OpenCV: Confusion about solvePnP

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I have a small confusion regarding the use of the solvePnP function in OpenCV.

I have the matrix for the intrinsic parameters of the camera and I have identified some keypoints in image and I am trying to estimate the extrinsic parameters for the calibration.

The documentation for solvePnP says:

cv2.solvePnP(objectPoints, imagePoints, cameraMatrix, 
distCoeffs[, rvec[, tvec[, useExtrinsicGuess[, flags]]]]) → retval, rvec, tvec

I am guesisng my imagePoints parameters are the keypoints I have detected. and these arte specified in pixels as (x1, y1), (x2, y2), (x3, y3).

I am totally confused about the objectPoints. So, the documentation says:

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<Point3f> can be also passed here.

How can I generate these object points from my image points? What does it mean when they say the object coordinate space in here?

perspectivecamera camera-calibration opencv
1 Answers

The cv2.solvePnP() method is generally used in pose estimation, or in other words, it can be used to estimate the orientation of a 3D object in a 2D image. So for this you need to tag some key-points in the 3D model of the object(objectPoints) and also detect those key-points in the 2D image(imagePoints).

You can consult this answer, to get the application of this technique for face pose estimation.

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