findChessboardCorners - fails to detect corners

Attempting to calibrate a stereo rig, I am failing at the very beginning … findChessboardCorners()

The images I am using for calibration are underwater photographs of a chessboard and I made them available at [1]

The code I use to create left/right images from the single frame is:

import os
import cv2
import pathlib
import glob
import numpy as np

imagepath = '/root/notebooks/PHD/STEREO/CALIBRATION/Images2015/'
imgs = list(sorted(glob.glob(f'{imagepath}/*.png')))
pathlib.Path(pathlib.Path(imagepath) / 'left').mkdir(parents=True, exist_ok=True)
pathlib.Path(pathlib.Path(imagepath) / 'right').mkdir(parents=True, exist_ok=True)

for i in imgs:
    image = cv2.imread(i)
    h, w, d = image.shape
    # left = cv2.cvtColor(image[:, 0:int(w/2), :], cv2.COLOR_BGR2RGB)
    # right = cv2.cvtColor(image[:, int(w/2):, :], cv2.COLOR_BGR2RGB)
    left = image[:, 0:int(w/2), :]
    right = image[:, int(w/2):, :]
    cv2.imwrite(str(pathlib.Path( pathlib.Path(pathlib.Path(imagepath) / 'left') / str(pathlib.Path(i).stem+'_left.png'))), left)
    cv2.imwrite(str(pathlib.Path( pathlib.Path(pathlib.Path(imagepath) / 'right') / str(pathlib.Path(i).stem+'_right.png'))), right)

I then attempt the first step of the calibration which should consist of finding the chessboard points with:

left_imgs = list(sorted(glob.glob('/root/notebooks/PHD/STEREO/CALIBRATION/Images2015/left/*.png')))
right_imgs = list(sorted(glob.glob('/root/notebooks/PHD/STEREO/CALIBRATION/Images2015/right/*.png')))
assert len(left_imgs) == len(right_imgs)

criteria = (cv2.TERM_CRITERIA_EPS + cv2.TERM_CRITERIA_MAX_ITER, 30, 1e-3)
left_pts, right_pts = [], []
img_size = None
for left_img_path, right_img_path in zip(left_imgs[1:], right_imgs[1:]):
    left_img = cv2.imread(left_img_path, cv2.IMREAD_GRAYSCALE)
    right_img = cv2.imread(right_img_path, cv2.IMREAD_GRAYSCALE)
    if img_size is None:
        img_size = (left_img.shape[1], left_img.shape[0])

    res_left, corners_left = cv2.findChessboardCorners(left_img, PATTERN_SIZE)
    res_right, corners_right = cv2.findChessboardCorners(right_img, PATTERN_SIZE)
    corners_left = cv2.cornerSubPix(left_img, corners_left, (10, 10), (-1,-1),
    corners_right = cv2.cornerSubPix(right_img, corners_right, (10, 10), (-1,-1),


But that steps fails with the following errro:

error: OpenCV(4.5.1-pre) /src/opencv/modules/imgproc/src/cornersubpix.cpp:58: error: (-215:Assertion failed) count >= 0 in function 'cornerSubPix'

I checked on the web for similar errors and seems the reason is a failure in findChessboardCorners

And that seems to be the cause of it:

for i in range(len(left_imgs)):

    image_left = left_imgs[i]
    image_right = right_imgs[i]

    res_left, corners_left = cv2.findChessboardCorners(cv2.imread(image_left, 
                                                       (6, 5))
    res_right, corners_right = cv2.findChessboardCorners(cv2.imread(image_right, 
                                                         (6, 5))

    print(i, res_left, res_right)

Is returning the following:

0 False True
1 False True
2 False False
3 False True
4 True False
5 False False
6 False False
7 False False
8 False False
9 False False

I double-checked the pattern size (number of inner corners in the chessboard), I also tried to invert the width/height size, as I wasn’t sure about the order … but that didn’t help)

Images used for calibration:
[1] (63.6 MB)

The zip file contains the ‘stereo frames’ as a single image for left+right frame, and two directories (left/right) whit the single images for convenience.

Have you any hints on what I am doing wrong?
Is it possible that my images need some sort of “pre-processing” due to the shade/colors in them? If that could be the case, what do you suggest?

Thanks for any help!

I may have a solution for this first error - I can gett all “True” by setting the CV_CALIB_USE_INTRINSIC_GUESS flag:

    res_left, corners_left = cv2.findChessboardCorners(left_img, PATTERN_SIZE, flags=cv2.CALIB_USE_INTRINSIC_GUESS)
    res_right, corners_right = cv2.findChessboardCorners(right_img, PATTERN_SIZE, flags=cv2.CALIB_USE_INTRINSIC_GUESS)

I can now go ahead with the calibration, finger crossed …