gaps in stacked symbols

[Luca Padovani <lpadovan cs unibo it>]

Hi all,

I've implemented a piece of code which stretches some math symbols like
parentheses so as to span a give height. Using pango API this is
restricted to the symbols for which Unicode characters representing
"pieces" of those symbols are available, for example U+239B is "left
parenthesis upper hook" and U+239d is "left parenthesis lower hook". The
symbols are stacked on top of each other according to their bounding box
(ink), however, as can be seen in the screenshot

http://www.cs.unibo.it/~lpadovan/secret/ss1.png

there is a small gap between the pieces. I wonder if this is due to some
unprecise metrics in the font or if the application is itself
responsible for putting the pieces a little bit closer than their boxes
tell. But then how much this "little bit" is?

A second screenshot taken on a different machine with different fonts
shows the same problem with the integral sign, but not with the
parentheses:

http://www.cs.unibo.it/~lpadovan/secret/ss2.png

Note however that the parentheses have been shaped with a non-scalable
font which I'm not able to identify!! (the gucharmap applications says:
Fixed, no more than that). Is there any way for querying which font was
actually used? I'm curious since I haven't been able to reproduce the
latter screenshot on any other machine but my laptop.

TIA,
--Luca