diff --git a/exp/hough.py b/exp/hough.py
--- a/exp/hough.py
+++ b/exp/hough.py
@@ -34,6 +34,10 @@ class LineBag:
 
 
 class HoughTransform:
+	"""Find line sequences with Hough transform.
+
+	Uses usual image coordinates on input and output, with [0,0] in the upper left corner and [height-1,width-1] in the lower right.
+	However, internally it uses the usual cartesian coordinates, centered at the image center. [-w/2,-h/2] in the upper left and [w/2,h/2] in the lower right."""
 	def __init__(self,img):
 		self._angleBandwidth=30 # degrees
 
@@ -58,10 +62,8 @@ class HoughTransform:
 			keys=self._readLineKeys(alpha,beta)
 			for k in peaks:
 				(alphaDeg,d)=keys[k]
-				alphaRad=alphaDeg*math.pi/180
-				baseLine=Line(alphaRad,0)
-				dd=baseLine.distanceTo(EPoint(*self._center)) # to shift d from the center to 0,0
-				res.append(Line(alphaRad, dd+d-self._diagLen//2))
+				line=Line(alphaDeg*math.pi/180,d-self._diagLen//2)
+				res.append(self._transformOutput(line))
 			i+=1
 
 		self.show(img)
@@ -77,17 +79,21 @@ class HoughTransform:
 					self._acc[(alphaDeg,d)]+=weight
 		log.debug("Hough updated in %s s",round(datetime.now().timestamp()-start,3))
 
+	def show(self,img=None):
+		if img is None: img=self._createImg()
+		show(img,"Hough transform accumulator")
+
 	def _computeDist(self,x,y,alphaDeg):
 		alphaRad=alphaDeg*math.pi/180
 		(x0,y0)=self._center
-		(dx,dy)=(x-x0,y-y0)
+		(dx,dy)=(x-x0,y0-y)
 		d=dx*math.cos(alphaRad)+dy*math.sin(alphaRad)
 		return round(d)
 
 	def _detectLines(self):
 		bag=LineBag()
-		for alpha in range(0,180,2):
-			for beta in range(max(alpha-60,0),alpha+60,2):
+		for alpha in range(0,180+60,2):
+			for beta in range(max(alpha-60,0),min(alpha+60,180+60),2):
 				accLine=[self._acc[key] for key in self._readLineKeys(alpha,beta)]
 				(peaks,props)=scipy.signal.find_peaks(accLine,prominence=0)
 				(prominences,peaks)=zip(*sorted(zip(props["prominences"],peaks),reverse=True)[:19])
@@ -99,16 +105,19 @@ class HoughTransform:
 		res=[]
 		for i in range(n+1):
 			k=round((alpha*(n-i)+beta*i)/n)
-			if k<0 or k>=180:
+			if k>=180:
 				k=k%180
-				i=n+1-i
+				i=n-i
 			res.append((k,i))
 		return res
 
-	def show(self,img=None):
-		if img is None: img=self._createImg()
-
-		show(img,"Hough transform accumulator")
+	def _transformOutput(self,line):
+		(x,y)=self._center
+		basis=EPoint(-x,y)
+		shiftedLine=line.shiftBasis(basis)
+		reflectedLine=Line(math.pi*2-shiftedLine.alpha,shiftedLine.d)
+		log.debug("%s -> %s",line,reflectedLine)
+		return reflectedLine
 
 	def _createImg(self):
 		maxVal=self._acc.max()