Stop background from refreshing? - processing

Question

I am trying to get my gif to do something similar to this gif.
I have been able to get the line to draw, and the 'planets' to orbit, but can't figure out how to keep the line connecting the two circles, like the gif does.
Here's the basic code:
int x = 500;
int y = 500;
int radius = y/2;
int cX = x/2;
int cY = y/2;
String text1;
int lg_xBall;
int lg_yBall;
int sm_xBall;
int sm_yBall;
void setup() {
size(x, y);
smooth();
colorMode(RGB);
}
void draw() {
background(0);
stroke(255);
float t = millis()/1000.0f;
drawSmBallOrbit(100);
drawLgBallOrbit(100);
moveSmBall(t);
moveLgBall(t);
sun();
// showMouse();
connectingLines();
}
void drawCircle() { // This will draw a simple circle
stroke(1);
// x1=a+r*cos t, y1=b+r*sin t
ellipse(x/2, y/2, x/2, y/2);
}
void drawLines() { // This will draw lines from the center of the circle.
stroke(1);
line(x/2, y/2, radius/2, radius); // line from 6 to center
line(x/2, y/2, x/2, y/4); // line from 12 to center
for (int i = 0; i <= 5; i+=2.5) {
float x1 = x/2+radius/2*cos(i);
float y1 = y/2+radius/2*sin(i);
line(x/2, y/2, x1, y1);
}
}
void moveSmBall(float ky) { // This will create, and move, a small 'planet'
pushStyle();
stroke(100);
sm_xBall = (int)(cX+radius*cos(ky));
sm_yBall = (int)(cY+radius*sin(ky));
fill(190, 0, 0);
// background(0);
ellipse(sm_xBall, sm_yBall, 10, 10);
popStyle();
}
void drawSmBallOrbit(float opacity) {
pushStyle();
stroke(255, opacity);
strokeWeight(1);
noFill();
ellipse(x/2, y/2, cX+radius, cY+radius);
popStyle();
}
void moveLgBall(float kx) {
kx = kx/.7;
pushStyle();
lg_xBall = (int)(cX+radius*cos(kx)*.6);
lg_yBall = (int)(cY+radius*sin(kx)*.6);
fill(0, 0, 230);
ellipse(lg_xBall, lg_yBall, 30, 30);
popStyle();
}
void drawLgBallOrbit(float opacity) {
pushStyle();
stroke(255, opacity);
strokeWeight(1);
noFill();
ellipse(x/2, y/2, (cX+radius)*.6, (cY+radius)*.6);
popStyle();
}
void sun() {
pushStyle();
fill(250, 250, 0);
ellipse(cX, cY, 40, 40);
popStyle();
}
void connectingLines() {
line(sm_xBall, sm_yBall, lg_xBall, lg_yBall);
}
void showMouse() {
text("X: " + mouseX, x/2, y/2-30);
text("Y: " + mouseY, x/2, y/2-50);
}
Thanks for any help/advice!

Answer 1

The problem is that you're calling background() during every frame, which will clear away anything you've already drawn.
So you either need to stop calling background(), or you need to redraw the old lines every frame.
If you simply move the call to background() out of your draw() function and into your setup() function, you're about 50% there already:
void setup() {
size(x, y);
smooth();
colorMode(RGB);
background(0);
}
void draw() {
// background(0);
stroke(255);
float t = millis()/1000.0f;
drawSmBallOrbit(100);
drawLgBallOrbit(100);
moveSmBall(t);
moveLgBall(t);
sun();
// showMouse();
connectingLines();
}
However, the original animation does not show the previous positions of the ellipses. So you need to clear away the previous frame by calling the background() function, and then redraw previous line positions. You'd do that by having an ArrayList that holds those previous positions.
Here's a simple example that uses an ArrayList to redraw anywhere the mouse has been:
ArrayList<PVector> points = new ArrayList<PVector>();
void setup() {
size(500, 500);
}
void draw() {
background(0);
stroke(255);
points.add(new PVector(mouseX, mouseY));
for(PVector p : points){
ellipse(p.x, p.y, 10, 10);
}
}
You would need to do something very similar, but you'd have to keep track of two points at a time instead of one, since you're tracking two ellipses and not just the mouse position.