diff --git a/src/analyzer/corners.py b/src/analyzer/corners.py
--- a/src/analyzer/corners.py
+++ b/src/analyzer/corners.py
@@ -8,6 +8,8 @@ log=logging.getLogger(__name__)
 class Corners:
 	def __init__(self,cornerList=[]):
 		self._corners=cornerList[:]
+		self._is_canon=False
+		self._canonizeOrder()
 
 	## Adds a new corner if there are less than four, replaces the closest otherwise.
 	def add(self,x,y):
@@ -29,6 +31,7 @@ class Corners:
 				index,minDist=i,a.dist(c)
 
 		self._corners[index]=a
+		self._canonizeOrder()
 
 	## Order the corners (0,1,2,3) so they make a quadrangle with vertices KLMN in counter-clockwise order, K being in the upper left.
 	#
@@ -51,8 +54,8 @@ class Corners:
 	#  From these pick the one with the lowest slope (dy/dx) and declare its ending point the upper left corner. For the same slope pick the one further left.
 	#
 	#  @return True for a convex quadrangle, False for concave and degenerate cases.
-	def canonizeOrder(self):
-		if len(self._corners)!=4: return False # erroneus call
+	def _canonizeOrder(self):
+		if len(self._corners)!=4: self._is_canon=False
 
 		a,b,c,d=self._corners
 		abc=doubleTriangleArea(a,b,c)
@@ -60,7 +63,7 @@ class Corners:
 		acd=doubleTriangleArea(a,c,d)
 		bcd=doubleTriangleArea(b,c,d)
 
-		if any(x==0 for x in (abc,abd,acd,bcd)): return False # collinear degenerate
+		if any(x==0 for x in (abc,abd,acd,bcd)): self._is_canon=False # collinear degenerate
 
 		swaps=[(1,3),(0,1),(1,2),(0,3),(2,3),(0,0)]
 		index=(8 if abc>0 else 0)|(4 if abd>0 else 0)|(2 if acd>0 else 0)|(1 if bcd>0 else 0)
@@ -82,7 +85,7 @@ class Corners:
 		self._corners= self._corners[kIndex:] + self._corners[:kIndex] # rotate the upper left corner to the first place
 
 		log.debug(self._corners)
-		return True # success
+		self._is_canon=True # success
 
 	def scale(self,scale):
 		self._corners=[c * scale for c in self._corners]
@@ -93,6 +96,9 @@ class Corners:
 	def __len__(self):
 		return len(self._corners)
 
+	def is_canon(self):
+		return self._is_canon
+
 
 ## Computes twice the area of the triangle formed by points a,b,c.
 #