Question

I have a fine, working system in C# that draws with Cairo commands in a render method. However, sometimes I would like to draw into a pixmap, rather than dynamically when the screen needs to be updated. For example, currently I have:

public override void render(Cairo.Context g) {
  g.Save();
  g.Translate(x, y);
  g.Rotate(_rotation);
  g.Scale(_scaleFactor, _scaleFactor);
  g.Scale(1.0, ((double)_yRadius)/((double)_xRadius));
  g.LineWidth = border;
  g.Arc(x1, y2, _xRadius, 0.0, 2.0 * Math.PI); 
  g.ClosePath();
}

But I would like, if I choose, to render the Cairo commands to a Gtk.Pixbuf. Something like:

 g = GetContextFromPixbuf(pixbuf);
 render(g);

Is that possible? It would be great if I didn’t have to turn the context back into a pixbuf, but that the cairo drawing would go directly to the pixbuf. Any hints on this would be appreciated!

Answer 1

The answer is actually quite easy: when you render the objects, render them to a context created from a saved surface. Then when you render the window, insert a context based on the same saved surface.

Create a surface:

  surface = new Cairo.ImageSurface(Cairo.Format.Argb32, width, height);

Render a shape to the surface:

using (Cairo.Context g = new Cairo.Context(surface)) {
  shape.render(g); // Cairo drawing commands
}

Render the window:

  g.Save();
  g.SetSourceSurface(surface, 0, 0);
  g.Paint();
  g.Restore();
      ... // other Cairo drawing commands

That’s it!