How to remove previous shape after draw() in Processing

Question

I cant figure this out. I have a sketch with little rotating rectangles on it. They rotate on every draw(). However the previous rectangle remains visible. I tried moving backgr

Answer 1

You've got a few things going on in your code that I've seen in your previous posts. The way you're doing your drawing doesn't make a ton of sense, and I'll explain why.

Here's what most Processing sketches do:

• Use the setup() function to setup any data structures you'll use in your program. Don't do any drawing from the setup() function.
• Call background() every frame to clear out old frames.
• Draw everything you want to be drawn in the frame in the draw() function.
• Modify the data structures to change what you're drawing on the screen.

Your code is a bit too long for an MCVE, so here's a little example that handles the drawing in a more standard way:

ArrayList<PVector> circles = new ArrayList<PVector>();

void setup() {
  size(500, 500);
  ellipseMode(RADIUS);

  //setup your data structures here
  circles.add(new PVector(250, 250));

  //don't do any drawing yet
}

void mousePressed() {
  //modify the data structure whenever you want to change what's on the screen
  circles.add(new PVector(mouseX, mouseY));
}

void keyPressed() {
  //modify the data structure whenever you want to change what's on the screen
  if (!circles.isEmpty()) {
    circles.remove(0);
  }
}

void draw() {
  //call background every frame to clear out old frames
  background(0);

  //draw everything
  for (PVector p : circles) {
    ellipse(p.x, p.y, 20, 20);
  }
}

Notice how this is different from what you're doing. Here's what you do:

• You use the setup() function to setup your data structures, but then you draw the background and some of the objects to the screen.
• You then don't call background() from draw(), so you're always stuck with whatever has already been drawn.
• You then only draw a subset of what you want on the screen, so you can't redraw your whole scene.

You have to modify your code to no longer draw anything from setup(), to call the background() function every frame, and to draw everything you want on the screen every frame.

Answer 2

What you are doing is printing every single circle or line...ect. You need to have a timer that removes them every so often. If you do it too fast you get a strobe like look. So have a timer that removes the first rect from the array list every so often.