OpenCV - Sizes of input arguments do not match - addWeighted

Question

I am trying to apply the Canny operator in a certain location of an image with the following code:

//region of interest from my RGB image
Mat devilROI = img(         


        

Answer 1

Easy. You do not have the same number of channels in the 2 images to merge.

cvtColor(devilROI, canny, CV_RGB2GRAY);

Is taking your 3 channel image and turning it into a 1 channel greyscale image. You need the same number of channels to use addWeighted