Python Pose Estimation Example Error

Question

I'm beginner at opencv and I'm trying to run tutorial codes at: http://docs.opencv.org/trunk/doc/py_tutorials/py_calib3d/py_table_of_contents_calib3d/py_table_of_contents_calib3d.html

My code and problem:

import scipy.io
import numpy as np
import cv2
import time

# data comes from calibration program same as tutorial
data=scipy.io.loadmat('distm.mat')
dist=data['dist000']
mtx=data['mtx']
data=None

#takes the corners in the chessboard  
def draw(img, corners, imgpts):
    corner = tuple(corners[0].ravel())
    img = cv2.line(img, corner, tuple(imgpts[0].ravel()), (255,0,0), 5)
    img = cv2.line(img, corner, tuple(imgpts[1].ravel()), (0,255,0), 5)
    img = cv2.line(img, corner, tuple(imgpts[2].ravel()), (0,0,255), 5)
    return img

criteria = (cv2.TERM_CRITERIA_EPS + cv2.TERM_CRITERIA_MAX_ITER, 30, 0.001)
objp = np.zeros((6*7,3), np.float32)
objp[:,:2] = np.mgrid[0:7,0:6].T.reshape(-1,2)

axis = np.float32([[3,0,0], [0,3,0], [0,0,-3]]).reshape(-1,3)

# cam open and wait a second to deny black or dark images                                                                              
cap = cv2.VideoCapture(2)
ret, frame = cap.read()
print "Checking camera read:"+str(ret)
while ret ==False:
    cap = cv2.VideoCapture(2)
    ret, frame = cap.read()
time.sleep(1)

# takes live photos from camera
for i in range(0,20):
    print i

    ret, img = cap.read()
    gray = cv2.cvtColor(img,cv2.COLOR_BGR2GRAY)
    ret, corners = cv2.findChessboardCorners(gray, (7,6),None)
    print ret

    if ret == True:

        corners2 = cv2.cornerSubPix(gray,corners,(11,11),(-1,-1),criteria)
        # 'corners2' returns "none" and below codes not working so I use 'corners'
        print corners2
        # Find the rotation and translation vectors.
        rvecs, tvecs, inliers = cv2.solvePnPRansac(objp, corners, mtx, dist)

        # project 3D points to image plane
        imgpts, jac = cv2.projectPoints(axis, rvecs, tvecs, mtx, dist)

        img = draw(img,corners,imgpts)  # img returns None
        print img                       
        cv2.imshow('pose',img)           # error line 
        k = cv2.waitKey(0) & 0xff

and gives error at "cv2.imshow('pose',img)" as:

OpenCV Error: Assertion failed (size.width>0 && size.height>0) in
cv::imshow, file ..\..\..\modules\highgui\src\window.cpp, line 269
Traceback (most recent call last):   File "<stdin>", line 1, in
<module>   File
"C:\Python27\lib\site-packages\spyderlib\widgets\externalshell\sitecustomize.py",
line 540, in runfile
    execfile(filename, namespace)   File "C:/Users/ACS/Documents/Python/Pose Estimation.py", line 49, in
<module>
    cv2.imshow('pose',img) cv2.error: ..\..\..\modules\highgui\src\window.cpp:269: error: (-215)
size.width>0 && size.height>0 in function cv::imshow

Thank you for your attention.

Answer 1

Thanks for Arnaud's interest.

I fix the problem. Working code:

import scipy.io
import numpy as np
import cv2
import time

data=scipy.io.loadmat('distm.mat')
dist=data['dist000']
mtx=data['mtx']
data=None

#takes the corners in the chessboard 
def draw(img, corners, imgpts):
    corner = tuple(corners[0].ravel())
    cv2.line(img, corner, tuple(imgpts[0].ravel()), (255,0,0), 5)
    cv2.line(img, corner, tuple(imgpts[1].ravel()), (0,255,0), 5)
    cv2.line(img, corner, tuple(imgpts[2].ravel()), (0,0,255), 5)
    return img

criteria = (cv2.TERM_CRITERIA_EPS + cv2.TERM_CRITERIA_MAX_ITER, 30, 0.001)
objp = np.zeros((6*7,3), np.float32)
objp[:,:2] = np.mgrid[0:7,0:6].T.reshape(-1,2)

axis = np.float32([[3,0,0], [0,3,0], [0,0,-3]]).reshape(-1,3)

cap = cv2.VideoCapture(2)
ret, frame = cap.read()
print "Checking camera read:"+str(ret)
while ret ==False:
    cap = cv2.VideoCapture(2)
    ret, frame = cap.read()
time.sleep(1)

for i in range(0,20):
    time.sleep(1)
    ret, img = cap.read()
    gray = cv2.cvtColor(img,cv2.COLOR_BGR2GRAY)
    ret, corners = cv2.findChessboardCorners(gray, (7,6),None)
    print str(i)+"'th image check board corners "+str(ret)
    if ret :
        corners2 = cv2.cornerSubPix(gray,corners,(11,11),(-1,-1),criteria)
        print corners2
        # Find the rotation and translation vectors.
        rvecs, tvecs, inliers = cv2.solvePnPRansac(objp, corners, mtx, dist)

        # project 3D points to image plane
        imgpts, jac = cv2.projectPoints(axis, rvecs, tvecs, mtx, dist)

        img = draw(img,corners,imgpts)
        cv2.imshow('pose',img)
        k = cv2.waitKey(0) & 0xff

Answer 2

One thing that may cause trouble is the fact that the success of cap.read() is not checked : the variable ret is re-assigned to a new value without having been checked the first time.

Surrounding the code below the cap.read() with another if ret: may help. You will notice there's no need for == True in Python