I use the Opencv python interface and got the homography matrix H. It seems to work correctly, since I can use the warp perspective to get the distorted image from the original image. Now I tried to use H and Inverse H to convert a point (not an image) back and forth between two coordinates and not getting the expected results.
To get the matrix, I did the following:
pts1 = np.float32(corners)
pts2 = np.float32([[0,0], [400,0], [400,400], [0,400]])
self.transform_matrix = cv2.getPerspectiveTransform(pts1, pts2)
Given this matrix, I use the following to convert back and forth:
def transformPoints(self, x, y, reverse=False, integer=True):
if reverse == False:
H = self.transform_matrix
else:
val, H = cv2.invert(self.transform_matrix)
h0 = H[0,0]
h1 = H[0,1]
h2 = H[0,2]
h3 = H[1,0]
h4 = H[1,1]
h5 = H[1,2]
h6 = H[2,0]
h7 = H[2,1]
h8 = H[2,2]
tx = (h0*x + h1*y + h2)
ty = (h3*x + h4*x + h5)
tz = (h6*x + h7*y + h8)
if integer==True:
px = int(tx/tz)
py = int(ty/tz)
Z = int(1/tz)
else:
px = tx/tz
py = ty/tz
Z = 1/tz
return (px, py)
Now if I do this:
s, t = 100,200
print "s=%d, t=%d" % (s,t)
a, b = pt.transformPoints(s,t)
print "a=%d, b=%d" % (a,b)
c, d = pt.transformPoints(a, b, True)
print "c=%d, d=%d" % (c,d)
This is what it prints: a = 395, b = 169 c = 91, d = 226
I expected c = 100 and d = 200, or at least something close.
This is a matrix and it is inverse. H matrix
[[ -1.01486350e-01 -1.99156329e+01 8.44058060e+02]
[ 1.82486862e+00 3.62765073e-01 -1.49259809e+03]
[ -4.43678849e-03 -4.28012674e-02 1.00000000e+00]]
Inverse:
[[ 4.13378829e-01 1.05495739e-01 -1.91452995e+02]
[ -3.12201095e-02 -2.37099792e-02 -9.03788455e+00]
[ 4.97814178e-04 -5.46754880e-04 -2.36269358e-01]]
, , , ok:
[[ 1.00000000e+00 1.77635684e-15 -5.68434189e-14]
[ -6.93889390e-18 1.00000000e+00 5.32907052e-15]
[ -2.16840434e-19 1.73472348e-18 1.00000000e+00]]
.