opencv cvtColor assertion failed C++

Question

I am getting an assertion error when I try to convert an input image to grayscale. I have seen lots of posts around the same issue but none seem to give a reason why it is erroring. In the code below, input is NOT NULL and has been loaded from an image "test.jpg" on my drive using imgRead.

Mat img_gray;
cvtColor(input, img_gray, CV_BGR2GRAY);

Assertion Error:

OpenCV Automatic Number Plate Recognition working with file: test OpenCV Error: Assertion failed (scn == 3 || scn == 4) in cvtColor, file /opt/local/var/macports/build/_opt_mports_dports_graphics_opencv/opencv/work/opencv-2.4.9/modules/imgproc/src/color.cpp, line 3737 libc++abi.dylib: terminate called throwing an exception

The code is taken from https://github.com/MasteringOpenCV/code/blob/master/Chapter5_NumberPlateRecognition/DetectRegions.cpp

check out line 72.

I have tried multiple pictures with no luck. Any suggestions would be appreciated. Thanks

Answer 1

That assertion message means that the image you passed in is not 3 or 4 channel. Eg if you pass in a one channel grayscale image it will fail.

try putting in

std::cerr << "image chans : " << image.channels() << std::endl;

immediately before cvtColor() . I am sure that it won't output 3 or 4.