Re: [sdljava-users] Usage example for glGetDoublev

[Ivan Z. G. <iva...@ya...>]

Gregor,

I had a look in the code and intact SDLSurface isn't a good example
because the buffer is created in the native code.  Have a look at
ByteBufferPool instead.

-Ivan/

Gregor M=FCckl wrote:

>Hi!
>
>I"m trying to use glGetDoublev to read back the modelview and projection=
=20
>matrices from OpenGL. However, it looks like I'm doing something wrong w=
ith=20
>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)=20
>    {
>	super();
>
>        matrixBuffer=3DDoubleBuffer.allocate(16);
>        modelViewMatrix=3Dnew Matrix4();
>        projectionMatrix=3Dnew Matrix4();
>	=09
>	currentRenderMode=3DRenderMode.Solid;
>
>        initGLContext();
>
>       // snip: unrelated code
>
>        addListener(new LifeCycleListener() {
>
>            public void lifeCycleEvent(LifeCycleEvent e)
>            {
>                // TODO Auto-generated method stub
>                if(e.getType()=3D=3DLifeCycleEvent.Type.REALIZE) {
>                    setupViewingAreaInitial();
>                }
>                   =20
>            }
>	}
>
>        // snip: unrelated code
>    }
>
>    protected void setupViewingAreaInitial()=20
>    {
>        viewFieldOfView=3D45.0;
>       =20
>        glBegin();
>        GlewJNI.SWIG_glew_init();
>        gl=3Dnew GlewImpl();
>        int width =3D getWidth();
>        int height =3D getHeight();
>        gl.glViewport(0, 0, width, height);    =20
>        gl.glMatrixMode(GL.GL_PROJECTION);      // select the projection=
=20
>matrix
>        gl.glLoadIdentity();                            // reset the=20
>projection matrix
>        float fAspect =3D (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);
>       =20
>        glEnd();
>    }
>
>What this does:
>
>The constructor allocates the DoubleBuffer in question and registers an =
event=20
>handler to do the rest of the initialilsation once the opengl context ca=
n=20
>actually be used. When this is the case, setupViewingAreaInitial is call=
ed to=20
>perform the init of the gl context. This fails with a NullPointerExcepti=
on in
>the first call to  gl.glGetDoublev(). The console output for this except=
ion is=20
>as follows:
>
>Exception in thread "main" java.lang.NullPointerException: null address=20
>returned from GetDirectBufferAddress() call.  Make sure the buffer is a=20
>_direct_ buffer.
>	at org.gljava.opengl.x.swig.GlewJNI.glGetDoublev(Native Method)
>	at org.gljava.opengl.impl.glew.GlewImpl.glGetDoublev(GlewImpl.java:143)
>	at=20
>ml.ui.plugins.view.ViewWindowSpace.setupViewingAreaInitial(ViewWindowSpa=
ce.java:322)
>	at=20
>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 que=
stion=20
>badly? Is there any example that shows how it should be done? Any help w=
ould=20
>be appreciated.
>
>Regards,
>Gregor
>
>
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