diff --git a/src/gui/imgview.py b/src/gui/imgview.py
--- a/src/gui/imgview.py
+++ b/src/gui/imgview.py
@@ -5,6 +5,7 @@ from PIL import ImageTk
 import config
 from .resizablecanvas import ResizableCanvas
 from corners import Corners
+from epoint import EPoint
 from grid import Grid
 import imageanalyzer
 
@@ -19,7 +20,6 @@ class ImgView(ResizableCanvas):
 
 		self._img=None
 		self._tkImg=None
-		self._imgSizeCoef=1
 
 		self.configure(width=480,height=360)
 		self.bind('<1>',lambda e: self.addCorner(e.x,e.y))
@@ -54,25 +54,17 @@ class ImgView(ResizableCanvas):
 					self.create_rectangle(r1,c1,r2,c2,outline="#00ffff")
 
 	def setImg(self,img):
-		w=int(self._width)
-		h=int(self._height)
-		wo,ho=img.size # o for original
-		widthRatio=wo/w
-		heightRatio=ho/h
-		self._imgSizeCoef=max(widthRatio,heightRatio)
-
 		self._img=img
 
 	## Stores a grid corner located at x,y coordinates.
 	def addCorner(self,x,y):
 		self._corners.add(x,y)
 		log.debug("click on %d,%d",x,y)
-		log.debug("sizeCoef: %f",self._imgSizeCoef)
 		if self._corners.canonizeOrder():
 			# transform corners from show coordinates to real coordinates
 			log.debug(self._corners.corners)
 			self._boardGrid=Grid(self._corners.corners)
-			corners=[(c*self._imgSizeCoef) for c in self._corners.corners]
+			corners=[self._transformPoint(c) for c in self._corners.corners]
 			self._parent.sendMsg("setCorners",(corners,))
 
 		self.redraw()
@@ -86,5 +78,17 @@ class ImgView(ResizableCanvas):
 		w=self._width
 		super()._onResize(event)
 		self._corners.scale(self._width/w)
-		self._boardGrid=Grid(self._corners.corners)
+		if len(self._corners.corners)==4:
+			self._boardGrid=Grid(self._corners.corners)
 		self.redraw()
+
+	def _transformPoint(self,point):
+		w=int(self._width)
+		h=int(self._height)
+		wo,ho=self._img.size # o for original
+		widthRatio=wo/w
+		heightRatio=ho/h
+		self._imgSizeCoef=max(widthRatio,heightRatio)
+		# shift compensates possible horizontal or vertical empty margins from unmatching aspect ratios
+		self._imgShift=EPoint(wo-w*self._imgSizeCoef,ho-h*self._imgSizeCoef)/2
+		return EPoint(self.canvasx(point.x),self.canvasy(point.y)) * self._imgSizeCoef + self._imgShift