Re: Fast ways to draw brush outlines on a DrawingArea

[jcupitt gmail com]

On 9/30/07, jcupitt gmail com <jcupitt gmail com> wrote:
> On 9/30/07, Keith Feesh <gummybassist gmail com> wrote:
> > The way that I do things now includes redrawing the entire canvas every time
> > the mouse is moved, in order to clear the old
> > outline's drawing, and then to redraw the outline once again. This brings my
> > system to it's knees and I figure there must be
> > a better method, just I can't think of any method. Any hints would be great.
>
> The animation is done with gtk_widget_queue_draw_area().

I was thinking about this again, and it seems to me that there are two
separate problems. How to quickly draw a brush outline (XOR lines or
repaint areas of your window) and how best to do rubberbanding in GTK
(drawing tracks mouse movement). (2) is actually the more difficult
part, IMO.

GTK only really supports drawing in the expose handler. You can draw
elsewhere (eg. in the mouse movement handler) but it may not be
portable to non-X11 backends, it's very hard to control multiple
drawing events that happen together, you lose all of GTK's nice
framework for doublebuffering and for event compression, and you lose
the decoupling between input and output that will make you app work
well under load.

So ... I would very strongly urge a solution like this:

1) make your expose handler work well for small repaints (ie. don't
redraw the whole view on every expose). For a vector drawing
application you might need some fancy datastructure for efficient
culling. Take a look at gdk_region_get_rectangles() too, if you want
to get finer-grained expose events
2) put the rubberband (or brush outline) position and size into your
model and draw it as part of your expose handler
3) in the mouse motion handler, queue a redraw for the current
position of the outline, update the model to the new position, queue a
draw there as well
4) use mouse motion compression to make sure you don't get lag when
your machine is running slowly

John