diff --git a/exp/board_detect.py b/exp/board_detect.py
--- a/exp/board_detect.py
+++ b/exp/board_detect.py
@@ -13,7 +13,7 @@ import scipy.cluster
 import scipy.ndimage
 import scipy.signal
 
-from polar_hough import PolarHough
+from geometry import Line
 from annotations import DataFile,computeBoundingBox
 from hough import show,prepareEdgeImg,HoughTransform
 from analyzer.epoint import EPoint
@@ -111,8 +111,13 @@ class BoardDetector:
 			cv.circle(stonesImg,(int(point.x),int(point.y)),2,255,-1)
 		# cv.drawContours(stonesImg,[c for (point,c) in stones],-1,255,-1)
 		show(stonesImg,"detected stones")
-		hough.update(stonesImg,5)
-		hough.extract()
+		hough.update(stonesImg,10)
+		lines=hough.extract()
+
+		linesImg=np.copy(rect)
+		for line in lines:
+			self._drawLine(linesImg,line)
+		show(linesImg,"detected lines")
 
 		# # detect vanishing points of the lines
 		# imgCenter=EPoint(w//2-x1, h//2-y1)
@@ -222,6 +227,24 @@ class BoardDetector:
 
 		return matrix
 
+	def _drawLine(self,img,line):
+		(h,w)=img.shape[:2]
+		corners=[EPoint(0,0),EPoint(w,0),EPoint(0,h),EPoint(w,h)] # NW NE SW SE
+		borders=[
+			[Line.fromPoints(corners[0],corners[1]), Line.fromPoints(corners[2],corners[3])], # N S
+			[Line.fromPoints(corners[0],corners[2]), Line.fromPoints(corners[1],corners[3])] # W E
+		]
+
+		(a,b)=(line.intersect(borders[0][0]), line.intersect(borders[0][1]))
+		log.debug("%s %s",line,(a,b))
+		if not a or not b:
+			(a,b)=(line.intersect(borders[1][0]), line.intersect(borders[1][1]))
+			log.debug("* %s %s",line,(a,b))
+		if any(abs(x)>10**5 for x in [*a,*b]):
+			log.debug("ignored")
+			return
+		cv.line(img,(int(a.x),int(a.y)),(int(b.x),int(b.y)),[0,255,0])
+
 
 if __name__=="__main__":
 	detector=BoardDetector(sys.argv[2])