[cairo] Cairo + GTK animation - high Xorg load

[Stefan Salewski]

On Wed, 2016-10-26 at 15:51 -0400, David J. Herzfeld wrote:
> I must be doing something wrong...

Nice example.

I think that drawing 1000 lines with 60 Hz in fullscreen mode is indeed
a high load for Cairo.

I have created a schematics editor with GTK3 and cairo (http://ssalewsk
i.de/PetEd.html.en) and my observation was very similar. I have
restricted the redraw with bounding boxes to only the areas which have
changed, but still drawing is not fully smooth. OK, that tool is still
coded in Ruby, moving to a fast compiled language like Nim, Rust or
similar would increase speed, but not too much.

What you can do is of course: Shift the already drawn curve to the left
and draw only the new part. Cairo offers fast routines for copying
areas, so for that case cpu load should be really low. For the trivial
case of a plain sine wave, you can even draw the size extended curve to
a backup area and then just copy the desired area to your window, so no
drawing is necessary at all. That is what I did for my schematics
editor for the background grid. (Of course complete redraw is easier,
and when copying areas there is a minimal risk that you can get
artefacts at the borders of old and new area.)

Of course you don't have to code in plain C in these days, my Nim
bindings are available,

https://github.com/ngtk3
http://forum.nim-lang.org/t/2198#15801

and I think for Rust, Go and some other nice and fast languages there
should exists bindings too.

Other backends: As far as I know there is no hope for much speedup when
using the not well supported opengl backend -- at least that was what I
found some years ago when doing some google research. With GTK 3.16 we
get better OpenGL support by GTK GlArea I think. So maybe situation has
improved since then, my feeling is that GlArea is more for real 3D
objects and not for fast line drawings, but I have not done
investigations in this direction yet.