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// 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;
  }
}