OpenCV::solvePNP() - Assertion failed

Question

I am trying to get the pose of the camera with the help of solvePNP() from OpenCV.

After running my program I get the following errors:

OpenCV Error:         


        

Answer 1

The types don't seem right, at least in the code that worked for me I used different types(as mentioned in the documentation).

objectPoints – Array of object points in the object coordinate space, 3xN/Nx3 1-channel or 1xN/Nx1 3-channel, where N is the number of points. vector can be also passed here.

imagePoints – Array of corresponding image points, 2xN/Nx2 1-channel or 1xN/Nx1 2-channel, where N is the number of points. vector can be also passed here.

cameraMatrix – Input camera matrix A = \vecthreethree{fx}{0}{cx}{0}{fy}{cy}{0}{0}{1} .

distCoeffs – Input vector of distortion coefficients (k_1, k_2, p_1, p_2[, k_3[, k_4, k_5, k_6]]) of 4, 5, or 8 elements. If the vector is NULL/empty, the zero distortion coefficients are assumed.

rvec – Output rotation vector (see Rodrigues() ) that, together with tvec , brings points from the model coordinate system to the camera coordinate system.

tvec – Output translation vector.

useExtrinsicGuess – If true (1), the function uses the provided rvec and tvec values as initial approximations of the rotation and translation vectors, respectively, and further optimizes them.

Documentation from here.

vector<Mat> rvec, tvec should be Mat rvec, tvec instead.

vector<vector<Point2f> > imagePoints(1) should be vector<Point2f> imagePoints(1) instead.

vector<vector<Point3f> > boardPoints(1) should be vector<Point3f> boardPoints(1) instead.

Note: I encountered the exact same problem, and this worked for me(It is a little bit confusing since calibrateCamera use vectors). Haven't tried it for imagePoints or boardPoints though.(but as it is documented in the link above, vector,vector should work, I thought I'd better mention it), but for rvec,trec I tried it myself.

Answer 2

I run in exactly the same problem with solvePnP and opencv3. I tried to isolate the problem in a single test case. I seams passing a std::vector to cv::InputArray does not what is expected. The following small test works with opencv 2.4.9 but not with 3.2.

And this is exactly the problem when passing a std::vector of points to solvePnP and causes the assert at line 63 in solvepnp.cpp to fail !

Generating a cv::mat out of the vector list before passing to solvePnP works.

//create list with 3 points
std::vector<cv::Point3f> vectorList;
vectorList.push_back(cv::Point3f(1.0, 1.0, 1.0));
vectorList.push_back(cv::Point3f(1.0, 1.0, 1.0));
vectorList.push_back(cv::Point3f(1.0, 1.0, 1.0));

//to input array
cv::InputArray inputArray(vectorList);
cv::Mat mat = inputArray.getMat();
cv::Mat matDirect = cv::Mat(vectorList);

LOG_INFO("Size vector: %d mat: %d matDirect: %d", vectorList.size(), mat.checkVector(3, CV_32F), matDirect.checkVector(3, CV_32F));

QVERIFY(vectorList.size() == mat.checkVector(3, CV_32F));

Result opencv 2.4.9 macos:

TestObject: OpenCV
Size vector: 3 mat: 3 matDirect: 3

Result opencv 3.2 win64:

TestObject: OpenCV
Size vector: 3 mat: 9740 matDirect: 3

Answer 3

Step into the function in a debugger and see exactly which assertion is failing. ( Probably it requires values in double (CV_64F) rather than float. )

OpenCVs new "inputarray" wrapper issuppsoed to allow you to call functions with any shape of mat, vector of points, etc - and it will sort it out. But a lot of functions assume a particular inut format or have obsolete assertions enforcing a particular format.

The stereo/calibration systems are the worst for requiring a specific layout, and frequently succesive operations require a different layout.