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I'm trying to write a java opengl (JOGL) method that writes to an offscreen drawable and then writes that to an image. I have verified this works when using an onscreen drawable as well as GLP buffers, but the output image in its current state is just solid black. The code is below.

GLProfile glp = GLProfile.getDefault();
GLCapabilities caps = new GLCapabilities(glp);
caps.setOnscreen(false);

// create the offscreen drawable
GLDrawableFactory factory = GLDrawableFactory.getFactory(glp);
GLOffscreenAutoDrawable drawable = factory.createOffscreenAutoDrawable(null,caps,null,width,height);
drawable.display();
drawable.getContext().makeCurrent();

// a series of x/y coordinates
FloatBuffer buffer = generateData();

GL2 gl = drawable.getGL().getGL2();

// use pixel coordinates
gl.glMatrixMode(GLMatrixFunc.GL_PROJECTION);
gl.glLoadIdentity();    
gl.glOrtho(0d, width, height, 0d, -1d, 1d);

// draw some points to the drawable
gl.glPointSize(4f);
gl.glColor3f(1f,0f,0f);    
gl.glEnableClientState(GL2.GL_VERTEX_ARRAY);
gl.glVertexPointer(2, GL2.GL_FLOAT, 0, buffer);
gl.glDrawArrays(GL2.GL_POINTS, 0, numPoints);

BufferedImage im = new AWTGLReadBufferUtil(drawable.getGLProfile(), false).readPixelsToBufferedImage(drawable.getGL(), 0, 0, width, height, true /* awtOrientation */);
ImageIO.write(im,"png",new File("im.png"));
Jeff Storey
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  • Have you looked at our Github project jogl-demos and our unit tests? GLPbuffer is deprecated as far as I know. Which version of JOGL do you use? Rather ask your question on the official forum and provide a complete source code that we can run: http://forum.jogamp.org Maybe there is a missing bind or something very trivial, I did a similar mistake some months ago. – gouessej Apr 10 '14 at 10:16
  • Thanks. GLPBuffer is deprecated, but I was having trouble getting this to work with the createOffscreenDrawable. I'll post the example in the jogamp forums. Thanks. – Jeff Storey Apr 10 '14 at 12:00
  • We would be very happy if you posted here on our forum the working source code so that someone making a similar mistake can find the solution: http://forum.jogamp.org/Render-offscreen-buffer-to-image-td4032144.html – gouessej Apr 13 '14 at 10:45
  • I will do that. Sorry I haven't quite done it yet... – Jeff Storey Apr 13 '14 at 17:00
  • Hi @JeffStorey, you should avoid legacy opengl – elect Aug 12 '15 at 09:05

2 Answers2

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This is a bit old, but I found a solution to the problem that seems to work for me. I just added an ordinary GLEventListener object right before calling .display() on the drawable, as such:

//...
drawable.addGLEventListener(new OffscreenJOGL());
drawable.display();

//Move drawing code to OffscreenJOGL

BufferedImage im = new AWTGLReadBufferUtil(drawable.getGLProfile(), false).readPixelsToBufferedImage(drawable.getGL(), 0, 0, width, height, true /* awtOrientation */);
ImageIO.write(im,"png",new File("im.png"));

The code to draw now should be in your custom OffscreenJOGL class, under the init(...), reshape(...) and display(...) methods. Note that setting the current context must be in the init(...) method of OffscreenJOGL. I get an exception thrown otherwise.

class OffscreenJOGL implements GLEventListener {
    public void init(GLAutoDrawable drawable) {
        drawable.getContext().makeCurrent();
        //Other init code here
    }

    public void display(GLAutodrawable drawable) {
        //Code for drawing here
    }

    public void reshape(GLAutoDrawable drawable, int x, int y, int width, int height) {
        //Called at least once after init(...) and before display(...)
    }

    public void dispose(GLAutoDrawable drawable) {
        //Dispose code here
    }
}
Zoodinger
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Most likely you might have found the required answer for your query.
If not, I suggest to add a line, for example: 

gl.glClearColor(GLfloat red, GLfloat green, GLfloat blue, GLfloat alpha)

I have tested it, and it works.

Oliver
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Chinnaswamy Gounder
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