diff --git a/exp/kerokero/prepare_data.py b/exp/kerokero/prepare_data.py
--- a/exp/kerokero/prepare_data.py
+++ b/exp/kerokero/prepare_data.py
@@ -41,7 +41,7 @@ class Sample:
 		img=np.float32(img)/128-1
 		grid=Corners(c.transform(m) for c in self.grid)
 		grid=list(map(lambda p: 2*p/self.SIDE-EPoint(1,1), grid))
-		return (img,grid,list(itertools.chain.from_iterable(grid)))
+		return (img,grid)
 
 	def _getCenter(self):
 		(a,b,c,d)=self.grid
@@ -88,7 +88,7 @@ def harvestDir(path):
 		for b in boards:
 			sample=Sample(img,b.grid)
 			# sample.show()
-			(transformedImg,transformedGrid,label)=sample.transform()
+			(transformedImg,label)=sample.transform()
 			# Sample(np.uint8((transformedImg+1)*128),map(lambda c: (c+EPoint(1,1))*Sample.SIDE/2,transformedGrid)).show()
 			yield (transformedImg,label)