Bug 975 - GLJPanel's OffscreenDrawable shall not double swap - Use default auto-swap mechanims

Refines commit 908ebd99d1eb57ce773a1fdd67c76886da86b9e6

Note that the test case decide whether to auto-swap (after read-pixels)
or not auto-swap (manual swap before read-pixels).

See UITestCase.swapBuffersBeforeRead(GLCapabilitiesImmutable chosenCaps):
   Determines whether the chosen GLCapabilitiesImmutable requires a swap-buffers before reading pixels.

   Usually one uses the default-read-buffer, i.e. GL.GL_FRONT for single-buffer
   and GL.GL_BACK for double-buffer GLDrawables
   and GL.GL_COLOR_ATTACHMENT0 for offscreen framebuffer objects.
   Here swap-buffers shall happen after calling reading pixels, the default.

   However, multisampling offscreen GLFBODrawables utilize swap-buffers to downsample
   the multisamples into the readable sampling sink.
   In this case, we require a swap-buffers before reading pixels.

   Returns:  chosenCaps.isFBO() && chosenCaps.getSampleBuffers()

+++

- GLJPanel:
  - Remove SurfaceUpdatedListener mechanism in favor of
    default auto-swap-buffer via GLDrawableHelper.
    This removes complexity.

  - postGL does not need to perform explicit swapBuffer operation,
    but rely on GLDrawableHelper and the default mechanism.
    This is also compatible w/ J2D backend.

  - Use GLDrawableHelper for setAutoSwapBufferMode(..) and getAutoSwapBufferMode()

+++

UnitTests:

- UITestCase:
  - Add 'boolean swapBuffersBeforeRead(GLCapabilitiesImmutable chosenCaps)'
    to determine whether swapBuffers() must occure before read-pixels. See above.

- GLReadBuffer00Base*
  - remove explicit addSnapshotGLEL/removeSnapshotGLEL

  - add TextRendererGLEL, to display frame-count and -dimension

  - SnapshotGLEL*
    - simply toggle auto-swap in their init(..) and dispose(..) method!

    - clear back-buffer if 'swapBuffersBeforeRead'
      to test whether the right buffer is being used for read-pixels.

5 files changed, 265 insertions, 113 deletions

diff --git a/src/jogl/classes/javax/media/opengl/awt/GLJPanel.java b/src/jogl/classes/javax/media/opengl/awt/GLJPanel.java
index 66a757d10..c80b405b5 100644
--- a/src/jogl/classes/javax/media/opengl/awt/GLJPanel.java
+++ b/src/jogl/classes/javax/media/opengl/awt/GLJPanel.java
@@ -60,7 +60,6 @@ import java.util.List;
 
 import javax.media.nativewindow.AbstractGraphicsDevice;
 import javax.media.nativewindow.NativeSurface;
-import javax.media.nativewindow.SurfaceUpdatedListener;
 import javax.media.nativewindow.WindowClosingProtocol;
 import javax.media.opengl.GL;
 import javax.media.opengl.GL2;
@@ -355,7 +354,6 @@ public class GLJPanel extends JPanel implements AWTGLAutoDrawable, WindowClosing
     this.chooser = chooser;
 
     helper = new GLDrawableHelper();
-    helper.setAutoSwapBufferMode(false); /** Always handles buffer swapping in Backend! */
     if( null != shareWith ) {
         helper.setSharedContext(null, shareWith);
     }
@@ -945,18 +943,13 @@ public class GLJPanel extends JPanel implements AWTGLAutoDrawable, WindowClosing
   }
 
   @Override
-  public void setAutoSwapBufferMode(boolean onOrOff) {
-    // In the current implementation this is a no-op.
-    // All Backend's require control of buffer swapping,
-    // i.e. as required for MSAA offscreen FBO buffer.
+  public void setAutoSwapBufferMode(boolean enable) {
+    helper.setAutoSwapBufferMode(enable);
   }
 
   @Override
   public boolean getAutoSwapBufferMode() {
-    // In the current implementation this is a no-op.
-    // All Backend's require control of buffer swapping,
-    // i.e. as required for MSAA offscreen FBO buffer.
-    return true;
+    return helper.getAutoSwapBufferMode();
   }
 
   @Override
@@ -1402,8 +1395,6 @@ public class GLJPanel extends JPanel implements AWTGLAutoDrawable, WindowClosing
     private volatile GLContextImpl offscreenContext; // volatile: avoid locking for read-only access
     private boolean flipVertical;
 
-    private boolean needsSwapBuffers = true;
-
     // For saving/restoring of OpenGL state during ReadPixels
     private final GLPixelStorageModes psm =  new GLPixelStorageModes();
 
@@ -1439,12 +1430,6 @@ public class GLJPanel extends JPanel implements AWTGLAutoDrawable, WindowClosing
                                                     chooser,
                                                     panelWidth, panelHeight);
           offscreenDrawable.setRealized(true);
-          offscreenDrawable.getNativeSurface().addSurfaceUpdatedListener(new SurfaceUpdatedListener() {
-              @Override
-              public final void surfaceUpdated(Object updater, NativeSurface ns, long when) {
-                  needsSwapBuffers = false;
-              } } );
-
           offscreenContext = (GLContextImpl) offscreenDrawable.createContext(shareWith[0]);
           offscreenContext.setContextCreationFlags(additionalCtxCreationFlags);
           if( GLContext.CONTEXT_NOT_CURRENT < offscreenContext.makeCurrent() ) {
@@ -1564,7 +1549,7 @@ public class GLJPanel extends JPanel implements AWTGLAutoDrawable, WindowClosing
 
