Question

All,
I have a python script that uses OpenCV to calibrate a camera. It now works with much help from stackoverflow help. But, I’m having problems with the distortion constants. I can not seem to control the number of constants fit. I want only 4 distoriton constants, not 5. In the documentation it states “That is, if the vector contains four elements, it means that K3=0”, that is what I want. So I initiate dist_const = np.zeros(4), pass it to cv2.calibrateCamera, but the returned dist_const has 5 constants returned. I’ve also tried the flag CV_CALIB_FIX_K3, but keep getting a ‘an iteger must be asigned’ error. Has anyone seen this behavior before, any help would be appreciated greatly.

import cv2 
from cv2 import cv 
import numpy as np

obj_points = [[-9.7,3.0,4.5],[-11.1,0.5,3.1],[-8.5,0.9,2.4],[-5.8,4.4,2.7],[-4.8,1.5,0.2],[-6.7,-1.6,-0.4],[-8.7,-3.3,-0.6],[-4.3,-1.2,-2.4],[-12.4,-2.3,0.9],[-14.1,-3.8,-0.6],[-18.9,2.9,2.9],[-14.6,2.3,4.6],[-16.0,0.8,3.0],[-18.9,-0.1,0.3],[-16.3,-1.7,0.5],[-18.6,-2.7,-2.2]]

img_points = [[993.0,623.0],[942.0,705.0],[1023.0,720.0],[1116.0,645.0],[1136.0,764.0],[1071.0,847.0],[1003.0,885.0],[1142.0,887.0],[886.0,816.0],[827.0,883.0],[710.0,636.0],[837.0,621.0],[789.0,688.0],[699.0,759.0],[768.0,800.0],[697.0,873.0]]

obj_points = np.array(obj_points,'float32')
img_points = np.array(img_points,'float32')

w = 1680
h = 1050
size = (w,h)

camera_matrix = np.zeros((3,3),'float32')
camera_matrix[0,0]= 2200.0
camera_matrix[1,1]= 2200.0
camera_matrix[2,2]=1.0

camera_matrix[0,2]=750.0 
camera_matrix[1,2]=750.0

dist_coefs = np.zeros(4,'float32')

retval,camera_matrix,dist_coefs,rvecs,tvecs = cv2.calibrateCamera([obj_points],[img_points],size,camera_matrix,dist_coefs,flags=cv.CV_CALIB_USE_INTRINSIC_GUESS)

print camera_matrix
print dist_coefs

link to my other question, has results for dist_coefs, from the caltech matlab code I’m trying to replace with python/opencv.

john

Answer 1

Change your calibrateCamera call to:

retval,camera_matrix,dist_coefs,rvecs,tvecs = cv2.calibrateCamera([obj_points],[img_points],size,camera_matrix,dist_coefs,flags=cv2.CALIB_USE_INTRINSIC_GUESS + cv2.CALIB_FIX_K3)

You don’t want to me mixing the cv2 and cv namespaces like you did in your example.