Python, OpenCv Array Error

Question

I am trying to draw contours where I see motion in an image using python and cv2. I followed the tutorial here: http://www.steinm.com/blog/motion-detection-webcam-python-opencv-differential-images/ to get the motion part working, but I keep running into different array errors when I try different approaches to getting the contours drawn.

Here is my approach. I find the differential image and then find the contours on that image. Then I draw the contours on the differential image and then show it.

Here is the code:

import cv2
import numpy as np

#Find the differential image
def diffImg(t0, t1,t2):
    d1 = cv2.absdiff(t2, t1)
    d2 = cv2.absdiff(t1, t0)
    d_final = cv2.bitwise_and(d1,d2)
    d_binary = cv2.threshold(d_final, 35, 255,cv2.THRESH_BINARY)[1]
    d_blur =  cv2.blur(d_binary, (15,15))
    return d_blur

#Capture Video from camera
cam = cv2.VideoCapture(0)
s, img = cam.read()

window_name = "Movement Visual"
cv2.namedWindow(window_name, cv2.CV_WINDOW_AUTOSIZE)

#Read the first three images to find the differential image
t_minus = cv2.cvtColor(cam.read()[1], cv2.COLOR_RGB2GRAY)
t = cv2.cvtColor(cam.read()[1], cv2.COLOR_RGB2GRAY)
t_plus = cv2.cvtColor(cam.read()[1], cv2.COLOR_RGB2GRAY)

while s:
    #draw contours
    contours, hierarchy = cv2.findContours(diffImg(t_minus,t,t_plus),cv2.RETR_TREE,cv2.CHAIN_APPROX_SIMPLE)
    final_contour = cv2.drawContours(img,contours,-1,(250,250,250),2)
    cv2.imshow(window_name,final_contour)
    t_minus = t
    t = t_plus
    t_plus = cv2.cvtColor(cam.read()[1],cv2.COLOR_RGB2GRAY)

    key = cv2.waitKey(10)
    if key == 27: 
        cv2.destroyWindow(window_name)
        break
print "Bye"

#cv2.imwrite("image.png", diffImg(t_minus,t,t_plus)

Here is the error I am getting:

 line 28, in <module>
    cv2.imshow(window_name,final_contour)
error: C:\slave\WinInstallerMegaPack\src\opencv\modules\core\src\array.cpp:2482: error: (-206) Unrecognized or unsupported array type

Line 28 is where final_contour is declared in the while loop. I don't understand why I am getting this since it seems like I am essentially just swapping images between functions.

Thanks for any advice.

Did you trace your code!? What was the type of final_contour? — Hadi
– Hadi, Commented Apr 15, 2014 at 11:17
Yeah. I can print out the sizes. I believe contours is coming out as a list, but it fails before I see what final_contour is. — JustBlossom
– JustBlossom, Commented Apr 15, 2014 at 15:34

JustBlossom · Accepted Answer · 2014-04-15 16:47:44Z

Okay so, figured it out with props to @Constantine for reminding me to trace my code. When I printed out what diffImg(t_minus,t,t_plus)and contour and hierarchy, I found that they were an array of zeros, [] and None respectively. So, there was (atleast the way I see it) no image to draw contours on. Thus the error. I changed the code to draw contours on a copy of a color image being read in directly from the camera. So, basically, if I find contours on the diffImg(t_minus,t,t_plus) and then I draw the contours on the image feed from the camera and show it on a new screen. Here is a section of the code to clarify:

while s:
    #draw contours
    contours, hierarchy = cv2.findContours(diffImg(t_minus,t,t_plus),cv2.RETR_TREE,cv2.CHAIN_APPROX_SIMPLE)
    cv2.drawContours(img,contours,-1,(250,250,250),2)
    cv2.imshow('Contours',img)
    cv2.imshow(window_name,diffImg(t_minus,t,t_plus))
    t_minus = t
    t = t_plus
    t_plus = cv2.cvtColor(cam.read()[1],cv2.COLOR_RGB2GRAY)
    ...

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