     @Override
     public final boolean preGL(Graphics g) {
-      needsSwapBuffers = true;
+      // Empty in this implementation
       return true;
     }
 
@@ -1579,6 +1564,9 @@ public class GLJPanel extends JPanel implements AWTGLAutoDrawable, WindowClosing
     @Override
     public final void postGL(Graphics g, boolean isDisplay) {
       if (isDisplay) {
+        // offscreenDrawable is already swapped,
+        // either by GLDrawableHelper.invoke or user's GLEL according to auto-swap-buffer-mode.
+
         final GL gl = offscreenContext.getGL();
 
         final int componentCount;
@@ -1666,10 +1654,6 @@ public class GLJPanel extends JPanel implements AWTGLAutoDrawable, WindowClosing
             gl2es3.glReadBuffer(gl2es3.getDefaultReadBuffer());
         }
 
-        if( needsSwapBuffers ) {
-            offscreenDrawable.swapBuffers();
-        }
-
         if(null != glslTextureRaster) { // implies flippedVertical
             final boolean viewportChange;
             final int[] usrViewport = new int[] { 0, 0, 0, 0 };
diff --git a/src/test/com/jogamp/opengl/test/junit/jogl/acore/GLReadBuffer00Base.java b/src/test/com/jogamp/opengl/test/junit/jogl/acore/GLReadBuffer00Base.java
index c5885d6b4..2ad622d14 100644
--- a/src/test/com/jogamp/opengl/test/junit/jogl/acore/GLReadBuffer00Base.java
+++ b/src/test/com/jogamp/opengl/test/junit/jogl/acore/GLReadBuffer00Base.java
@@ -27,8 +27,10 @@
  */
 package com.jogamp.opengl.test.junit.jogl.acore;
 
+import java.io.IOException;
 import java.lang.reflect.InvocationTargetException;
 
+import javax.media.opengl.GL2ES2;
 import javax.media.opengl.GLAutoDrawable;
 import javax.media.opengl.GLCapabilities;
 import javax.media.opengl.GLCapabilitiesImmutable;
@@ -40,7 +42,15 @@ import org.junit.FixMethodOrder;
 import org.junit.Test;
 import org.junit.runners.MethodSorters;
 
+import com.jogamp.graph.curve.Region;
+import com.jogamp.graph.curve.opengl.RenderState;
+import com.jogamp.graph.curve.opengl.TextRenderer;
+import com.jogamp.graph.font.Font;
+import com.jogamp.graph.font.FontFactory;
+import com.jogamp.graph.geom.opengl.SVertex;
+import com.jogamp.opengl.math.geom.AABBox;
 import com.jogamp.opengl.test.junit.util.UITestCase;
+import com.jogamp.opengl.util.glsl.ShaderState;
 
 /**
  * Multiple GLJPanels in a JFrame
@@ -48,24 +58,128 @@ import com.jogamp.opengl.test.junit.util.UITestCase;
 @FixMethodOrder(MethodSorters.NAME_ASCENDING)
 public abstract class GLReadBuffer00Base extends UITestCase {
 
-    @BeforeClass
-    public static void initClass() {
-        GLProfile.initSingleton();
+    public static abstract class TextRendererGLELBase implements GLEventListener {
+        final float[] textPosition = new float[] {0,0,0};
+        final int[] texSize = new int[] { 0 };
+        final int fontSize = 24;
+
+        final Font font;
+        final RenderState rs;
+        final TextRenderer renderer;
+
+        boolean flipVerticalInGLOrientation = false;
+
+        public TextRendererGLELBase() {
+            {
+                Font _font = null;
+                try {
+                    _font = FontFactory.get(FontFactory.UBUNTU).getDefault();
+                } catch (IOException e) {
+                    e.printStackTrace();
+                }
+                this.font = _font;
+            }
+            if( null != font ) {
+                this.rs = RenderState.createRenderState(new ShaderState(), SVertex.factory());
+                this.renderer = TextRenderer.create(rs, Region.VBAA_RENDERING_BIT);
+            } else {
+                this.rs = null;
+                this.renderer = null;
+            }
+        }
+
+        public void setFlipVerticalInGLOrientation(boolean v) { flipVerticalInGLOrientation=v; }
+        public final TextRenderer getRenderer() { return renderer; }
+
+        public void init(GLAutoDrawable drawable) {
+            if( null != renderer ) {
+                final GL2ES2 gl = drawable.getGL().getGL2ES2();
+                renderer.init(gl);
+                renderer.setAlpha(gl, 0.99f);
+                renderer.setColorStatic(gl, 1.0f, 1.0f, 1.0f);
+                final ShaderState st = rs.getShaderState();
+                st.useProgram(gl, false);
+            }
+        }
+
+        public void reshape(GLAutoDrawable drawable, int x, int y, int width, int height) {
+            if( null != renderer ) {
+                final GL2ES2 gl = drawable.getGL().getGL2ES2();
+                final ShaderState st = rs.getShaderState();
+                st.useProgram(gl, true);
+                // renderer.reshapePerspective(gl, 45.0f, width, height, 0.1f, 1000.0f);
+                renderer.reshapeOrtho(gl, width, height, 0.1f, 1000.0f);
+                st.useProgram(gl, false);
+                texSize[0] = width * 2;
+            }
+        }
+
+        public abstract void display(GLAutoDrawable drawable);
+
+        public void dispose(GLAutoDrawable drawable) {
+            if( null != renderer ) {
+                final GL2ES2 gl = drawable.getGL().getGL2ES2();
+                renderer.destroy(gl);
+            }
+        }
+
+        int lastRow = -1;
+
+        public void renderString(GLAutoDrawable drawable, String text, int column, int row, int z0) {
+            if( null != renderer ) {
+                final GL2ES2 gl = drawable.getGL().getGL2ES2();
+                final int height = drawable.getHeight();
+
+                int dx = 0;
+                int dy = height;
+                if(0>row) {
+                    row = lastRow + 1;
+                }
+                AABBox textBox = font.getStringBounds(text, fontSize);
+                dx += font.getAdvanceWidth('X', fontSize) * column;
+                dy -= (int)textBox.getHeight() * ( row + 1 );
+
+                final ShaderState st = rs.getShaderState();
+                st.useProgram(gl, true);
+                gl.glEnable(GL2ES2.GL_BLEND);
+                renderer.resetModelview(null);
+                renderer.translate(gl, dx, dy, z0);
+                if( flipVerticalInGLOrientation && drawable.isGLOriented() ) {
+                    renderer.scale(gl, 1f, -1f, 1f);
+                }
+                renderer.drawString3D(gl, font, text, textPosition, fontSize, texSize);
+                st.useProgram(gl, false);
+                gl.glDisable(GL2ES2.GL_BLEND);
+
+                lastRow = row;
+            }
+        }
     }
+    public static class TextRendererGLEL extends TextRendererGLELBase {
+        int frameNo = 0;
 
-    protected abstract void test(final GLCapabilitiesImmutable caps, final boolean useSwingDoubleBuffer, final boolean skipGLOrientationVerticalFlip);
+        public TextRendererGLEL() {
+            super();
+        }
 
-    private boolean defAutoSwapBufferMode = true;
-    protected void addSnapshotGLEL(final GLAutoDrawable glad, final GLEventListener snapshotGLEL) {
-        defAutoSwapBufferMode = glad.getAutoSwapBufferMode();
-        glad.setAutoSwapBufferMode(false);
-        glad.addGLEventListener(snapshotGLEL);
+        @Override
+        public void display(GLAutoDrawable drawable) {
+            frameNo++;
+            final String text = String.format("Frame %04d: %04dx%04d", frameNo, drawable.getWidth(), drawable.getHeight());
+            if( null != renderer ) {
+                renderString(drawable, text, 0,  0, -1);
+            } else {
+                System.err.println(text);
+            }
+        }
     }
-    protected void removeSnapshotGLEL(final GLAutoDrawable glad, final GLEventListener snapshotGLEL) {
-        glad.removeGLEventListener(snapshotGLEL);
-        glad.setAutoSwapBufferMode(defAutoSwapBufferMode);
+
+    @BeforeClass
+    public static void initClass() throws IOException {
+        GLProfile.initSingleton();
     }
 
+    protected abstract void test(final GLCapabilitiesImmutable caps, final boolean useSwingDoubleBuffer, final boolean skipGLOrientationVerticalFlip);
 
     @Test
     public void test00_MSAA0_DefFlip() throws InterruptedException, InvocationTargetException {
@@ -78,17 +192,17 @@ public abstract class GLReadBuffer00Base extends UITestCase {
     }
 
     @Test
-    public void test10_MSAA4_DefFlip() throws InterruptedException, InvocationTargetException {
+    public void test10_MSAA8_DefFlip() throws InterruptedException, InvocationTargetException {
         final GLCapabilities caps = new GLCapabilities(null);
-        caps.setNumSamples(4);
+        caps.setNumSamples(8);
         caps.setSampleBuffers(true);
         test(caps, false /*useSwingDoubleBuffer*/, false /* skipGLOrientationVerticalFlip */);
     }
 
     @Test
-    public void test11_MSAA4_UsrFlip() throws InterruptedException, InvocationTargetException {
+    public void test11_MSAA8_UsrFlip() throws InterruptedException, InvocationTargetException {
         final GLCapabilities caps = new GLCapabilities(null);
-        caps.setNumSamples(4);
+        caps.setNumSamples(8);
         caps.setSampleBuffers(true);
         test(caps, false /*useSwingDoubleBuffer*/, true /* skipGLOrientationVerticalFlip */);
     }
diff --git a/src/test/com/jogamp/opengl/test/junit/jogl/acore/TestGLReadBuffer01GLJPanelAWT.java b/src/test/com/jogamp/opengl/test/junit/jogl/acore/TestGLReadBuffer01GLJPanelAWT.java
index 4ad84aa73..ee9eea725 100644
--- a/src/test/com/jogamp/opengl/test/junit/jogl/acore/TestGLReadBuffer01GLJPanelAWT.java
+++ b/src/test/com/jogamp/opengl/test/junit/jogl/acore/TestGLReadBuffer01GLJPanelAWT.java
@@ -52,6 +52,7 @@ import org.junit.runners.MethodSorters;
 
