I tried to simplify the problem by using triangles instead of textured quads. The result is the same:
package JOGL;
import java.awt.*;
import java.awt.image.*;
import java.awt.event.*;
import javax.swing.*;
import net.java.games.jogl.*;
public class PBDemo extends JFrame implements GLEventListener {
private static int maxPbufferWidth = 720;
private static int maxPbufferHeight = 576;
private GLDrawable pbuffer;
private BufferedImage pbImage;
private DataBufferInt dbInt;
private JLabel label;
private ImageIcon image;
private JScrollPane scrollPane;
public PBDemo() {
setTitle( "Pbuffer Demo" );
setSize( 640, 480 );
// Create a temporary window to create a GLCanvas
// that is never displayed on screen.
Window tempWindow = new Window( this );
tempWindow.setSize( 0, 0 );
GLCanvas tempCanvas = GLDrawableFactory.getFactory().createGLCanvas( new GLCapabilities() );
tempWindow.add( tempCanvas );
tempWindow.setVisible( true );
// Create a GLCapabilities object for the pbuffer
GLCapabilities pbCaps = new GLCapabilities();
pbCaps.setAlphaBits( 8 );
pbCaps.setDoubleBuffered( false );
pbuffer = tempCanvas.createOffscreenDrawable( pbCaps, maxPbufferWidth, maxPbufferHeight );
// PBuffers are lazily created so the tempCanvas needs to run
// through its display method once.
tempCanvas.display();
// Create the BufferedImage that will hold the pixel data
// from the pbuffer.
pbImage = new BufferedImage( maxPbufferWidth, maxPbufferHeight, BufferedImage.TYPE_INT_ARGB );
// dbByte is the array that the pbuffer pixel data is stored in.
dbInt = (DataBufferInt)pbImage.getRaster().getDataBuffer();
pbuffer.addGLEventListener( this );
pbuffer.display();
// Display the pbuffer in a JScrollPane
image = new ImageIcon( pbImage );
label = new JLabel( image );
scrollPane = new JScrollPane( label );
getContentPane().add( scrollPane );
addWindowListener(new WindowAdapter() {
public void windowClosing(WindowEvent e) {
dispose();
System.exit(0);
}
});
setVisible( true );
}
public void displayChanged(GLDrawable gLDrawable, boolean modeChanged, boolean deviceChanged) {
}
public void init( GLDrawable drawable ) {
GL gl = drawable.getGL();
gl.glClearColor( 0.0f, 0.0f, 0.0f, 0.0f );
gl.glColor4f( 1.0f, 1.0f, 1.0f, 1.0f );
System.out.println("init");
reshape( drawable, 0, 0, maxPbufferWidth, maxPbufferHeight );
}
public void display( GLDrawable drawable ) {
GL gl = drawable.getGL();
System.out.println("display");
gl.glClear(GL.GL_COLOR_BUFFER_BIT | GL.GL_DEPTH_BUFFER_BIT);
gl.glLoadIdentity(); // Reset The View
gl.glTranslatef(0.0f,0.0f,-5.0f);
gl.glEnable(GL.GL_BLEND);
gl.glBlendFunc(GL.GL_SRC_ALPHA,GL.GL_ONE_MINUS_SRC_ALPHA);
gl.glColor4f(0f,0f,0f,1f);
gl.glBegin( GL.GL_TRIANGLES );
gl.glVertex3f( 0.0f, 0.0f, 0.0f );
gl.glVertex3f( 1.0f, 0.0f, 0.0f );
gl.glVertex3f( 0.0f, 1.0f, 0.0f );
gl.glEnd();
gl.glTranslatef(.5f,0.0f,0.0f);
gl.glColor4f(0f,0f,0f,.5f);
gl.glBegin( GL.GL_TRIANGLES );
gl.glVertex3f( 0.0f, 0.0f, 0.0f );
gl.glVertex3f( 1.0f, 0.0f, 0.0f );
gl.glVertex3f( 0.0f, 1.0f, 0.0f );
gl.glEnd();
gl.glLoadIdentity(); // Reset The View
// Because opengl's origin is the lower left corner and java's origin
// is the upper right corner that image will be flipped upside down.
gl.glReadBuffer( GL.GL_FRONT );
gl.glReadPixels( 0, 0, maxPbufferWidth, maxPbufferHeight, GL.GL_BGRA, GL.GL_UNSIGNED_BYTE, dbInt.getData() );
gl.glFlush();
}
public void reshape( GLDrawable drawable, int x, int y, int width, int height ) {
GL gl = drawable.getGL();
GLU glu = drawable.getGLU();
if (height <= 0) // avoid a divide by zero error!
height = 1;
final float h = (float)width / (float)height;
gl.glViewport(0, 0, width, height);
gl.glMatrixMode(GL.GL_PROJECTION);
gl.glLoadIdentity();
glu.gluPerspective(45.0f, h, 1.0, 20.0);
gl.glMatrixMode(GL.GL_MODELVIEW);
gl.glLoadIdentity();
}
public static void main( String[] args ) {
PBDemo demo = new PBDemo();
}
}
The second triangle is 50% opaque and should not appear over the black 100% opaque triangle.
opengl:
java2d:
