The copy() function is not working with OPENGL, but the copy() method of
PImage is. So...

This code is working:

import processing.opengl.*;

PImage img;

void setup() {
size(100, 100, OPENGL);
img = loadImage("arch.jpg");
}

void draw() {
img.copy(50, 0, 50, 100, 0, 0, 50, 100);
image(img, 0, 0);
}


But this code is not working:

import processing.opengl.*;

PImage img;

void setup() {
size(150, 150, OPENGL); // Copy does not work
//size(150, 150); // Copy works fine
img = loadImage("arch.jpg");
}

void draw() {
image(img, 0, 0, 100, 100);
copy(50, 0, 50, 100, 0, 0, 50, 100);
}

edit]
Sample code showing the problem

this actually goes with bug #91, but is this something that is preventing you from doing book
work, if so, then it gets bumped up on the priority list.

It's not preventing the book work, it came up in class while working on a
project. I'll check out bug #91.

*** This bug has been marked as a duplicate of 91 ***