 import com.jogamp.opengl.test.junit.jogl.demos.es2.GearsES2;
 import com.jogamp.opengl.test.junit.util.MiscUtils;
+import com.jogamp.opengl.test.junit.util.UITestCase;
 import com.jogamp.opengl.util.Animator;
 import com.jogamp.opengl.util.awt.AWTGLReadBufferUtil;
 import com.jogamp.opengl.util.texture.TextureIO;
@@ -72,6 +73,7 @@ public class TestGLReadBuffer01GLJPanelAWT extends GLReadBuffer00Base {
         final JFrame frame = new JFrame();
         final Dimension d = new Dimension(320, 240);
         final GLJPanel glad = createGLJPanel(skipGLOrientationVerticalFlip, useSwingDoubleBuffer, caps, d);
+        final SnapshotGLELAWT snapshotGLEL = new SnapshotGLELAWT(awtGLReadBufferUtil, skipGLOrientationVerticalFlip);
         try {
             javax.swing.SwingUtilities.invokeAndWait(new Runnable() {
                 public void run() {
@@ -81,10 +83,14 @@ public class TestGLReadBuffer01GLJPanelAWT extends GLReadBuffer00Base {
                     panel.setDoubleBuffered(useSwingDoubleBuffer);
                     frame.getContentPane().add(panel);
 
-                    final GearsES2 gears = new GearsES2(0);
+                    final GearsES2 gears = new GearsES2(1);
                     gears.setFlipVerticalInGLOrientation(skipGLOrientationVerticalFlip);
                     gears.setVerbose(false);
                     glad.addGLEventListener(gears);
+                    final TextRendererGLEL textRendererGLEL = new TextRendererGLEL();
+                    textRendererGLEL.setFlipVerticalInGLOrientation(skipGLOrientationVerticalFlip);
+                    glad.addGLEventListener(textRendererGLEL);
+                    glad.addGLEventListener(snapshotGLEL);
                     panel.add(glad);
                     frame.pack();
                     frame.setVisible(true);
@@ -93,18 +99,11 @@ public class TestGLReadBuffer01GLJPanelAWT extends GLReadBuffer00Base {
             throwable.printStackTrace();
             Assume.assumeNoException( throwable );
         }
-        final GLEventListener snapshotGLEL = new SnapshotGLEL(awtGLReadBufferUtil, skipGLOrientationVerticalFlip);
+        glad.display(); // trigger initialization to get chosen-caps!
         final Dimension size0 = frame.getSize();
         final Dimension size1 = new Dimension(size0.width+100, size0.height+100);
         final Dimension size2 = new Dimension(size0.width-100, size0.height-100);
         try {
-            final Animator anim = new Animator(glad);
-            anim.start();
-            try { Thread.sleep(2*duration); } catch (InterruptedException e) { }
-            anim.stop();
-            addSnapshotGLEL(glad, snapshotGLEL);
-            glad.display();
-
             try { Thread.sleep(duration); } catch (InterruptedException e) { }
             javax.swing.SwingUtilities.invokeAndWait(new Runnable() {
                     public void run() {
@@ -125,7 +124,8 @@ public class TestGLReadBuffer01GLJPanelAWT extends GLReadBuffer00Base {
                     } } );
             try { Thread.sleep(duration); } catch (InterruptedException e) { }
 
-            removeSnapshotGLEL(glad, snapshotGLEL);
+            glad.disposeGLEventListener(snapshotGLEL, true /* remove */);
+            final Animator anim = new Animator(glad);
             anim.start();
             try { Thread.sleep(2*duration); } catch (InterruptedException e) { }
             anim.stop();
@@ -152,21 +152,31 @@ public class TestGLReadBuffer01GLJPanelAWT extends GLReadBuffer00Base {
         return canvas;
     }
 
-    private class SnapshotGLEL implements GLEventListener {
+    private class SnapshotGLELAWT implements GLEventListener {
         final AWTGLReadBufferUtil glReadBufferUtil;
         final boolean skipGLOrientationVerticalFlip;
+        boolean defAutoSwapMode;
+        boolean swapBuffersBeforeRead;
         int i;
 
-        SnapshotGLEL(final AWTGLReadBufferUtil glReadBufferUtil, final boolean skipGLOrientationVerticalFlip) {
+        SnapshotGLELAWT(final AWTGLReadBufferUtil glReadBufferUtil, final boolean skipGLOrientationVerticalFlip) {
             this.glReadBufferUtil = glReadBufferUtil;
             this.skipGLOrientationVerticalFlip = skipGLOrientationVerticalFlip;
+            this.defAutoSwapMode = true;
+            this.swapBuffersBeforeRead = false;
             i = 0;
         }
 
