How to find mean value for a given part of an Image

Question

I have "n" number of contour detected images(frame). I wants to find mean value for the rectangle portion of that image.
(Instead of finding mean value for a full image, i need to calculate the mean value for the rectangle portion of that image.)


I have rectangle's x,y position and width, height values. First Image x,y,w,h is 109,45 ,171,139 and second image x,y,w,h is 107,71,175,110.

I get the values using the below code.
cv2.rectangle(frame, (x,y),(x+w,y+h), (0,0,255), 3)
I know using "ROI" concept we can do mean calculation.
So, I referred some links.
Ex. Get the ROI of two binary images and find difference of the mean image intesities between 2 ROI in python.

But, I have confused with the parameter settings. Can anyone help me to resolve my problem ? Thanks in advance...

Answer 1

There's easier way to get rectangle from an image in Python. Since cv2 operates on NumPy arrays, you can use normal slicing (note, that i corresponds to y and j - to x, not the other way):

rect = image[i:i+h, j:j+w]

And taking mean is even simpler:

rect.mean()