imread returns None, violating assertion !_src.empty() in function 'cvtColor' error

I am trying to do a basic colour conversion in python however I can't seem to get past the below error. I have re-installed python, opencv and tried on both python 3.4.3 (latest) and python 2.7 (which is on my Mac).

I installed opencv using python's package manager opencv-python.

Here is the code that fails:

frame = cv2.imread('frames/frame%d.tiff' % count)
frame_HSV= cv2.cvtColor(frame,cv2.COLOR_RGB2HSV)

This is the error message:

cv2.error: OpenCV(3.4.3) /Users/travis/build/skvark/opencv-python/opencv/modules/imgproc/src/color.cpp:181: error: (-215:Assertion failed) !_src.empty() in function 'cvtColor'

Solution 1:

This error happened because the image didn't load properly. So you have a problem with the previous line cv2.imread. My suggestion is :

• check if the image exists in the path you give

• check if the count variable has a valid number

Solution 2:

If anyone is experiencing this same problem when reading a frame from a webcam:

Verify if your webcam is being used on another task and close it. This wil solve the problem.

I spent some time with this error when I realized my camera was online in a google hangouts group. Also, Make sure your webcam drivers are up to date

Solution 3:

I kept getting this error too:

Traceback (most recent call last):
  File "face_detector.py", line 6, in <module>
    gray_img=cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
cv2.error: OpenCV(4.1.0) C:\projects\opencv-python\opencv\modules\imgproc\src\color.cpp:182: error: (-215:Assertion failed) !_src.empty() in function 'cv::cvtColor

My cv2.cvtColor(...) was working fine with \photo.jpg but not with \news.jpg. For me, I finally realized that when working on Windows with python, those escape characters will get you every time!! So my "bad" photo was being escaped because of the file name beginning with "n". Python took the \n as an escape character and OpenCV couldn't find the file!

Solution:
Preface file names in Windows python with r"...\...\" as in

cv2.imread(r".\images\news.jpg")