I have a simple sketch that clears the background, shifts the location of a vertical line horizontally and draws that line:

Code:

int x;

void setup()
{
 size(400,400)
 frameRate(30);
}

void draw() {
  background(0);
  stroke(255);
  strokeWeight(4);
  x += 20;
  line(x, 0, x, 400);
  if (x > 400) {
    x = 0;
  }
}
 

When I shift the line one pixel each frame, the animation looks smooth at 30fps. However, when I shift the line 20 pixels each frame, two blinking lines are rendered.

I've read on these boards that enabling vsync has fixed these issues for some. I tried this, but got the same result.
Code:

import processing.opengl.*;
import javax.media.opengl.GL;

PGraphicsOpenGL pgl;
GL gl;

int x;

void setup()
{
 size(400,400, OPENGL);
 pgl = (PGraphicsOpenGL) g; //processing graphics object
 gl = pgl.beginGL(); //begin opengl
 gl.setSwapInterval(1); //set vertical sync on
// pgl.endGL(); //end opengl
 frameRate(30);
 smooth();
}

void draw() {
  background(0);
  stroke(255);
  strokeWeight(4);
  x += 20;
  line(x, 0, x, 400);
  if (x > 400) {
    x = 0;
  }
}
 

System details:
MacBook Pro C2D 2.8ghz, 4gm ram
Nvidia GeForce 9600 GT
OS 10.6.2, Processing 1.0.9

I also tested this on a Windows XP machine running 1.0.9 and got the same result. I did notice here that I got tearing without vsync.

Thanks in advance!