OpenCV Grey scale Conversion Error

Question

I am trying to open a video source file in OpenCV to be processed. I am following a tutorial given here: http://docs.opencv.org/trunk/doc/py_tutorials/py_video/py_lucas_kanade/py_lucas_kanade.html#lucas-kanade

Each time I attempt to open a .avi video file to be processed I receive the following error:

    Traceback (most recent call last):
  File "C:\Users\Adam\Desktop\LastProjectAI\sources\samples\python2\lines.py", line 22, in <module>
    old_gray = cv2.cvtColor(old_frame, cv2.COLOR_BGR2GRAY)
error: ..\..\..\src\opencv\modules\imgproc\src\color.cpp:3402: error: (-215) scn == 3 || scn == 4

I have read from other posts that such an error is thrown when the source video file is neither 3 or 4 channels. However, I am using videos provided by the OpenCV tutorial so I don't see why they do not work.

Here is the full code I am working with:

import numpy as np
import cv2

cap = cv2.VideoCapture('walking.avi')

# params for ShiTomasi corner detection
feature_params = dict( maxCorners = 100,
                       qualityLevel = 0.3,
                       minDistance = 7,
                       blockSize = 7 )

# Parameters for lucas kanade optical flow
lk_params = dict( winSize  = (15,15),
                  maxLevel = 2,
                  criteria = (cv2.TERM_CRITERIA_EPS | cv2.TERM_CRITERIA_COUNT, 10, 0.03))

# Create some random colors
color = np.random.randint(0,255,(100,3))

# Take first frame and find corners in it
ret, old_frame = cap.read()
old_gray = cv2.cvtColor(old_frame, cv2.COLOR_BGR2GRAY)
p0 = cv2.goodFeaturesToTrack(old_gray, mask = None, **feature_params)

# Create a mask image for drawing purposes
mask = np.zeros_like(old_frame)

while(1):
    ret,frame = cap.read()
    frame_gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)

    # calculate optical flow
    p1, st, err = cv2.calcOpticalFlowPyrLK(old_gray, frame_gray, p0, None, **lk_params)

    # Select good points
    good_new = p1[st==1]
    good_old = p0[st==1]

    # draw the tracks
    for i,(new,old) in enumerate(zip(good_new,good_old)):
        a,b = new.ravel()
        c,d = old.ravel()
        mask = cv2.line(mask, (a,b),(c,d), color[i].tolist(), 2)
        frame = cv2.circle(frame,(a,b),5,color[i].tolist(),-1)
    img = cv2.add(frame,mask)

    cv2.imshow('frame',img)
    k = cv2.waitKey(30) & 0xff
    if k == 27:
        break

    # Now update the previous frame and previous points
    old_gray = frame_gray.copy()
    p0 = good_new.reshape(-1,1,2)

cv2.destroyAllWindows()
cap.release()

The following should be noted:

I am using the Python editor IDLE with windows 8.
The video file which I am trying to load is in the same directly as the Python file.

I have tried multiple video files with no luck. Does anyone have any suggestions to go about fixing this error?

EDIT:

I am able to show the frame directly after it is read from teh video capture object. However, if I try to load the frame after the for loop which draws the track, I get this error:

OpenCV Error: Bad flag (parameter or structure field) (Unrecognized or unsupported array type) in cvGetMat, file /build/buildd/opencv-2.4.5+dfsg/modules/core/src/array.cpp, line 2482
Traceback (most recent call last):
  File "prac.py", line 55, in <module>
    cv2.imshow('frame2', frame)
cv2.error: /build/buildd/opencv-2.4.5+dfsg/modules/core/src/array.cpp:2482: error: (-206) Unrecognized or unsupported array type in function cvGetMat

which opencv version you are using? In opencv 2.x cv2.line() and cv2.circle() returns empty so just remove the "mask =" and "frame=" from your code — soumyadeep debnath

skm skm · Accepted Answer · 2014-03-31T06:48:53

I think that the program is not able to find your video files. Whenever you try to load a video or image file, don't forget to make a if condition to check whether the video/image has been loaded or not.

if frame data is nothing -->> print "frame is empty"

else to be sure that a frame from the video has been loaded successfully, you can try to show the frame immediately once it loaded but don't forget to put a waitKey(0) after the imshow statement

First of all check if the video frames are loaded or not and let us know about it.

OpenCV Grey scale Conversion Error

1 Answers