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

Answer 2

Ok, I think I got it.

I tried using the Mat::copyTo, then I got the:

 (scn ==1 && (dcn == 3 || dcn == 4))

error.

Then I found this Stackoveflow topic, which gave me the idea of converting back to RGB, then I tried the following and it worked:

Mat devilROI = img(Rect(r->x+lowerRect.x, 
                        r->y + lowerRect.y, 
                        lowerRect.width, 
                        lowerRect.height));
Mat canny;
cvtColor(devilROI, canny, CV_BGR2GRAY);
Canny(canny, canny, low_threshold, high_threshold);
cvtColor(canny, canny, CV_GRAY2BGR);
addWeighted(devilROI, 1.0, canny, 0.3, 0., devilROI);

So, if anyone has any other suggestion, I would be grateful.

Thank you!