Question

Something appears to be afoul with “getPerspectiveTransform” in the python2 wrapper for opencv. For starters, it takes two arguments instead of the expected three.

So instead of the first wrapper’s:

cv.GetPerspectiveTransform(source,destination,3x3mat)

It’s now:

cv2.getPerspectiveTransform(?,?)

If I try to put in two sets of four quad vert coordinates as the arguments like so:

cv2.getPerspectiveTransform(first_set_of_corners, second_set_of_corners)

it spits out the following error:

cv2.error: C:\slave\WinInstallerMegaPack\src\opencv\modules\imgproc\src\imgwarp.
cpp:3194: error: (-215) src.checkVector(2, CV_32F) == 4 && dst.checkVector(2, CV
_32F) == 4

If the checkVectors shouldn’t equal four (hence the “quad” in quadrangle) then I’m not certain what it wants from me. As with everything else in the python2 wrapper this feature is completely undocumented so I don’t know if it’s broken or, more likely, that I’m doing it wrong. Has anyone here managed to get this to work properly?

Answer 1

cv2.getPerspectiveTransform is not broken anyway.

May be, your points are of not length 4 or they may not be float32 numbers.

Try following :

import cv2
import numpy as np

img = cv2.imread('1original.jpg',0)

src = np.array([[50,50],[450,450],[70,420],[420,70]],np.float32)
dst = np.array([[0,0],[299,299],[0,299],[299,0]],np.float32)

ret = cv2.getPerspectiveTransform(src,dst)
print ret

Result is :

[[  8.36097696e-01  -4.51944700e-02  -3.95451613e+01]
 [ -4.51944700e-02   8.36097696e-01  -3.95451613e+01]
 [  6.45161290e-05   6.45161290e-05   1.00000000e+00]]