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Disparity Map to Point Cloud gone wrong

asked 2020-03-26 13:48:53 -0500

Ratoone gravatar image

Hello everyone! I'm trying to do a face reconstruction using 3 images in Python using OpenCV, but I've run into some issues, namely generating the point cloud from the disparity map. The steps I followed so far:

  • Intrinsic calibration, using checkerboard images cv2.calibrateCamera, with the flag cv2.CALIB_FIX_PRINCIPAL_POINT (although removing the flag didn't seem to change anything)

  • Extrinsic calibration, using cv2.stereoCalibrate, with the flag cv2.CALIB_FIX_INTRINSIC, and cv2.stereoRectify, with the final image size the same as the initial one (for some reason the flag cv2.CALIB_ZERO_DISPARITY didn't seem to change anything as well)

  • Normalization of the new images, and then rectification using cv2.initUndistortRectifyMap, and afterwards cropping the two images using some combination of the ROI parameters (maybe it's worth noting that the sizes of the 2 ROIs are different)

  • cv2.StereoSGBM algorithm to compute the stereo map. I changed the parameters until I got the image shown below

  • cv2.reprojectImageTo3D on the resulting disparity map using the Q matrix from cv2.stereoRectify. I then used the Python open3d library to plot the resulting point cloud.

The results are presented here:

  • the rectified and cropped images
    left right

  • the obtained disparity map (variant)
    disparity

  • the point cloud visualized in open3d (it looks like the disparity from a very specific angle)
    point cloud

I also tried reimplementing the cv2.reprojectImageTo3D without success. Following the advice from here I also converted the disparity map into a float and divided by 16 (which resulted in some better results for the disparity map, however the point cloud still looks the same). Maybe I should mention that for a minDisparity of 0 I get a -16 as the minimum value in the disparity (which I found very weird).

I also tried using cv2.CALIB_USE_INTRINSIC_GUESS in the cv2.stereoCalibrate, but that resulted in worse rectified images (and automatically worse disparity). I also tried following the link above to not crop the images, and use cv2.getValidDisparityROI to crop the disparity map afterwards, but then the resulting images don't really look epipolar, and the resulting disparity map is way worse.

What am I doing wrong? Can you please help me with this?

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The output of the stereo disparity computation is a 2D Mat of 16-bit signed values corresponding to the first input array's (typ. left camera) rectified image. The least significant 4 bits are a fraction, so you are correct to convert the array float and divide by 16.0. The -16 value (-1.0 after division) is the 'not a number' value - it means 'disparity could not be determined at this cell'. The only valid disparities then range from min disparity to min disparity + num disparities.

opalmirror gravatar imageopalmirror ( 2020-03-27 14:22:59 -0500 )edit

You might try googling for 'opencv stereo depth to point cloud example' and reading thru example code to look for inconsistencies with your implementation.

opalmirror gravatar imageopalmirror ( 2020-03-27 14:33:45 -0500 )edit

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answered 2020-03-28 15:16:55 -0500

Ratoone gravatar image

Thank you @opalmirror for your explanation! After a couple more tries and research, I managed to solve the problem by replacing the matrix Q from cv2.stereoRectify with this value:

Q = np.float32([[1, 0, 0, 0],
               [0, -1, 0, 0],
               [0, 0, focal_length * 0.05, 0],
               [0, 0, 0, 1]])

Hope this would help others with a similar problem!

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Asked: 2020-03-26 13:48:53 -0500

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Last updated: Mar 28