[sdljava-users] Usage example for glGetDoublev

[Gregor <Gre...@gm...>]

Hi!

I"m trying to use glGetDoublev to read back the modelview and projection 
matrices from OpenGL. However, it looks like I'm doing something wrong with 
the DoubleBuffers required for that.

What I'm doing basically is in this code:

    GlewImpl gl;
    private ml.math.Matrix4 modelViewMatrix;
    private ml.math.Matrix4 projectionMatrix;
    private double viewFieldOfView;
    private DoubleBuffer matrixBuffer;

    public ViewWindowSpace(Widget parent) 
    {
	super();

        matrixBuffer=DoubleBuffer.allocate(16);
        modelViewMatrix=new Matrix4();
        projectionMatrix=new Matrix4();
		
	currentRenderMode=RenderMode.Solid;

        initGLContext();

       // snip: unrelated code

        addListener(new LifeCycleListener() {

            public void lifeCycleEvent(LifeCycleEvent e)
            {
                // TODO Auto-generated method stub
                if(e.getType()==LifeCycleEvent.Type.REALIZE) {
                    setupViewingAreaInitial();
                }
                    
            }
	}

        // snip: unrelated code
    }

    protected void setupViewingAreaInitial() 
    {
        viewFieldOfView=45.0;
        
        glBegin();
        GlewJNI.SWIG_glew_init();
        gl=new GlewImpl();
        int width = getWidth();
        int height = getHeight();
        gl.glViewport(0, 0, width, height);     
        gl.glMatrixMode(GL.GL_PROJECTION);      // select the projection 
matrix
        gl.glLoadIdentity();                            // reset the 
projection matrix
        float fAspect = (float) width / (float) height;
        gl.gluPerspective(viewFieldOfView, fAspect, 0.5f, 400.0f);
        gl.glMatrixMode(GL.GL_MODELVIEW);       // select the modelview matrix
        gl.glLoadIdentity();

        gl.glGetDoublev(GL.GL_MODELVIEW_MATRIX,matrixBuffer);
        modelViewMatrix.setFromOpenGLMatrix(matrixBuffer);
        gl.glGetDoublev(GL.GL_PROJECTION_MATRIX,matrixBuffer);
        projectionMatrix.setFromOpenGLMatrix(matrixBuffer);
        
        glEnd();
    }

What this does:

The constructor allocates the DoubleBuffer in question and registers an event 
handler to do the rest of the initialilsation once the opengl context can 
actually be used. When this is the case, setupViewingAreaInitial is called to 
perform the init of the gl context. This fails with a NullPointerException in
the first call to  gl.glGetDoublev(). The console output for this exception is 
as follows:

Exception in thread "main" java.lang.NullPointerException: null address 
returned from GetDirectBufferAddress() call.  Make sure the buffer is a 
_direct_ buffer.
	at org.gljava.opengl.x.swig.GlewJNI.glGetDoublev(Native Method)
	at org.gljava.opengl.impl.glew.GlewImpl.glGetDoublev(GlewImpl.java:143)
	at 
ml.ui.plugins.view.ViewWindowSpace.setupViewingAreaInitial(ViewWindowSpace.java:322)
	at 
ml.ui.plugins.view.ViewWindowSpace$2.lifeCycleEvent(ViewWindowSpace.java:277)
	at org.gnu.gtk.Widget.fireLifeCycleEvent(Widget.java:735)
	at org.gnu.gtk.Widget.handleRealize(Widget.java:764)
	at org.gnu.gtk.Widget.gtk_widget_show(Native Method)
	at org.gnu.gtk.Widget.show(Widget.java:119)
	at ml.core.State.createMainWindow(State.java:158)
	at ml.ML3D.<init>(ML3D.java:49)
	at ml.ML3D.main(ML3D.java:66)

I have no clue why this happens. Do I initialize the DoubleBuffer in question 
badly? Is there any example that shows how it should be done? Any help would 
be appreciated.

Regards,
Gregor