Re: drawing on drawables

[John Vetterli <jvetterli linux ca>]

On Thu, 7 Apr 2005, Olexiy Avramchenko wrote:
> Joachim Mathes wrote:
> > While programming gtk-applications which contain drawable-widgets e.g. 
> > pixmaps, I realized that it is possible to draw a rectangle on coordinates 
> > that are actually outside the allocated region. For example: If you create 
> > a pixmap with a height and width of 200 and you draw a rectangle on 
> > coordinates (250,250) no exception is raised. Even negative coordinates are 
> > possible: gdk_draw_rectangle (pixmap, gc, TRUE, -50, -50, 100, 100); and 
> > only the rest of the rectangle is plotted. This is very comfortable and 
> > reduces the amont of checks for validate coordinates.
> > So my question is if this is a feature I can rely on. Is this not just a 
> > matter of implementation but defined in the specificaton of GTK?
> 1) GDK doesn't clip drawing requests at drawable boundaries, but Xserver do 
> (I assume that you're using X11 backend).
> 2) X11 protocol specification tells nothing about that (or I missed it), so I 
> think that this is the pure Xserver implementation question.
> 3) But any server has to clip, because the unclipped drawing request (like 
> line -10,-10,10x10 to off-screen pixmaps in system memory) is a huge security 
> hole.
> So you may rely on it :) Just try not to use coordinates outside +-16K, if 
> you want to have a portable app.

If you want to be sure you can set clipping in the graphics context.  See 
gdk_gc_set_clip_rectangle() and the like.

My 2 cents.
JV