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Hi everyone,
Does anyone know what rvec and tvec corresponds to? I read the corresponding opencv pages but it just indicates that rvec is the rotation and tvec is the translation vector. But with respect to what?
Best regards..
References:
In tod_detecting (I think we tell about this transformation) after running recognizer->match you get vector of guesses. Each guess contains PoseRT (rvec, tvec) field (aligned_pose). It is transformation from some abstract coordinate system (defined by fiducial.yaml configuration file: in this system coordinates of corners of fiducial markers must be equals described ones in config) to coordinate system of camera for test image. If you want to find projection of this guess to the test image, you should take any observation of this object from training base(recommend to choose observation with ImageId equals imageIndex from guess->imageIndices). Next you should transform 3d points related with the observation to the abstract coord. system using inverse features3d->camera->pose transformation (for example with using Tools::invert and project3dPoints). Then you should transform obtained 3d points with guess->aligned_pose and project them to the test image (for this purpose you should know intrinsics parameters of the camera, now we assume, that test and train images was photographed with the same camera; we use projectPoints function for this purpose).
Let consider a case of solvePnP or solvePnPRansac:
void solvePnP(const Mat& objectPoints, const Mat& imagePoints, const Mat& cameraMatrix, const Mat& distCoeffs, Mat& rvec, Mat& tvec, bool useExtrinsicGuess=false)
The function returns rvec and tvec. This transformation converts objectPoints to the coordinate system of the camera such a way, that re-projection error between imagePoints and projected points after transformation would be a minimal. In other words we try to find rvec-tvec pair to minimize distance between projection (u, v) of transformed point (x,y,z) calculated on (X,Y,Z) point from objectPoints and corresponding point (x, y) from imagePoints. ki and pi are from distCoeffs, fx, fy, cx, cy are from cameraMatrix.
See attached image:
rvec and tvec determine transformation between coordinate systems. rvec uniquely determines rotation matrix 3x3 (let it be named R). Transformation equation (T - transpose): (x y z)T = R*(X Y Z)T + tvec
Rodrigues function from OpenCV can convert rvec to R or R to rvec. Direction of rvec vector is the same with the axis of rotation, magnitude of the vector is angle of rotation.
Also OpenCV contains solvePnPRansac function in calib3d module. If you have incorrect matches you should use it.
As an example in tod_detecting we end up with a candidate object pose inside an ObjectInfo object. The output of object.tvec is as follows, tvec = [-0.0661444212290265; -0.0178236515058925; 0.01091917408303104]. So it means the object coordinate frame is moved -6 CM in x direction, -1 CM in y direction and 1 CM in z direction with respect to what? Where this translation will get me from object origin to WHICH coordinate frame's ORIGIN?
Guess contains rvec and tvec transformed some 3d points from abstract coordinate system (that set by fidycial.yaml config and rigidly attached to fidycial markers) to the coordinate system rigidly attached to test camera. It is not in centimeters. Scale of 3d translation depends on calibration parameters of your device. I don't know what is it in the case of the Kinect.
But in terms of camera coordinate system or chessboard coordinate system?
In terms of camera coordinate system.
a) This is really an OpenCV question not a ROS question and thus answers.ros.org is probably not the best place to ask this.
b) In the links you gave there are descriptions of both of those:
rvec – The output rotation vector (see Rodrigues2) that (together with tvec) brings points from the model coordinate system to the camera coordinate system
tvec – The output translation vector
If you need help in interpreting those descriptions of rvec and tvec you should probably look to the OpenCV community.
Asked: 2011-03-22 01:07:56 -0600
Seen: 26,137 times
Last updated: Mar 24 '11
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