System information (version)

• OpenCV => 4.1.2
• Operating System / Platform => Linux / Ubuntu 18.04
• Compiler => ❔

Detailed description

When running the code below that call solvePnPGeneric from Python the program crashes because the C++ code expect a different type for reprojectionError from the one passed by solvePnPGeneric by default. The argument is however documented as optional. Passing the argument with the correct type avoid the error.

I suspect that the binding of the solvePnPGeneric function do not pass the correct argument type for reprojectionError by default.

The error:

Traceback (most recent call last):
  File "error.py", line 15, in <module>
    obj_points, img_points, cameraMatrix, distCoeffs #, reprojectionError=r
cv2.error: OpenCV(4.1.2) /io/opencv/modules/calib3d/src/solvepnp.cpp:1017: error: (-2:Unspecified error) in function 'int cv::solvePnPGeneric(cv::InputArray, cv::InputArray, cv::InputArray, cv::InputArray, cv::OutputArrayOfArrays, cv::OutputArrayOfArrays, bool, cv::SolvePnPMethod, cv::InputArray, cv::InputArray, cv::OutputArray)'
> Type of reprojectionError must be CV_32FC1 or CV_64FC1!:
>     'type == CV_32FC1 || type == CV_64FC1'
> where
>     'reprojectionError.type()' is 0 (CV_8UC1)

This might be closely linked to #16040 which is about Java binding of solvePnPGeneric

Steps to reproduce

Run this code (OpenCV 4.1.2 / Python 3.6.8)

import numpy as np
import cv2

obj_points = np.array([[0, 0, 0], [0, 1, 0], [1, 1, 0], [1, 0, 0]], dtype=np.float32)
img_points = np.array(
    [[700, 400], [700, 600], [900, 600], [900, 400]], dtype=np.float32
)

cameraMatrix = np.array(
    [[712.0634, 0, 800], [0, 712.540, 500], [0, 0, 1]], dtype=np.float32
)
distCoeffs = np.array([[0, 0, 0, 0]], dtype=np.float32)
#r = np.array([], dtype=np.float32)
x, r, t, e = cv2.solvePnPGeneric(
    obj_points, img_points, cameraMatrix, distCoeffs #, reprojectionError=r
)
print(e)
print(t)

The error can be bypassed by uncommenting the #r = np.array([], dtype=np.float32) lines.

Thanks for the amazing work and happy to help or provide more information!