         @Override
-        public void init(GLAutoDrawable drawable) { }
+        public void init(GLAutoDrawable drawable) {
+            defAutoSwapMode = drawable.getAutoSwapBufferMode();
+            swapBuffersBeforeRead = UITestCase.swapBuffersBeforeRead(drawable.getChosenGLCapabilities());
+            drawable.setAutoSwapBufferMode( !swapBuffersBeforeRead );
+        }
         @Override
-        public void dispose(GLAutoDrawable drawable) { }
+        public void dispose(GLAutoDrawable drawable) {
+            drawable.setAutoSwapBufferMode( defAutoSwapMode );
+        }
         @Override
         public void reshape(GLAutoDrawable drawable, int x, int y, int width, int height) { }
         @Override
@@ -178,10 +188,17 @@ public class TestGLReadBuffer01GLJPanelAWT extends GLReadBuffer00Base {
             final String filenameAWT = getSnapshotFilename(sn, "awt",
                                                            drawable.getChosenGLCapabilities(), drawable.getWidth(), drawable.getHeight(),
                                                            glReadBufferUtil.hasAlpha(), fileSuffix, destPath);
-            // gl.glFinish(); // just make sure rendering finished ..
-            drawable.swapBuffers();
+            if( swapBuffersBeforeRead ) {
+                drawable.swapBuffers();
+                // Just to test whether we use the right buffer,
+                // i.e. back-buffer shall no more be required ..
+                gl.glClear(GL.GL_COLOR_BUFFER_BIT);
+            } else {
+                gl.glFinish(); // just make sure rendering finished ..
+            }
+
             final boolean awtOrientation = !( drawable.isGLOriented() && skipGLOrientationVerticalFlip );
-            System.err.println(Thread.currentThread().getName()+": ** screenshot: awtOrient/v-flip "+awtOrientation+", "+filenameAWT);
+            System.err.println(Thread.currentThread().getName()+": ** screenshot: awtOrient/v-flip "+awtOrientation+", swapBuffersBeforeRead "+swapBuffersBeforeRead+", "+filenameAWT);
 
             final BufferedImage image = glReadBufferUtil.readPixelsToBufferedImage(gl, awtOrientation);
             final File fout = new File(filenameAWT);
diff --git a/src/test/com/jogamp/opengl/test/junit/jogl/acore/TestGLReadBuffer01GLWindowNEWT.java b/src/test/com/jogamp/opengl/test/junit/jogl/acore/TestGLReadBuffer01GLWindowNEWT.java
index 598ec1021..dde4c32fa 100644
--- a/src/test/com/jogamp/opengl/test/junit/jogl/acore/TestGLReadBuffer01GLWindowNEWT.java
+++ b/src/test/com/jogamp/opengl/test/junit/jogl/acore/TestGLReadBuffer01GLWindowNEWT.java
@@ -34,7 +34,6 @@ import javax.media.nativewindow.util.DimensionImmutable;
 import javax.media.opengl.GL;
 import javax.media.opengl.GLAutoDrawable;
 import javax.media.opengl.GLCapabilitiesImmutable;
-import javax.media.opengl.GLDrawable;
 import javax.media.opengl.GLEventListener;
 import javax.media.opengl.GLProfile;
 
@@ -46,6 +45,7 @@ import org.junit.runners.MethodSorters;
 import com.jogamp.newt.opengl.GLWindow;
 import com.jogamp.opengl.test.junit.jogl.demos.es2.GearsES2;
 import com.jogamp.opengl.test.junit.util.MiscUtils;
+import com.jogamp.opengl.test.junit.util.UITestCase;
 import com.jogamp.opengl.util.Animator;
 import com.jogamp.opengl.util.GLReadBufferUtil;
 import com.jogamp.opengl.util.texture.TextureIO;
@@ -67,29 +67,26 @@ public class TestGLReadBuffer01GLWindowNEWT extends GLReadBuffer00Base {
         }
         final GLReadBufferUtil glReadBufferUtil = new GLReadBufferUtil(false, false);
         final GLWindow glad= GLWindow.create(caps);
+        final SnapshotGLEL snapshotGLEL = new SnapshotGLEL(glReadBufferUtil);
         try {
             glad.setPosition(64, 64);
             glad.setSize(320, 240);
-            final GearsES2 gears = new GearsES2(0);
+            final GearsES2 gears = new GearsES2(1);
             gears.setVerbose(false);
             glad.addGLEventListener(gears);
+            final TextRendererGLEL textRendererGLEL = new TextRendererGLEL();
+            textRendererGLEL.setFlipVerticalInGLOrientation(skipGLOrientationVerticalFlip);
+            glad.addGLEventListener(textRendererGLEL);
+            glad.addGLEventListener(snapshotGLEL);
             glad.setVisible(true);
         } catch( Throwable throwable ) {
             throwable.printStackTrace();
             Assume.assumeNoException( throwable );
         }
-        final GLEventListener snapshotGLEL = new SnapshotGLEL(glReadBufferUtil);
         final DimensionImmutable size0 = new Dimension(glad.getWidth(), glad.getHeight());
         final DimensionImmutable size1 = new Dimension(size0.getWidth()+100, size0.getHeight()+100);
         final DimensionImmutable size2 = new Dimension(size0.getWidth()-100, size0.getHeight()-100);
         try {
-            final Animator anim = new Animator(glad);
-            anim.start();
-            try { Thread.sleep(2*duration); } catch (InterruptedException e) { }
-            anim.stop();
-            addSnapshotGLEL(glad, snapshotGLEL);
-            glad.display();
-
             try { Thread.sleep(duration); } catch (InterruptedException e) { }
             glad.setSize(size1.getWidth(), size1.getHeight());
             try { Thread.sleep(duration); } catch (InterruptedException e) { }
@@ -98,7 +95,8 @@ public class TestGLReadBuffer01GLWindowNEWT extends GLReadBuffer00Base {
             glad.setSize(size0.getWidth(), size0.getHeight());
             try { Thread.sleep(duration); } catch (InterruptedException e) { }
 
