diff --git a/src/gui/imgview.py b/src/gui/imgview.py
--- a/src/gui/imgview.py
+++ b/src/gui/imgview.py
@@ -13,10 +13,10 @@ log=logging.getLogger(__name__)
 
 
 class ImgView(ResizableCanvas):
-	def __init__(self,master=None,parent=None):
+	def __init__(self,gui,master=None):
 		super().__init__(master)
 
-		self._parent=parent
+		self._gui=gui
 		self._corners=Corners()
 		self._boardGrid=None
 
@@ -31,7 +31,9 @@ class ImgView(ResizableCanvas):
 		self.delete("all")
 
 		if self._img:
-			img=self._img.resize((int(self._width),int(self._height)))
+			w,h=self._img.size
+			ratio=min(self._width/w, self._height/h)
+			img=self._img.resize((int(w*ratio),int(h*ratio)))
 			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)
 
@@ -66,7 +68,8 @@ class ImgView(ResizableCanvas):
 			log.debug(self._corners.corners)
 			self._boardGrid=Grid(self._corners.corners)
 			corners=[self._transformPoint(c) for c in self._corners.corners]
-			self._parent.detector.setCorners(corners)
+			self._gui.detector.setCorners(corners)
+			self._gui.preview()
 
 		self.redraw()
 
@@ -96,9 +99,10 @@ class ImgView(ResizableCanvas):
 		w=int(self._width)
 		h=int(self._height)
 		wo,ho=self._img.size # o for original
+		log.debug("image: %sx%s, view: %sx%s",wo,ho,w,h)
 		widthRatio=wo/w
 		heightRatio=ho/h
-		self._imgSizeCoef=max(widthRatio,heightRatio)
+		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
+		imgShift=EPoint(wo-w*imgSizeCoef,ho-h*imgSizeCoef)/2
+		return EPoint(self.canvasx(point.x),self.canvasy(point.y)) * imgSizeCoef + imgShift