This example is for Processing 3+. If you have a previous version, use the examples included with your software. If you see any errors or have suggestions, please let us know.

Follow 2 based on code from Keith Peters.

A two-segmented arm follows the cursor position. The relative angle between the segments is calculated with atan2() and the position calculated with sin() and cos().

float[] x = new float[2];
float[] y = new float[2];
float segLength = 50;

void setup() {
  size(640, 360);
  stroke(255, 100);

void draw() {
  dragSegment(0, mouseX, mouseY);
  dragSegment(1, x[0], y[0]);

void dragSegment(int i, float xin, float yin) {
  float dx = xin - x[i];
  float dy = yin - y[i];
  float angle = atan2(dy, dx);  
  x[i] = xin - cos(angle) * segLength;
  y[i] = yin - sin(angle) * segLength;
  segment(x[i], y[i], angle);

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