(mgp-users 00126) Re: embarassingly slow tfont handling...

[Luigi Rizzo <luigi@labinfo.iet.unipi.it>]

> >i think we have a small performance problem here... it is most
> >evident when i run mgp with truetype fonts and a not-solid background
> >(e.g. an image):
> >    rendering the slides is very slow!
> 
> 	This is because we do anti-aliasing with freetype, and it is not
> 	trivial to draw anti-aliased text on non-solid background.
> 	(you will need to compute the mixed color, basically)


Ok,
i am not sure what is the problem, but running a profiled version of
mgp shows that a lot of time is spent in talking to the X server,
possibly doing XGetPixel()/XPutPixel.

A little bit of investigation shows that a non-solid background
causes the first part of tfont.c::tfc_image() to be run, which
calls:

 * malloc() (possibly causing some huge memory fragmentation although
   no apparent leak),
 * a large number of XGetPixel()
 * XQueryColors()

somehow these calls seem to trash the behaviour of the X server (at
least on my machine with 2.2.x and XFree3.2

I actually wonder why there appears to be no way write a rectangular
region into an XImage and instead one has to resort to calls to
XPutPixel...

In any case a quick and extremely dirty workaround seems to be the
addition, near the beginning of tfont.c::tfc_image(), of

	back = 0 ;

which basically fools the rendering code into believing that there is
a solid black background.
This way at least rendering times are stable over time...

	luigi