code: function(x, y) {
      x = (x - this.viewport[0]) / this.viewport[2];
      y = 1 - (y - this.viewport[1]) / this.viewport[3];
      var ray0 = Vector.lerp(this.ray00, this.ray10, x);
      var ray1 = Vector.lerp(this.ray01, this.ray11, x);
      return Vector.lerp(ray0, ray1, y).unit();
    }
docs: ### .getRayForPixel(x, y)  Returns the ray originating from the camera and traveling through the pixel `x, y`.