CODEWITHSUNDEEP
pythonopencvnumpy
user2039916
user2039916

Reputation: 21

Python openCV 2.4.3 cv2.SolvePnP error

I am new to Python, OpenCV, and Numpy. I have been attempting to implement a python version of C++ code which determines the relative pose of a camera to a known marker. The code uses the cv2.SolvePnP function, which I have been at wits end to get to run. I have searched the web, but find nothing but similar confusion for this function. It seems that no matter what form I pass my data, the function is unhappy. The test case I've been using is: 

## target image points

tPoints = np.zeros((4,2),dtype=np.float64)
tPoints[0,0] = 384.3331
tPoints[0,1] = 162.23618
tPoints[1,0] = 385.27521
tPoints[1,1] = 135.21503
tPoints[2,0] = 409.36746
tPoints[2,1] = 165.64435

## actual marker point set

mPoints = np.zeros((4,3),dtype=np.float64)
mPoints[0,0] = -88.0
mPoints[0,1] = 88.0
mPoints[0,2] = 0
mPoints[1,0] = -88.0
mPoints[1,1] = -88.0
mPoints[1,2] = 0
mPoints[2,0] = 88.0
mPoints[2,1] = -88.0
mPoints[2,2] = 0
mPoints[3,0] = 88.0
mPoints[3,1] = 88.0
mPoints[3,2] = 0

camMatrix = np.zeros((3,3),dtype=np.float64 )
camMatrix[0][0] = 519.0
camMatrix[0][2] = 320.0
camMatrix[1][1] = 522.0
camMatrix[1][2] = 240.0
camMatrix[2][2] = 1.0

retval, rvec, tvec = cv2.solvePnP(objectPoints = tPoints, imagePoints = mPoints, cameraMatrix = camMatrix, distCoeffs = None)

The error returned is:

cv2.error: C:\slave\WinInstallerMegaPack\src\opencv\modules\calib3d\src\solvepnp.cpp:52: >error: (-215) npoints >= 0 && npoints == std::max(ipoints.checkVector(2, CV_32F), >ipoints.checkVector(2, CV_64F))

Help getting this to execute, as well as information as to where I am going wrong will be greatly appreciated. Lots still to learn!

Upvotes: 2

Views: 9436

Answers (2)

Ben
Ben

Reputation: 993

If you try to slice an array as input this will also cause problems. Taken from here

World = array([[-0.5, -0.5,  3. ],
               [ 0.5, -0.5,  3. ],
               [ 0.5,  0.5,  3. ],
               [-0.5,  0. ,  3. ]])
keyPoints = array([[ 279.03286469,  139.80463604,    1.        ],
                     [ 465.40665724,  136.70519839,    1.        ],
                     [ 465.40665724,  325.1505936 ,    1.        ],
                     [ 279.03286469,  230.927896  ,    1.        ]])

objectPoints = World
imagePoints = keyPoints[:,:2] # <--- THIS SLICE IS A PROBLEM CAUSER!!!
cv2.solvePnP(objectPoints, imagePoints, np.eye(3), np.zeros(5))

Upvotes: 3

Warren Weckesser
Warren Weckesser

Reputation: 114781

The shapes of the arguments that you've given to solvePnP are not correct. objectPoints should be Nx3, and imagePoints should be Nx2. If I switch how tPoints and mPoints are assigned to these arguments in your example, it does not raise the exception.

Upvotes: 4

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