFT IN!
Page 012 Letter 008
Page 018 Letter 003
YO! SHIFT OUT!
Letter 018
Letter 012
etc., with none of the numbers > 256
Where Unicode is more like:
Letter 02345
Letter 00002
Letter 73459
and so on with none of the numbers greater than umptybazillion.
OK. So what, right? ;-)
Well, with all that shiftystuff, traditional doublebyte is fragile. Lose one
byte at the wrong time and your text is hosed.
And there were quite a few different doublebyte encoding systems; in Japanese,
they were all similar enough that it was VERY difficult (impossible in some
cases) to tell accurately which you were looking at. And all just different
enough that if you guessed wrong (or your PC did) you got gibberish.
So I'm guessing that MS is trying to guarantee first and foremost that they
don't mung our text. That'd mean not allowing us to substitute just any
doublebyte font for any other. The encoding would have to match or at least be
compatible.
And of course, substituting a single byte for a double byte font would
virtually guarantee that the text would get hosed, so they don't want to do
that either.
I suppose they could let you choose a different font on a provisional basis,
then read through all the text in the file and if there's so much as one non-
latin character that isn't available in the new font, kibosh the deal, but
that'd be slow, error prone, almost as frustrating to the user and would take a
lot of dev time to implement; time they could put to use against features/bugs
that'd affect far more people.
And why not let us sub Unicode fonts for any ol' double byte font? Because
there's no one-to-one match between characters and there's no guarantee that a
unicode font would even HAVE the needed characters. Hardly any of them have
the whole universe of possible characters, y'see. MS Arial Uni's the exception
(and wouldja lookit the SIZE of that thing!).
Arial has 1674 characters, Arial UNI 50,000+
Unicode fonts also have named groups and the font properties tell you whether
the font contains the characters in a given group, so PPT might be looking at
the original font and only presenting possible replacements that support the
same groups. If the original font is available; if not, we're back to the
original problem of working out what encoding's used, if we can, and whether
the characters needed by each character in each bit of text in the presentation
are supported by the chosen substitute font. And again, I figure that's too
much work and too likely to fail. So we get the PPT "Nuh-uh, not gonna"
instead.
Not that any of this helps solve the problem of course ... but I guess it's
interesting to know the breed of dog that's sunk its teeth into your butt and
why he's mad at you if you can't get him to turn loose in any case.
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