Linear

/**
 * Linear Motion.
 *
 * Changing a variable to create a moving line.
 * When the line moves off the edge of the window,
 * the variable is set to 0, which places the line
 * back at the bottom of the screen.
 */

float a;

void setup() {
  size(640, 360);
  stroke(255);
  a = height/2;
}

void draw() {
  background(51);
  line(0, a, width, a);

  a = a - 0.5;
  if (a < 0) {
    a = height;
  }
}