Re: [Freefont-bugs] Large rectangle with stretchy U+2F/U+2211/U+27E8/U+27E9 and GNUFreeFont

[Jonathan Kew]

On 17/4/15 16:07, Frédéric Wang wrote:
> Hi all,
>
> Here is a testcase with a dev version of GNUFreeFont:
> http://fred-wang.github.io/MathFonts/GNUFreeFont/
>
> As I understand, stretchy U+2F/U+2211/U+27E8/U+27E9 do not have a "glue"
> so we connect them using a rule
>
> https://dxr.mozilla.org/mozilla-central/source/layout/mathml/nsMathMLChar.cpp#855
>
> but the rule thickness has the width of the parts, which leads to these
> ugly large rectangles.

I don't see how a rule of *any* thickness would look good in the middle 
of a character such as / or ∑. See below...

> What do you think? Is it a bug due to our limited implementation of the
> MathVariants table (bug 963147)? Or is it incorrect for a font to
> specify a stretchy operator construction without any extender?

ISTM this is incorrect. The spec[1] says that glyph assemblies are used by:

1. Assemble all parts by overlapping connectors by maximum amount, and 
removing all extenders. This gives the smallest possible result.

2. Determine how much extra width/height can be distributed into all 
connections between neighboring parts. If that is enough to achieve the 
size goal, extend each connection equally by changing overlaps of 
connectors to finish the job.

3. If all connections have been extended to minimum overlap and further 
growth is needed, add one of each extender, and repeat the process from 
the first step.

Step 3 here implies that at least one extender glyph should exist in the 
assembly. Otherwise, it cannot be extended beyond the maximum size 
achieved by step 2.

Note also that extension can only happen, AFAICT, in a straight vertical 
or horizontal line. It doesn't look possible to extend diagonals; and 
therefore only variant glyphs, not extensible assemblies, should be used 
for characters such as slash or Sigma.

JK


[1] http://www.microsoft.com/typography/otspec/math.htm