On my MacBook Pro, the hint(ENABLE_OPENGL_2X_SMOOTH) function seems to be
calling background(). I'm not able to accumulate pixels into the display
window. The card is ATI Radeon X1600. On my Windows box with an nVidia
card, the behavior of this hint behaves as expected.

Here's the text code:

import processing.opengl.*;

void setup() {
size(400, 400, OPENGL);
hint(ENABLE_OPENGL_2X_SMOOTH);
smooth();
}

void draw() {
line(frameCount, 0, 0, height);
}

yep, this is covered in the reference.