// rotatedemo D. Parson Feb. 13, 2018
float rotateAmount = 0, rotateSpeed = 1 ;
void setup() { size(500,500); background(255); }
void draw() {
  background(255);
  fill(255,255,0);
  stroke(0);
  // If I want to rotate around the center of a triangle,
  // easiest way is to translate to cernter of the triangle.
  pushMatrix();
  translate(width/2, height/4);
  rotate(radians(rotateAmount));
  triangle(0,-20, -10, 10, 10, 10);
  rotateAmount = rotateAmount + rotateSpeed ;
  if (rotateAmount < 0 || rotateAmount >= 360) {
    rotateAmount = 0 ;
  }
  popMatrix();
}