[Mesa3d-dev] X renderer speedup (write_span_mono_pixmap optimization)

[Jeff E. <je...@ds...>]

I'm currently trying to move an older piece of software from using
PEX/X11 to Mesa/X11.  (We use(d) a modified version of PEX with zbuffer
support, and it's becoming too much of a chore to propogate these
patches to new versions of XFree86)

Our software draws exclusively flat-shaded, zbuffered polygons.

Out of the box (Mesa 3.2/RedHat 6.2) or using Mesa 3.4.1, the Mesa
renderer is rather slower than our PEX-based renderer.  However, a
simple change to write_span_mono_pixmap brings the two renderers to
nearly equal speed.  

Basically, we discovered in past iterations of software renderers that
coalescing as many pixels as possible into single X calls is a very
important optimization (in fact, another dead renderer actually unifies
multiple single-color spans into a single XDrawSegments call).
write_span_mono_pixmap performs this optimization using
XMesaFillRectangle when no pixels in the span were excluded (due to
depth check or other factors)

However, if any pixels were excluded, then it falls back to
XMesaDrawPoint for all points.

This patch accumulates consecutive "drawn" pixels into a single
XMesaFillRectangle call.  This is a big speed boost in our application,
and a slight pessimization in the case of e.g., a 50% stipple.
(XFillRectangle is likely to be optimized for the 1x1 case, as well as
the Nx1 case)  A slightly more intelligent patch would probably
disable the optimization for very short spans, or spans with large
numbers of sub-spans.

I'll include the patch in my message.  Is submitting it to the
sourceforge patch tracker appropriate?  It doesn't look like that SF
feature is well-used.

Jeff