[cairo] Very slow performance
keithp at keithp.com
Thu Dec 23 10:20:36 PST 2004
Around 9 o'clock on Dec 23, Chris wrote:
> I made a simple test program that loads this file and renders it to an
> image buffer in Cairo. It takes over 30 seconds to do this. That's
> incredibly slow and makes me wonder about the usability of Cairo in
> everyday applications.
Performance hasn't been a critical concern for cairo development until
now; we've been much more focused on API structure and cross-language
There isn't anything inherant in the structure of the cairo implementation
causing these performance problems. As we get more experience with how
cairo is used in real applications, and do some real performance
measurements, we'll be able to find the bottlenecks in cairo and all of
the underlying systems.
This week alone saw some analysis from the xpdf cairo back-end development
speed text drawing up by a huge factor (10000) caused by a small
collection of easily fixed bugs.
There are some obvious places where code improvements will yield
One such place is in the primitive image compositing code which was written
as quickly as possible a few years ago to complete the Render extension
inside the X server. Two pieces are needed here; the first is some
mechanism for easily and accurately plugging in new acceleration functions,
the second is a replacement for the pixel-at-a-time software fallback
Another place is in the polygon rasterization code. Two pieces are also
needed here; the first is a new tesselator. Carl has done the research
necessary to build what we need, so it should be only a matter of
programming time before this task is complete. The second piece is an
efficient primitive trapezoid filler. We want to compute 8 bits of alpha
information, and the traditional methods for drawing anti-aliased polygons
all scale linearally in the number of alpha values. 255 is a large scale
factor. I have some ideas on how to make this faster, but if they don't
pan out, we may end up with 'fast' and 'pretty' modes, something which
we've avoided successfully in other areas of cairo until now.
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