@Override
	public Set<int[]> threshold(int thresh) throws IllegalStateException {
		if(!isDone) throw new IllegalStateException("accumulate() must be called before threshold(int)");
		Set<int[]> circles = new HashSet<int[]>();
		for(int r = 0; r < acc.length; ++r) {
			for(int x = 0; x < acc[r].length; ++x) {
				for(int y = 0; y < acc[r][x].length; ++y) {
					if(acc[r][x][y] >= thresh) circles.add(new int[]{r +minR, x, y});
				}
			}
		}
		return circles;
	}