Re: Flicker free GtkLabel

[Owen Taylor <otaylor redhat com>]

Miguel de Icaza <miguel@nuclecu.unam.mx> writes:

> Hello Owen,
> 
> > It wouldn't be possible to make the GtkLabel flicker-free without
> > modifications to the core GTK+ drawing code for flicker free
> > drawing. (In the future, it will be possible create a global
> > option "draw everything flicker free").
> 
> Wouldnt a widget with a window be able to achieve this?  The user
> should be aware of the implications of using a GdkWindows for the
> label. 

Yes a widget with a window can be made "flicker-free", but a
widget with window cannot have the current semantics of the
GtkLabel, which draws only the foreground, and not the
background. Essentially, you are talking about a GtkImageLabel.
(In analogy to XDrawImageString)
 
> In the future when the "draw everything flicker free" code is written
> we can make this option a no-op.
> 
> btw, would you care to detail what are your plans for implementing
> this "flicker free drawing" engine?  I am very interested in this.

The basic idea is very simple. When an expose event is handled on
a !NO_WINDOW widget, widget->window is temporarily replaced
with a pixmap, all the drawing code draws onto the pixmap, and
then the relevant area of the pixmap is copied back onto the screen.
(Just like the GnomeCanvas, essentially)

There are a number of niggly details that make this non-trivial
to implement - for instance, you need to be able to handle
gdk_window_clear_area()'s on a pixmap. (XClearArea only works
on a window), but it is certainly feasible.

Regards,
                                        Owen