-            removeSnapshotGLEL(glad, snapshotGLEL);
+            glad.disposeGLEventListener(snapshotGLEL, true /* remove */);
+            final Animator anim = new Animator(glad);
             anim.start();
             try { Thread.sleep(2*duration); } catch (InterruptedException e) { }
             anim.stop();
@@ -110,32 +108,48 @@ public class TestGLReadBuffer01GLWindowNEWT extends GLReadBuffer00Base {
 
     private class SnapshotGLEL implements GLEventListener {
         final GLReadBufferUtil glReadBufferUtil;
+        boolean defAutoSwapMode;
+        boolean swapBuffersBeforeRead;
         int i;
 
         SnapshotGLEL(final GLReadBufferUtil glReadBufferUtil) {
             this.glReadBufferUtil = glReadBufferUtil;
+            this.defAutoSwapMode = true;
+            this.swapBuffersBeforeRead = false;
             i = 0;
         }
 
         @Override
-        public void init(GLAutoDrawable drawable) { }
+        public void init(GLAutoDrawable drawable) {
+            defAutoSwapMode = drawable.getAutoSwapBufferMode();
+            swapBuffersBeforeRead = UITestCase.swapBuffersBeforeRead(drawable.getChosenGLCapabilities());
+            drawable.setAutoSwapBufferMode( !swapBuffersBeforeRead );
+        }
         @Override
-        public void dispose(GLAutoDrawable drawable) { }
+        public void dispose(GLAutoDrawable drawable) {
+            drawable.setAutoSwapBufferMode( defAutoSwapMode );
+        }
         @Override
         public void reshape(GLAutoDrawable drawable, int x, int y, int width, int height) { }
         @Override
         public void display(GLAutoDrawable drawable) {
-            snapshot(i++, drawable.getGL(), TextureIO.PNG, null);
+            snapshot(i++, drawable, TextureIO.PNG, null);
         }
-        public void snapshot(int sn, GL gl, String fileSuffix, String destPath) {
-            final GLDrawable drawable = gl.getContext().getGLReadDrawable();
+        public void snapshot(int sn, GLAutoDrawable drawable, String fileSuffix, String destPath) {
+            final GL gl = drawable.getGL();
             final String filenameJGL = getSnapshotFilename(sn, "jgl",
                                                            drawable.getChosenGLCapabilities(), drawable.getWidth(), drawable.getHeight(),
                                                            glReadBufferUtil.hasAlpha(), fileSuffix, destPath);
-            // gl.glFinish(); // just make sure rendering finished ..
-            drawable.swapBuffers();
+            if( swapBuffersBeforeRead ) {
+                drawable.swapBuffers();
+                // Just to test whether we use the right buffer,
+                // i.e. back-buffer shall no more be required ..
+                gl.glClear(GL.GL_COLOR_BUFFER_BIT);
+            } else {
+                gl.glFinish(); // just make sure rendering finished ..
+            }
             final boolean mustFlipVertically = !drawable.isGLOriented();
-            System.err.println(Thread.currentThread().getName()+": ** screenshot: v-flip "+mustFlipVertically+", "+filenameJGL);
+            System.err.println(Thread.currentThread().getName()+": ** screenshot: v-flip "+mustFlipVertically+", swapBuffersBeforeRead "+swapBuffersBeforeRead+", "+filenameJGL);
 
             if(glReadBufferUtil.readPixels(gl, mustFlipVertically)) {
                 glReadBufferUtil.write(new File(filenameJGL));
diff --git a/src/test/com/jogamp/opengl/test/junit/util/UITestCase.java b/src/test/com/jogamp/opengl/test/junit/util/UITestCase.java
index 82247a59b..79c76e49f 100644
--- a/src/test/com/jogamp/opengl/test/junit/util/UITestCase.java
+++ b/src/test/com/jogamp/opengl/test/junit/util/UITestCase.java
@@ -3,14 +3,14 @@
  *
  * Redistribution and use in source and binary forms, with or without modification, are
  * permitted provided that the following conditions are met:
- * 
+ *
  *    1. Redistributions of source code must retain the above copyright notice, this list of
  *       conditions and the following disclaimer.
- * 
+ *
  *    2. Redistributions in binary form must reproduce the above copyright notice, this list
  *       of conditions and the following disclaimer in the documentation and/or other materials
  *       provided with the distribution.
- * 
+ *
  * THIS SOFTWARE IS PROVIDED BY JogAmp Community ``AS IS'' AND ANY EXPRESS OR IMPLIED
  * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
  * FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL JogAmp Community OR
@@ -20,12 +20,12 @@
  * ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
  * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
  * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
- * 
+ *
  * The views and conclusions contained in the software and documentation are those of the
  * authors and should not be interpreted as representing official policies, either expressed
  * or implied, of JogAmp Community.
  */
- 
+
 package com.jogamp.opengl.test.junit.util;
 
 import java.io.BufferedReader;
@@ -69,15 +69,15 @@ public abstract class UITestCase {
 
     public static final String SINGLE_INSTANCE_LOCK_FILE = "UITestCase.lock";
     public static final int SINGLE_INSTANCE_LOCK_PORT = 59999;
-    
+
     public static final long SINGLE_INSTANCE_LOCK_TO   = 6*60*1000; // wait up to 6 mins
     public static final long SINGLE_INSTANCE_LOCK_POLL =      1000; // poll every 1s
 
