Webgl, Rotate object from different rotation origin?

p5.js Beginners
7

here is an example in processing Java and in 3D, using box instead of rect

float angleRightArm=0.0; 
float angleRightArmAdd=2; 

void setup() {
  size(1600, 900, P3D);
}

void draw() {
  background(255);
  lights(); 
  rightArm();
}

void rightArm() {
  // draw right arm

  pushMatrix(); // store matrix
  float armLength=300.0; 

  translate(255, 170, 0);  // arm position 
  rotateX(radians(angleRightArm));    // rotate  
  sphere();                // for testing only 
  translate(0, 0, armLength/2-20);  // that's the offset between arm center and where we want to have the shoulder / rotation !!!  

  // draw box/arm at 0,0,0 (box's center!) with the following width, height and depth: 
  fill(255); 
  stroke(0);
  box (15, 35, armLength);

  popMatrix();  // restore matrix

  // manage angle and angle bouncing:  
  angleRightArm += angleRightArmAdd;
  if (angleRightArm>12) {
    angleRightArmAdd=-2;
  }
  if (angleRightArm<-152) {
    angleRightArmAdd=2;
  }
}

void sphere() {

  // draw a sphere a the current 0,0,0 point

  fill(255, 0, 0);  // RED 
  noStroke();  
  sphere(11);
}
//
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