Re: Re-drawing GtkDrawingArea surface on motion-notify-event

[rbd <rbd soest hawaii edu>]

Hello Marcin,

I have written a number of gtk3 apps that perform the type of rubberband 
line drawing you have described. Although I was originally porting apps 
from an Xlib drawing environment where XOR was available to do 
lightning-fast 'undraws' of existing line segments and was skeptical of 
Cairo performance possibilities due to its lack of an XOR drawing op, I 
was very pleasantly surprised by its rendering speed (as Eric noted). In 
one of my apps, for instance, on a 2009 Macbook Pro laptop I can drag a 
wireframe with a dozen line segments around on a full-screen background 
field with only barely noticeable lag.

My suggestion to you would be to do the very easiest thing first and see 
if performance is acceptable -- if not, then do as Dov suggested and try 
redrawing only the smallest rectangle that encloses your line segment. You 
could also try using Cairo's clipping path, although I have no experience 
with that performance-wise.

Roger Davis
Univ. of Hawaii


> From: Marcin Kolny <marcin kolny gmail com>
> To: gtk-app-devel-list gnome org
> Subject: Re-drawing GtkDrawingArea surface on motion-notify-event
> Message-ID:
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> Hi everyone,
> I'd like to write very simple application for drawing lines.
> 1) User press button - app saves x and y coordinates
> 2) User moves the mouse - app dynamically draws potential line on each
> mouse move event
> 3) User releases button - app "saves" the line on the canvas.
>
> I've tried to modify slightly the GTK example drawing application [1].
> 1) On button press event I paint the current state to the "base_surface"
> surface.
> 2) On each mouse move I paint to the "tmp_surface" surface "base_surface"
> and after that the actual line, so I avoid multiple lines on mouse move.
> 3) On mouse release event I paint "tmp_surface" surface to "base_surface".
>
> This works, and full code can be found on my gist [2]. However, I don't
> think it's the right approach, since I have to re-paint a whole image for
> each mouse-move, and latter on, on "draw" event application is doing almost
> the same, so I'm doing the re-paint twice per each mouse move. I'm afraid
> that for huge surfaces it might be very inefficient.
>
> Do you know what would be the right approach to solve this sort of problems?
>
> Thank you for help,
> Marcin