diff --git a/src/gui/imgview.py b/src/gui/imgview.py
--- a/src/gui/imgview.py
+++ b/src/gui/imgview.py
@@ -37,8 +37,8 @@ class ImgView(ResizableCanvas):
 			self._tkImg=ImageTk.PhotoImage(img) # just to save the image from garbage collector
 			self.create_image(self._width//2, self._height//2, anchor="center", image=self._tkImg)
 
-		for corner in self._corners.corners:
-			self.markPoint(corner.x,corner.y)
+		for c in self._corners:
+			self.markPoint(c.x,c.y)
 
 		if self._boardGrid!=None and config.gui.showGrid:
 			for r in range(19):
@@ -65,9 +65,8 @@ class ImgView(ResizableCanvas):
 		log.debug("click on %d,%d",x,y)
 		if self._corners.canonizeOrder():
 			# transform corners from show coordinates to real coordinates
-			log.debug(self._corners.corners)
-			self._boardGrid=Grid(self._corners.corners)
-			corners=[self._transformPoint(c) for c in self._corners.corners]
+			self._boardGrid=Grid(self._corners)
+			corners=[self._transformPoint(c) for c in self._corners]
 			self._gui.detector.setCorners(corners)
 			self._gui.preview()
 
@@ -91,8 +90,8 @@ class ImgView(ResizableCanvas):
 		w=self._width
 		super()._onResize(event)
 		self._corners.scale(self._width/w)
-		if len(self._corners.corners)==4:
-			self._boardGrid=Grid(self._corners.corners)
+		if len(self._corners)==4:
+			self._boardGrid=Grid(self._corners)
 		self.redraw()
 
 	def _transformPoint(self,point):