     private static volatile SingletonInstance singletonInstance;
-    
+
     private static volatile boolean testSupported = true;
     private static volatile boolean resetXRandRIfX11AfterClass = false;
-    
+
     private static volatile int maxMethodNameLen = 0;
 
     private static final synchronized void initSingletonInstance() {
@@ -89,11 +89,11 @@ public abstract class UITestCase {
             }
         }
     }
-    
+
     public static boolean isTestSupported() {
         return testSupported;
     }
-    
+
     public static void setTestSupported(boolean v) {
         System.err.println("setTestSupported: "+v);
         testSupported = v;
@@ -102,13 +102,13 @@ public abstract class UITestCase {
     public static void setResetXRandRIfX11AfterClass() {
         resetXRandRIfX11AfterClass = true;
     }
-    
+
     /**
      * Iterates through all outputs and sets the preferred mode and normal rotation using RandR 1.3.
      * <p>
      * With NV drivers, one need to add the Modes in proper order to the Screen's Subsection "Display",
      * otherwise they are either in unsorted resolution order or even n/a!
-     * </p> 
+     * </p>
      */
     @SuppressWarnings("unused")
     public static void resetXRandRIfX11() {
@@ -122,7 +122,7 @@ public abstract class UITestCase {
                     final ProcessBuilder pb = new ProcessBuilder("xrandr", "-q");
                     pb.redirectErrorStream(true);
                     System.err.println("XRandR Query: "+pb.command());
-                    final Process p = pb.start();                    
+                    final Process p = pb.start();
                     final MiscUtils.StreamDump dump = new MiscUtils.StreamDump( out, p.getInputStream(), ioSync );
                     dump.start();
                     while( !dump.eos() ) {
@@ -185,7 +185,7 @@ public abstract class UITestCase {
         }
         return null;
     }
-    
+
     public static int processCommand(String[] cmdline, OutputStream outstream, String outPrefix) {
         int errorCode = 0;
         final Object ioSync = new Object();
@@ -207,9 +207,9 @@ public abstract class UITestCase {
             e.printStackTrace();
             errorCode = Integer.MIN_VALUE;
         }
-        return errorCode;            
+        return errorCode;
     }
-    
+
     public int getMaxTestNameLen() {
         if(0 == maxMethodNameLen) {
             int ml = 0;
@@ -221,13 +221,13 @@ public abstract class UITestCase {
             }
             maxMethodNameLen = ml;
         }
-        return maxMethodNameLen; 
+        return maxMethodNameLen;
     }
-    
+
     public final String getTestMethodName() {
         return _unitTestName.getMethodName();
     }
-    
+
     public final String getSimpleTestName(String separator) {
         return getClass().getSimpleName()+separator+getTestMethodName();
     }
@@ -235,10 +235,10 @@ public abstract class UITestCase {
     public final String getFullTestName(String separator) {
         return getClass().getName()+separator+getTestMethodName();
     }
-    
+
     @BeforeClass
     public static void oneTimeSetUp() {
-        // one-time initialization code                
+        // one-time initialization code
         initSingletonInstance();
     }
 
@@ -259,14 +259,14 @@ public abstract class UITestCase {
             System.err.println(" - "+unsupportedTestMsg);
             Assume.assumeTrue(testSupported); // abort
         }
-        System.err.println();      
+        System.err.println();
     }
 
     @After
     public void tearDown() {
         System.err.println("++++ UITestCase.tearDown: "+getFullTestName(" - "));
     }
-    
+
     public static void waitForKey(String preMessage) {
         BufferedReader stdin = new BufferedReader(new InputStreamReader(System.in));
         System.err.println(preMessage+"> Press enter to continue");
@@ -274,9 +274,32 @@ public abstract class UITestCase {
             System.err.println(stdin.readLine());
         } catch (IOException e) { }
     }
-    
+
     static final String unsupportedTestMsg = "Test not supported on this platform.";
 
+    /**
+     * Determines whether the chosen {@link GLCapabilitiesImmutable}
+     * requires a {@link GLDrawable#swapBuffers() swap-buffers}
+     * before reading pixels.
+     * <p>
+     * Usually one uses the {@link GL#getDefaultReadBuffer() default-read-buffer}, i.e.
+     * {@link GL#GL_FRONT} for single-buffer and {@link GL#GL_BACK} for double-buffer {@link GLDrawable}s
+     * and {@link GL#GL_COLOR_ATTACHMENT0} for offscreen framebuffer objects.<br>
+     * Here {@link GLDrawable#swapBuffers() swap-buffers} shall happen <b>after</b> calling reading pixels, the default.
+     * </p>
+     * <p>
+     * However, <i>multisampling</i> offscreen {@link javax.media.opengl.GLFBODrawable}s
+     * utilize {@link GLDrawable#swapBuffers() swap-buffers} to <i>downsample</i>
+     * the multisamples into the readable sampling sink.
+     * In this case, we require a {@link GLDrawable#swapBuffers() swap-buffers} <b>before</b> reading pixels.
+     * </p>
+     * @param chosenCaps the chosen {@link GLCapabilitiesImmutable}
+     * @return chosenCaps.isFBO() && chosenCaps.getSampleBuffers()
+     */
+    public static final boolean swapBuffersBeforeRead(GLCapabilitiesImmutable chosenCaps) {
+        return chosenCaps.isFBO() && chosenCaps.getSampleBuffers();
+    }
+
     public String getSnapshotFilename(int sn, String postSNDetail, GLCapabilitiesImmutable caps, int width, int height, boolean sinkHasAlpha, String fileSuffix, String destPath) {
         if(null == fileSuffix) {
             fileSuffix = TextureIO.PNG;
@@ -284,7 +307,7 @@ public abstract class UITestCase {
         final int maxSimpleTestNameLen = getMaxTestNameLen()+getClass().getSimpleName().length()+1;
         final String simpleTestName = this.getSimpleTestName(".");
         final String filenameBaseName;
-        {            
+        {
             final String accel = caps.getHardwareAccelerated() ? "hw" : "sw" ;
             final String scrnm;
             if(caps.isOnscreen()) {
@@ -307,22 +330,22 @@ public abstract class UITestCase {
             final String aaext = caps.getSampleExtension();
             postSNDetail = null != postSNDetail ? "-"+postSNDetail : "";
 
