Re: cairo & pixmap

[Philip Withnall <philip tecnocode co uk>]

Hi,

On Sat, 2007-05-19 at 13:41 +0200, control H wrote:
> Hi,
>
> I'm planning to make my app use cairo instead of GDK based functions.
> I'm using a drawing area, which draws into a pixmap when the
> "configure" event is emitted, and copies the relevant part of the
> pixmap back on the screen on the "expose" event.
>
> When I'm doing the same with cairo, a black screen is the result. The
> ref. manual says in the FAQ that I should either turn off double
> buffering, or draw in the expose signal. This is also what the example
> testcairo (coming along with Gtk) does.
>
> I'm confused. Does this mean that drawing using cairo is slower since
> it has to redraw the whole thing again on each expose signal? Or is
> there some smart thing in Gtk that somehow knows that when the expose
> event is a result of a configure event, it doesn't really do the
> drawing but copies the invalidated part back?

The signal handler for the expose event is passed some parameters
specifying which part of the screen has become invalidated. If you use
these in a Cairo drawing mask, then redraw your canvas, it should be
quite fast, as Cairo will ignore all operations to areas outside the
mask.

Philip

> Thanks...
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