Proprosed break in libGL / DRI driver ABI

[Roland Mainz]

Brian Paul wrote:
> >>>Ian Romanick wrote:
> >>>When I look at xc/extras/Mesa/src/mesa/main/config.h I see more items on
> >>>my wishlist: Would it be possible to increase |MAX_WIDTH| and
> >>>|MAX_HEIGHT| (and the matching texture limits of the software
> >>>rasterizer) to 8192 to support larger displays (DMX, Xinerama and Xprint
> >>>come in mind) ?
> >>
> >>If you increase MAX_WIDTH/HEIGHT too far, you'll start to see
> >>interpolation errors in triangle rasterization (the software
> >>routines).  The full explanation is long, but basically there needs to
> >>be enough fractional bits in the GLfixed datatype to accomodate
> >>interpolation across the full viewport width/height.
> >
> > Will increasing MAX_WIDTH/HEIGHT affect applications which run in
> > small windows or only those which use resolutions exceeding the 4Kx4K
> > limit?
> 
> Increasing MAX_WIDTH/HEIGHT will result in more memory usage
> regardless of window size.

This is just memory which comes from the stack (excluding the |MALLOC()|
xc/extras/Mesa/src/mesa/drivers/x11/xm_api.c) , right ?

> As is, you can't exceed 4K x 4K resolution without increasing
> MAX_WIDTH/HEIGHT.  Your glViewport call will be clamped to those
> limits if you specify larger.

Which leads to some unfortunate problems for printing, think about a
ISO-A4 page at 600DPI where a GL window covering the whole paper surface
is beyond the current 4Kx4K limit (that's the reason why the print demo
in "glxgears" simply shrinks width/height obtained from
|XpGetPageDimensions()| (AFAIK to 1/2 or 1/3) and then centers the
window on the page before starting to print).

> >>In fact, I'm not sure that we've already gone too far by setting
> >>MAX_WIDTH/HEIGHT to 4096 while the GLfixed type only has 11 fractional
> >>bits.  I haven't heard any reports of bad triangles so far though.
> >
> > Do you know any specific application which may expose bad rendering
> > when the size gets to large?
> 
> No (there's far too many OpenGL apps out there for me to say).
> 
> >>But there probably aren't too many people generating 4Kx4K images.
> >
> > We've been running tests with the glutdemo applications and Xprint at
> > higher resolutions (6Kx8K window size) and did not notice any bad
> > rendering using the software rasterizer.
> 
> How large are your triangles?

See above... usually 8Kx8K is enought for DIN-A4/600DPI, for larger
paper sizes you either go down with the surface resolution or you have
to deal with larger coordinates (which leads directly to this
MAX_WIDTH/MAX_HEIGHT debate) ...

----

Bye,
Roland

-- 
  __ .  . __
 (o.\ \/ /.o) roland.mainz at nrubsig.org
  \__\/\/__/  MPEG specialist, C&&JAVA&&Sun&&Unix programmer
  /O /==\ O\  TEL +49 641 7950090
 (;O/ \/ \O;)