Moving an objects while others stay static in a click controlled sketch - processing

Question

So I'm trying to build an animation that I can step through with mouse clicks. Adding individual objects click by click is easy. Sequence I want is as follows:
One object(a) drawn initially.
First mouse click adds an object(b).
Second mouse click adds an object(c).
Third mouse click, object(c) should move across the screen and disappear.
I'm having a problem on the last part of the sequence. I can't figure out how to make the object move and still maintain the static part of the sketch. The normal way of doing movement is to change the coordinates of the object with each loop through the draw() function, and use the background to cover up the previous objects. Can't do that in this case because I need object(a) and object(b) to be persistent.
Code below. Thanks for your help!
var count = 0;
function setup() {
createCanvas(200, 200);
a = new Object1(20, 40);
b = new Object1(20, 85);
c = new Object1(20, 130);
}
function draw() {
background(200);
a.display();
if (count == 1) {
b.display();
}
if (count == 2) {
b.display();
c.display();
}
if (count == 3) { //this is where I have a problem
}
if (count > 3) {
count = 0;
}
}
function Object1(ix, iy, itext) {
this.x = ix;
this.y = iy;
this.text = itext;
this.display = function() {
fill(160);
rect(this.x, this.y, 40, 40);
}
}
function mousePressed() {
count++;
}

Answer 1

Generally how you'd do this is by drawing the static part of your scene to an off-screen buffer, which you can create using the createGraphics() function. From the reference:
var pg;
function setup() {
createCanvas(100, 100);
pg = createGraphics(100, 100);
}
function draw() {
background(200);
pg.background(100);
pg.noStroke();
pg.ellipse(pg.width/2, pg.height/2, 50, 50);
image(pg, 50, 50);
image(pg, 0, 0, 50, 50);
}
You'd draw the static part to the buffer, then draw the buffer to the screen, then draw the dynamic stuff on top of it each frame.
This has been discussed before, so I'd recommend doing a search for stuff like "processing pgraphics" and "p5.js buffer" to find a bunch more information.