# Finding original pixel values from undistorted image (re-distorting)

I am using a stereo camera set-up with a very wide angle (180 degree). I suspect that the camera calibration is not optimal, because I am not getting very accurate results when trying to find the world coordinates of a point using the following method:

`````` points4d = cv2.triangulatePoints( left_proj_mtx, right_proj_mtx , points1u, points2u)
points3d = (points4d[:3, :]/points4d[3, :]).T
``````

where points1u is the centre location of an object in the left image and points2u is the centre location of the same object in the right image. If I have a point on my undistorted image, for example (1000 pixels, 1200 pixels), how I can I easily compute the pixel location of that point in my original image ? I want to remap a list of of points so that I can estimate the bearing to the camera using the original points, since I am unable to extract bearing information using my undistorted images. I do not want to re-distort the whole image.

I don’t think there is a cv::distortPoints function for the normal camera models, but if you are using a fisheye model there appears to be a cv::fisheye::distortPoints() function. I haven’t used it, but maybe this is what you want?

For the regular camera models, a few ideas.

1. You might be able to come up with a way to compute distortion parameters that encode the inverse distortion. If so you could then just call cv::undistortPoints() (on your undistorted points) with this inverse model and get what you want.
2. You could try to use cv::projectPoints (which does 3D → 2D projection based on rvec, tvec, and camera matrix, and then applies the distortion model to get the distorted position) to compute the distortion for you. I think you would just have to use 0 vecs for rvec and tvec, and then assign a Z coordinate that is the same as your focal length.
3. you could borrow the code from the projectPoints function that is related to applying the distortion:
``````        r2 = x*x + y*y;
r4 = r2*r2;
r6 = r4*r2;
a1 = 2*x*y;
a2 = r2 + 2*x*x;
a3 = r2 + 2*y*y;
cdist = 1 + k*r2 + k*r4 + k*r6;
icdist2 = 1./(1 + k*r2 + k*r4 + k*r6);
xd0 = x*cdist*icdist2 + k*a1 + k*a2 + k*r2+k*r4;
yd0 = y*cdist*icdist2 + k*a3 + k*a1 + k*r2+k*r4;
``````

Hi,
My usecase is as follows: Given a rectilinera image, apply a barrel distortion to it.
I have my camera calibrated and have the K and D matrices. I use a mesh grid. Convert the coordinates to homogenous, multiply with K inv and then send them to cv2.distortPoints(). Now the output of this does not seem to be reasonable. How do I use this output get the distorted image?
Thanks