I'm trying to figure out how to write to and draw from opengl pixel buffer objects in a processing sketch, but 1) I'm a complete novice to opengl coding and 2) my code is not working, but not failing catastrophically and producing a weird error. I'd really appreciate it if anyone can throw some light on this....

The source code is below. When I run it I get the not-particularly-informative error "OpenGL error 1282 at top endDraw(): invalid operation", the screen open and the sketch keeps running, but doesn't draw anything. I was hoping for a screen with a back bit on top and a red bit below...

2. import javax.media.opengl.GL2;
3. import java.nio.*;
4. import java.nio.ByteBuffer;
5. import java.nio.MappedByteBuffer;
6. 
   
7. PGraphicsOpenGL pgl;
8. GL2 gl;
9. ByteBuffer bb;
10. int[] pboIds;
11. boolean buffReady;
12. 
    
13. void setup() {
14.   size(300,300,OPENGL);
15.   pgl = (PGraphicsOpenGL) g;
16.   pboIds = new int[2];
17.   buffReady = false;
18. }
19. 
    
20. void draw() {
21.   println(frameCount);
22.   background(0);
23.   
24.   if (!buffReady) {
25.     println("buffer not ready, building it now...");
26.     gl = pgl.beginPGL().gl.getGL2();
27.     
28.     //set the way the data is packed in and unpacked from memory - 1=byte alignment
29.     //see http://samplecodebank.blogspot.co.uk/2011/06/glpixelstorei-example-c-c-java-objc.html
30.     gl.glPixelStorei(GL2.GL_PACK_ALIGNMENT, 1);
31.     gl.glPixelStorei(GL2.GL_UNPACK_ALIGNMENT, 1);
32.     
33.     // generate two buffer handles and store them int he pboIds array - what the third argument 
34.     // is is anyones guess. http://samplecodebank.blogspot.co.uk/2011/05/glgenbuffers-example-c-c.html
35.     gl.glGenBuffers(2, pboIds, 0);
36.     
37.     // bind a PIXEL_UNPACK_BUFFER to one of the two ids and then populate it with data. In this case
38.     // the buffer size is 4*width*height) for storing 4-byte integers for each pixel in the sketch
39.     // window. The array is populated with nulls. The last arg of glBufferData is the draw mode
40.     // this can be on of a few things depending on whats being done with the data.
41.     // http://samplecodebank.blogspot.co.uk/2011/05/glbindbuffer-example-c-c.html
42.     // http://samplecodebank.blogspot.co.uk/2011/05/glbufferdata-example-c-c.html
43.     gl.glBindBuffer(GL2.GL_PIXEL_UNPACK_BUFFER, pboIds[0]);
44.     gl.glBufferData(GL2.GL_PIXEL_UNPACK_BUFFER, width*height*4, null, GL2.GL_STREAM_DRAW);
45.     
46.     // do the same with the second buffer
47.     gl.glBindBuffer(GL2.GL_PIXEL_UNPACK_BUFFER, pboIds[1]);
48.     gl.glBufferData(GL2.GL_PIXEL_UNPACK_BUFFER, width*height*4, null, GL2.GL_STREAM_DRAW);
49.     
50.     // get the fist of the buffers, map it to a byte array so we can edit it directly and the
51.     // add some ints to it.
52.     gl.glBindBuffer(GL2.GL_PIXEL_UNPACK_BUFFER, pboIds[0]);
53.     bb = gl.glMapBuffer(GL2.GL_PIXEL_UNPACK_BUFFER, GL2.GL_WRITE_ONLY);
54.     //FloatBuffer floatbuffer = bb.order( ByteOrder.nativeOrder() ).asFloatBuffer();
55.     
56.     println("buffer created and mapped to a ByteBuffer. Time to add data! ...");
57.     
58.     int thisint = 1;
59.     for (int i=0; i<width*height; i++) {
60.       if (i>300) {
61.         thisint = 300;
62.       }
63.       bb.putInt(thisint);
64.     }
65.             
66.     // unmap the buffer and bind it back to an empty value (which apparently resets stuff!)
67.     println("done.");
68.     gl.glUnmapBuffer(GL2.GL_PIXEL_UNPACK_BUFFER);
69.     gl.glBindBuffer(GL2.GL_PIXEL_UNPACK_BUFFER, 0);
70.     
71.     println("buffer ready!");
72.     buffReady = true;
73.   }
74.   
75.   gl.glBindBuffer(GL2.GL_PIXEL_UNPACK_BUFFER, pboIds[0]);
76.   gl.glDrawPixels(width, height, GL2.GL_COLOR_INDEX, GL2.GL_INT, pboIds[0]);
77.   gl.glBindBuffer( GL2.GL_PIXEL_UNPACK_BUFFER, 0 );
78. }