[Gtk-sharp-list] DrawingArea flickering - how to stop erasing background
martin at berriman.co.uk
Wed Jun 13 12:20:24 UTC 2012
I've searched and searched on DoubleBuffering with Gdk# but am still stuck...
I'm writing a test linux app in Monodevelop using Gdk# and Cairo.
I have on my form a DrawingArea widget onto which I draw.
In order to avoid flickering, I am creating a Cairo.ImageSurface where I do
the actual drawing. This is then painted onto the DrawingArea during the
Expose event handler.
I am finding that it is flickering badly but fail to understand why. In a
previous app on Windows, I had a similar problem because Windows tries to
erase the background before each paint. Since I am painting the entire
image from memory, there is no need to erase first. This was solved by
handling the WM_ERASEBKGND message - i.e. intercept the message and ignore
it so that the attempted erase does not occur.
It seems likely that the same thing is happening here (automatic erase
before my paint) however I don't know how to stop it.
I've included example code below (although can't see any way to use code
tags) and would appreciate any suggestions.
private Cairo.ImageSurface viewSurface; //surface used to double buffer
drawing to prevent flicker
private System.Timers.Timer timer; //used to cause a redraw
public MainWindow (): base (Gtk.WindowType.Toplevel)
timer = new System.Timers.Timer(10);
timer.Enabled = false;
timer.Elapsed += HandleTimerElapsed;
void HandleTimerElapsed (object sender, System.Timers.ElapsedEventArgs e)
protected void eDrawingAreaConfigure (object o, Gtk.ConfigureEventArgs
//we get here we we have a resize event
//create our sufrace in memory for later use
viewSurface = new ImageSurface(Format.ARGB32,
protected void eDrawingAreaExpose (object sender, Gtk.ExposeEventArgs args)
//we get here when we need to redraw the screen
DrawingArea viewArea = (DrawingArea)sender;
using (Context cr = Gdk.CairoHelper.Create(viewArea.GdkWindow))
cr.SetSourceSurface(viewSurface, 0, 0);
private void UpdateViewSurface()
//whenever we want
//draw onto our surface in memory
//white background with a blue circle which has a red border
using (Context cr = new Context(viewSurface))
int width, height;
width = Allocation.Width;
height = Allocation.Height;
cr.Rectangle(0, 0, width, height);
cr.SetSourceRGB(1, 1, 1);
cr.LineWidth = 9;
cr.SetSourceRGB(1, 0, 0);
cr.Arc(0, 0, (width < height ? width : height) / 2 - 10, 0, 2*Math.PI);
cr.SetSourceRGB(0, 0, 1);
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