Unable to resolve simple rigid translations

[wiersmalab]

I notice that if I translate IX by:

rows,cols = IX.shape[:-1]
M = np.float32([[1,0,100],[0,1,50]])
IY = cv2.warpAffine(IX,M,(cols,rows))

the registered image output is highly warped. One would think that it would detect a simple rigid body translation in this case...? Is there a way to force only rigid transformations?