Processing.js

Arm

The angle of each segment is controlled with the mouseX and mouseY position. The transformations applied to the first segment are also applied to the second segment because they are inside the same pushMatrix() and popMatrix() group.

Original Processing.org Example: Arm

// All Examples Written by Casey Reas and Ben Fry
// unless otherwise stated.
float x = 50;
float y = 100;
float angle1 = 0.0;
float angle2 = 0.0;
float segLength = 50;

void setup() {
  size(200, 200);
  smooth(); 
  strokeWeight(20.0);
  stroke(0, 100);
}

void draw() {
  background(226);
  
  angle1 = (mouseX/float(width) - 0.5) * -PI;
  angle2 = (mouseY/float(height) - 0.5) * PI;
  
  pushMatrix();
  segment(x, y, angle1); 
  segment(segLength, 0, angle2);
  popMatrix();
}

void segment(float x, float y, float a) {
  translate(x, y);
  rotate(a);
  line(0, 0, segLength, 0);
}