FreeJ/scripts/processing/topics/reflection1.pde

 float baseX1, baseY1, baseX2, baseY2;
float baseLength;
float[] xCoords, yCoords;
float ellipseX, ellipseY, ellipseRadius = 6;
float directionX, directionY;
float ellipseSpeed = 3.5;
float velocityX, velocityY; 

void setup(){
  size(200, 200);
  frameRate(30);
  fill(128);
  smooth();
  baseX1 = 0;
  baseY1 = height-150;
  baseX2 = width;
  baseY2 = height;

  // start ellipse at middle top of screen
  ellipseX = width/2;
  
  // calculate initial random direction
  directionX = random(0.1, 0.99);
  directionY = random(0.1, 0.99);

  // normalize direction vector
  float directionVectLength = sqrt(directionX*directionX + 
            directionY*directionY);
  directionX /= directionVectLength;
  directionY /= directionVectLength;
}

void draw(){
  // draw background
  fill(0, 12);
  noStroke();
  rect(0, 0, width, height);

  // calculate length of base top
  baseLength = dist(baseX1, baseY1, baseX2, baseY2);
  xCoords = new float[ceil(baseLength)];
  yCoords = new float[ceil(baseLength)];

  // fill base top coordinate array
  for (int i=0; i<xCoords.length; i++){
    xCoords[i] = baseX1 + ((baseX2-baseX1)/baseLength)*i;
    yCoords[i] = baseY1 + ((baseY2-baseY1)/baseLength)*i;
  }

  // draw base
  fill(200);
  quad(baseX1, baseY1, baseX2, baseY2, baseX2, height, 0, height);

  // calculate base top normal
  float baseDeltaX = (baseX2-baseX1)/baseLength;
  float baseDeltaY = (baseY2-baseY1)/baseLength;
  float normalX = -baseDeltaY;
  float normalY = baseDeltaX;

  // draw ellipse
  noFill();
  stroke(200);
  ellipse(ellipseX, ellipseY, ellipseRadius*2, ellipseRadius*2);

  // calculate ellipse velocity
  velocityX = directionX * ellipseSpeed;
  velocityY = directionY * ellipseSpeed;

  // move elipse
  ellipseX += velocityX;
  ellipseY += velocityY;

  // normalized incidence vector
  float incidenceVectorX = -directionX;
  float incidenceVectorY = -directionY;

  // detect and handle collision
  for (int i=0; i<xCoords.length; i++){
    // check distance between ellipse and base top coordinates
    if (dist(ellipseX, ellipseY, xCoords[i], yCoords[i]) < ellipseRadius){

      // calculate dot product of incident vector and base top normal 
      float dot = incidenceVectorX*normalX + incidenceVectorY*normalY;

      // calculate reflection vector
      float reflectionVectorX = 2*normalX*dot - incidenceVectorX;
      float reflectionVectorY = 2*normalY*dot - incidenceVectorY;

      // assign reflection vector to direction vector
      directionX = reflectionVectorX;
      directionY = reflectionVectorY;

      // draw base top normal at collision point
      stroke(255, 128, 0);
      line(ellipseX, ellipseY, ellipseX-normalX*100, 
            ellipseY-normalY*100);
    }
  }

  // detect boundary collision
  // right
  if (ellipseX > width-ellipseRadius){
    ellipseX = width-ellipseRadius;
    directionX *= -1;
  }
  // left 
  if (ellipseX < ellipseRadius){
    ellipseX = ellipseRadius;
    directionX *= -1;
  }
  // top
  if (ellipseY < ellipseRadius){
    ellipseY = ellipseRadius;
    directionY *= -1;
    // randomize base top
    baseY1 = random(height-100, height);
    baseY2 = random(height-100, height);
  }
}