This example is for Processing 3+. If you have a previous version, use the examples included with your software. If you see any errors or have suggestions, please let us know.
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;
}
}