Call glReadPixels Within display() JOGL

Question

I am trying to implement object picking. I have code to render the objects as solid, unlit colors, then I read the pixels in the screen buffer. I interpret the readings from glReadPixels() to determine which object the cursor is currently on. Finally, I re-render everything lit, textured, and colored.

    gl.glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    gl.glMatrixMode(GL_MODELVIEW);
    gl.glLoadIdentity();
    gl.glPushMatrix();

    //Picking render goes here...

    gl.glPopMatrix();



    //The following code reads the current pixel color at the center of the screen
    FloatBuffer buffer = FloatBuffer.allocate(4);

    gl.glReadBuffer(GL_FRONT);
    gl.glReadPixels(drawable.getWidth() / 2, drawable.getHeight() / 2, 1, 1, GL_RGBA, GL_FLOAT, buffer);
    float[] pixels = new float[3];
    pixels = buffer.array();
    float red = pixels[0];
    float green = pixels[1];
    float blue = pixels[2];
    System.out.println(red + ", " + green + ", " + blue);



    gl.glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    gl.glMatrixMode(GL_MODELVIEW);
    gl.glLoadIdentity();
    gl.glPushMatrix();

    //Final render goes here...

    gl.glPopMatrix();

The problem is that the call to glReadPixels() returns me the pixel color of the FINAL render, rather than the picking render. I would appreciate an explanation of how to read the picking render pixels.

I am aware that I can use the OpenGL name stack, but frankly I find it inconvenient and ridiculous.

Answer 1

The problem is that you render to the back buffer, but your glReadPixels() call reads from the front buffer. If you render to a double buffered surface (which you almost always want when using OpenGL), your rendering goes to the back buffer by default. You have this call in your code:

gl.glReadBuffer(GL_FRONT);

This tells the following glReadPixels() call to read from the front buffer, which is the buffer you are not drawing to.

You should be able to simply remove the glReadBuffer() call, as GL_BACK is the default read buffer for a double buffered default rendering surface.