How to rotate a square in processing? - animation

Question

I've been trying to rotate a square for a project, I've done research and think I have the right formula to calculate the rotated points. I calculate the points as if they're individual around the center of the square. How to fix it?
//Declaring variables
float x0, y0, xo, yo,x1,y1,x2,y2,x3,y3, theta, newx, newy, s, c;
void setup() {
size (800,800);
//To debug
//frameRate(1);
fill(0);
//Initializing variables
xo = 400;
yo = 400;
x0 = 350;
y0 = 450;
x1 = 350;
y1 = 350;
x2 = 450;
y2 = 350;
x3 = 450;
y3 = 450;
theta = radians(5);
s = sin(theta);
c = cos(theta);
}
void draw() {
//Reseting the background
background(255);
//Drawing the square
quad(x0,y0,x1,y1,x2,y2,x3,y3);
//Doing the rotations
x0 = rotateX(x0,y0);
y0 = rotateY(x0,y0);
x1 = rotateX(x1,y1);
y1 = rotateY(x1,y1);
x2 = rotateX(x2,y2);
y2 = rotateY(x2,y2);
x3 = rotateX(x3,y3);
y3 = rotateY(x3,y3);
}
//Rotate x coordinate method
float rotateX(float x, float y) {
x -= xo;
newx = x * c - y * s;
x = newx + xo;
return x;
}
//Rotate y coordinate method
float rotateY(float x, float y) {
y -= yo;
newy = x * s - y * c;
y = newy + yo;
return y;
}

Answer 1

There are two things:
1) You have a sign error in rotateY(). The y term should have a positive sign:
newy = x * s + y * c;
2) When you do this:
x0 = rotateX(x0,y0);
y0 = rotateY(x0,y0);
... then the first call modifies x0, which the second call then uses. But the second call needs the original coordinates to rotate correctly:
float x0Rotated = rotateX(x0, y0);
y0 = rotateY(x0, y0);
x0 = x0Rotated;
The same thing for the other points.