-            filenameBaseName = String.format("%-"+maxSimpleTestNameLen+"s-n%04d%s-%-6s-%s-%s-B%s-F%s_I%s-D%02d-St%02d-Sa%02d_%s-%04dx%04d.%s", 
-                    simpleTestName, sn, postSNDetail, caps.getGLProfile().getName(), accel, 
-                    scrnm, dblb, F_pfmt, pfmt, depthBits, stencilBits, samples, aaext,  
+            filenameBaseName = String.format("%-"+maxSimpleTestNameLen+"s-n%04d%s-%-6s-%s-%s-B%s-F%s_I%s-D%02d-St%02d-Sa%02d_%s-%04dx%04d.%s",
+                    simpleTestName, sn, postSNDetail, caps.getGLProfile().getName(), accel,
+                    scrnm, dblb, F_pfmt, pfmt, depthBits, stencilBits, samples, aaext,
                     width, height, fileSuffix).replace(' ', '_');
         }
-        return null != destPath ? destPath + File.separator + filenameBaseName : filenameBaseName;        
+        return null != destPath ? destPath + File.separator + filenameBaseName : filenameBaseName;
     }
-    
+
     /**
-     * Takes a snapshot of the drawable's current front framebuffer. Example filenames: 
+     * Takes a snapshot of the drawable's current front framebuffer. Example filenames:
      * <pre>
      * TestGLDrawableAutoDelegateOnOffscrnCapsNEWT.testES2OffScreenFBOSglBuf____-n0001-msaa0-GLES2_-sw-fbobject-Bdbl-Frgb__Irgba8888_-D24-St00-Sa00_default-0400x0300.png
      * TestGLDrawableAutoDelegateOnOffscrnCapsNEWT.testES2OffScreenPbufferDblBuf-n0003-msaa0-GLES2_-sw-pbuffer_-Bdbl-Frgb__Irgba8880-D24-St00-Sa00_default-0200x0150.png
      * TestGLDrawableAutoDelegateOnOffscrnCapsNEWT.testGL2OffScreenPbufferSglBuf-n0003-msaa0-GL2___-hw-pbuffer_-Bone-Frgb__Irgba5551-D24-St00-Sa00_default-0200x0150.png
      * </pre>
-     * @param sn sequential number 
+     * @param sn sequential number
      * @param postSNDetail optional detail to be added to the filename after <code>sn</code>
      * @param gl the current GL context object. It's read drawable is being used as the pixel source and to gather some details which will end up in the filename.
      * @param readBufferUtil the {@link GLReadBufferUtil} to be used to read the pixels for the screenshot.
@@ -330,20 +353,20 @@ public abstract class UITestCase {
      *                   If <code>null</code> the <code>"png"</code> as defined in {@link TextureIO#PNG} is being used.
      * @param destPath Optional platform dependent file path. It shall use {@link File#separatorChar} as is directory separator.
      *                 It shall not end with a directory separator, {@link File#separatorChar}.
-     *                 If <code>null</code> the current working directory is being used.  
+     *                 If <code>null</code> the current working directory is being used.
      */
     public void snapshot(int sn, String postSNDetail, GL gl, GLReadBufferUtil readBufferUtil, String fileSuffix, String destPath) {
         final GLDrawable drawable = gl.getContext().getGLReadDrawable();
-        final String filename = getSnapshotFilename(sn, postSNDetail, 
-                                                    drawable.getChosenGLCapabilities(), drawable.getWidth(), drawable.getHeight(), 
+        final String filename = getSnapshotFilename(sn, postSNDetail,
+                                                    drawable.getChosenGLCapabilities(), drawable.getWidth(), drawable.getHeight(),
                                                     readBufferUtil.hasAlpha(), fileSuffix, destPath);
         System.err.println(Thread.currentThread().getName()+": ** screenshot: "+filename);
         gl.glFinish(); // just make sure rendering finished ..
         if(readBufferUtil.readPixels(gl, false)) {
             readBufferUtil.write(new File(filename));
-        }                
+        }
     }
-    
+
     public class SnapshotGLEventListener implements GLEventListener {
         private final GLReadBufferUtil screenshot;
         private volatile boolean makeShot = false;
@@ -394,6 +417,6 @@ public abstract class UITestCase {
             postSNDetail = v;
         }
     };
-    
+
 }