[Freefont-bugs] Combining Diacritical Marks (CDMs)

[Josef W. Segur]

This Unicode range (U+0300 - U+036F) contains non-spacing marks
meant to be combined with almost any base character to add a
needed accent or other modification.

After the base character is drawn, the reference point is moved
by the width of the character. This implies that the outline of
a non-spacing mark should have negative horizontal values in
order to be combined with the base character. The "non-spacing"
also implies zero advance width for the mark so it does not move
the reference point.

TrueType fonts which support most of that range (Arial Unicode MS,
Caliban, Caslon, Code2000, Lucida Sans Unicode, Thryomanes) code
the glyphs as above with few exceptions. The Free fonts generally
don't.

It is possible for a text layout engine to ignore those metrics,
as Microsoft's Uniscribe seems to do for a dozen or so of the CDMs.
I put together a little test using the first 40 CDMs and showing
the results with Lucida Sans Unicode and Code2000 on Win95:
<URL: http://users.westelcom.com/jsegur/cdmtst4.html >
For comparison, what can be done on my system with Free Serif and
Uniscribe is in the image:
<URL: http://users.westelcom.com/jsegur/ubpfserf.png >

-- 
                                                 Joe