Re: problems understanding gtk3/gdk/cairo interaction

[jcupitt gmail com]

Hi Roger,

You should do all drawing in the expose handler and nowhere else.
Don't do any direct drawing in your data hander, instead update your
model and queue an expose event.

On 7 April 2012 02:16, Roger Davis <rbd soest hawaii edu> wrote:
> presumably this includes the GtkDrawingArea widget as well. If there is
> documentation on exactly how this double-buffering works with regard to
> GtkDrawingArea I would be greatly interested in seeing it to figure out if

Here's how it works in gtk2, I think this bit hasn't changed much for gtk3:

* an expose event comes in from X11
* it gets added to the set of pending expose events on your drawing
area -- gtk computes the minimal set of non-overlapping damage
rectangles
* when gtk feels the time is right to update your window, it allocates
a backing pixmap just large enough to hold all damage
* the backing pixmap is filled with the background colour for your widget
* the expose handler is triggered, passing in the backing pixmap as the drawable
* your expose handler draws to that --- you can fill the whole thing,
or you can loop over the (possibly smaller) list of damage rects that
make it up
* gtk clips against the damage outline and writes the new pixels to the display
* the temporary pixmap is deleted

As a former xlib programmer I was slightly horrified when I head about
all this, but it actually works pretty well on non-ancient machines.
My program disables the double buffering and does its own thing for my
main data display, since I don't want the "clear to background"
behaviour, but I use it everywhere else.

The discipline of only drawing in the expose handler is also helpful.
It forces you to decouple drawing from updating which will make your
performance degrade much more gracefully under load. Instead of
suddenly getting lag when you run out of cycles, you'll just see a
drop in the frame rate.

John