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# openCV perspective transformation doesn't work as expected

Binyamin Even
1#
Binyamin Even Published in 2018-02-14 08:23:45Z
 I have an image with a text. This is the image: What I'm trying to do is to straighten the text, using perspective transformation. The red dots in the corners are the detected boundaries. This is more or less my code (hard-coded, for simplicity): old_pts=np.float32([[2,41],[37,965],[1389,1121],[1389,0]]) bor=cv2.boundingRect(old_pts) #bounding_rect ul=[bor[0], bor[1]] #upper left ur=[bor[0], bor[1]+bor[3]] #upper right br=[bor[0]+bor[2],bor[1]+bor[3]] #bottom right bl=[bor[0]+bor[2],bor[1]] #bottom left new_pts=np.float32([ul,ur,br,bl]) #new pts=[[2,0],[2,1122],[1390,1122],[1390,0]] M = cv2.getPerspectiveTransform(old_pts,new_pts) transformed_img = cv2.warpPerspective(new_img,M,(bor[3],bor[2])) #bor[3] and bor[4] are the bounding rect height&width. transforemed_img=transformed_img.astype(int) cv2.imwrite('transformed.png',transformed_img)  Now, the result I'm getting is this: Why am I not getting a nice, straightened rectangle?? Any help will be appreciated!
api55
2#
 Take a look at your points: You have : old_pts=np.float32([[2,41],[37,965],[1389,1121],[1389,0]])  and: new_pts=np.float32([ul,ur,br,bl]) #new pts=[[2,0],[2,1122],[1390,1122],[1390,0]]  But, OpenCV manages POINTS as (x,y) values, yours are in (y,x)... I know this is confusing, since the matrix manipulation is done with (y,x) notation... The thing is that OpenCV sees the matrix manipulation as the rows and columns like a matrix, but points are seen as Cartesian coordinates... In conclusion, try flipping the axes for the points and check the results.