CODEWITHSUNDEEP
opencvcomputer-visionpython-3.4linear-algebra
ymoiseev
ymoiseev

Reputation: 436

opencv: undistort part of the image

I am trying to understand how to apply cv2.undistort function only on a subset of the image.

Camera calibration was done through cv2.findChessboardCorner and it seems to be working fine. I find that undistortion, however, is very slow, averaging around 9 fps on a 1080x1920 image. For the purpose of the project, I am interested only in the fixed subset of image, usually something like img[100:400].

What is the good way to approach this problem? It seems to be wasteful to undistort entire image only when stripe of 100 pixels is needed.

Upvotes: 3

Views: 2065

Answers (1)

Francesco Callari
Francesco Callari

Reputation: 11785

From the docs:

The function is simply a combination of cv::initUndistortRectifyMap (with unity R ) and cv::remap (with bilinear interpolation). See the former function for details of the transformation being performed.

So by calling undistort in a loop you are recomputing the un-distortion maps over and over - there is no caching, and their computation is expensive, since it involves solving a polynomial equation for every pixel. IIUC your calibration is fixed, so you should compute them only once using initUndistortRectifyMap(), and then pass the maps to remap() in your main loop.

The kind of "cropping" you describe is doable, but it may require some work and little experimentation because the undistortion maps used by OpenCV are in 1:1 correspondence with the un-distorted image, and store pixel (vector) offsets. This means that you need to crop out the maps portions corresponding to the rectangle of the image you care about, and then edit their values, which are x and y offsets from the un-distorted to the distorted image. Specifically, you need to apply to them the inverse of the 2D translation that brings that rectangle you care about to the top-left corner of the image canvas. Then you should be able to call remap() passing as destination image just an empty image the size of the undistorted cropped rectangle.

All in all, I'd first try the first recommendation (don't call undistort, separate map generation from remapping), and only try the second step if really can't keep up with the frame rate.

Upvotes: 2

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