amnon, in posting to a question of mine, offered this beautiful code.

https://forum.processing.org/topic/controlling-the-length-of-line-from-x-y-to-a-point-which-is-randomly-chosen

it appears a little  more than half-way down the page.

Because I am learning, I decided to work with the code piece by piece to see if I could learn what it was doing.

I'm leaving out all the camera stuff which is way beyond me. But, I ran into a problem when I wanted to confine, to the processing screen, the lines being drawn.

I first tried constrain with this code:

1. //    constrain(p.x, 0, width-20);
2. //    constrain(p.y, 0, height-20);
3. //    constrain(p.z, 0, height);

and that appeared to do nothing.

I then tried the forceful brute math method:

1.     if (p.x >= width) {p.x=width-10;}
2.     if (p.y >= height){ p.y=height-10;}
3.     if (p.x < 0) {p.x=10;}
4.     if (p.y < 0){ p.y=10;}

And still they disappear.

Although the line usually comes back on screen fairly fast, my main issue is with the immediate start up of the sketch -- it VERY often is off screen for quite some time and I stop and restart. But I would like to ALWAYS have the line start on the screen area.

I've no idea what's going on.  Here is the full sketch:

1. // a selection from amnon's 3D line code
   
2. ArrayList <PVector> points = new ArrayList <PVector> ();
3. PVector c = new PVector();
4. 
   
5. void setup() {
6.   size(700, 500, P3D);
7.   background(0);
8.   colorMode(HSB, 500, 100, 100);
9.   points.add( new PVector() );
10.   frameRate(5);
11.   smooth(8);
12.   strokeWeight(2);
13.   noFill();
14. }
15. 
    
16. void draw() {
17. background(0);
18.   int index=0;
19.   beginShape();
20.   for (PVector p : points) {
21.     stroke((++index+frameCount)%500, 100, 100);
22.     if (p.x >= width) {  p.x=width-10;   }
23.     if (p.y >= height) { p.y=height-10;  }
24.    if (p.x < 0) {  p.x=10;  }
25.     if (p.y < 0) {  p.y=10;  }
26.     curveVertex(p.x, p.y, p.z);
27.   }
28.   endShape();
29.   // new point at specified range
30.   PVector n = PVector.random3D();
31.   n.mult(100);
32.   n.add(points.get(points.size()-1));
33.   points.add( n );
34.   if (mousePressed == true) {
35.     saveFrame("mylines-####.tif");
36.   }
37. }