org.freedesktop.cairo
Class XlibSurface

Object
  org.freedesktop.bindings.Pointer
      org.freedesktop.bindings.Proxy
          org.freedesktop.cairo.Surface
              org.freedesktop.cairo.XlibSurface

public class XlibSurface
extends Surface

A Window as rendered by the X Server. Used internally when drawing to an application window.

Ordinarily you don't think about creating an XlibSurface; one is implicitly created for you when you create a Cairo context in order to draw in an Widget.ExposeEvent handler:

 gizmo.connect(new Widget.ExposeEvent() {
     public boolean onExposeEvent(Widget source, EventExpose event) {
         Context cr;
         Surface surface;
         
         cr = new Context(source.getWindow());
         
         // start drawing
         
         // for interest's sake
         surface = cr.getTarget();
         assert (surface instanceof XlibSurface);
     }
 }
 

Efficient caching of image data

There is a fair bit of work involved in translating from an image in bitmap form in application memory to the representation that will be used by the X server in video memory. Thus if you are drawing frequently with the same Pixbuf, you may find you are dramatically better off caching the image in the X server.

The best way to do this is to create an XlibSurface with this image in it. You set that image to be the source pattern with a call to setSource() and then paint that onto the Context where you are drawing. The key method involved is createSimilar(), which allows you to create an X resource as a cache:

 gizmo.connect(new Widget.ExposeEvent() {
     private Surface cache;
 
     public boolean onExposeEvent(Widget source, EventExpose event) {
         final Context cr, cr2;
         final Surface target;
         final Pixbuf pixbuf;
         final double x, y;
         
         cr = new Context(source.getWindow());
         
         // start drawing
         ...
         
         // cache image
         if (cache == null) {
             pixbuf = new Pixbuf(filename);
             
             target = cr.getTarget();
             cache = target.createSimilar(Content.COLOR_ALPHA, pixbuf.getWidth(), pixbuf.getHeight());
             cr2 = new Context(cache);
             cr2.setSource(pixbuf, 0.0, 0.0);
             cr2.paint();
         }
         
         // now we can draw the image
         cr.setSource(cache, x, y);
         cr.paint();
     }
 }
 

Obviously you are consuming X server memory doing this, and it can be overdone. You'll have to look at your application's performance to decide whether this is necessary and if so, how much caching to do. Nevertheless, modern graphics cards are very good at blitting images together so if you are doing anything image intensive you are best to let the X server do the work. Cairo is designed with this in mind.

Since:

4.0.7

Author:

Andrew Cowie

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