// call clearCache when changing parameters

sclass OCRCompass implements Renderable {

  // compass can look for dark or bright lines, set this flag accordingly:

  bool brightForeground;

  IBWIntegralImage image;

  Pt center; // point we're looking at

  double radius = 16; // radius of circle

  int sensorFootprint = 0; // size of each "pixel" (rectangle) to grab - defaults to radius

  int steps = 12; // how many segments of circle (=steps/2 line orientations). should be even

  int blur; // blur width

  double[] values; // brightnesses at each point (low=foreground)

  L<Rect> sensors;

  L<Rect> sensors() {

    if (sensors == null)

      sensors = squaresAroundCircle(center, steps, radius, sensorFootprint());

    ret sensors;

  }

  int angles() { ret steps/2; }

  int sensorFootprint() {

    ret sensorFootprint == 0 ? iceil(radius*12/max(steps, 12)) : sensorFootprint;

  }

  void compute {

    L<Rect> sensors = sensors();

    if (l(values) != angles()) values = new double[angles()];

    for i over values: values[i] = avg(

      image.getPixelAverage(sensors.get(i)),

      image.getPixelAverage(sensors.get(i+angles())));

  }

  void ensureComputed { if (values == null) compute(); }

  double mostLikelyAngle() { ensureComputed();

    ret indexOfMinOrMaxEntryInDoubleArray(brightForeground, values)*twoPi()/steps;

  }

  public void renderOn(Graphics2D g, int w, int h) { ensureComputed();

    renderRenderables(g, w, h, zipAndMap(sensors(), repeatList(2, asList(values)),

      (r, value) -> BoxWithColor(r, 0, 0, iround(value))));

    drawRotatedRoundEdgeLine(g, center, mostLikelyAngle(), radius*1.5, Color.green, radius/3f);

  }

  void clearCache {

    sensors = null;

  }

}