Region of Interest opencv python

Question

I am trying to get a region of an image (ROI) using opencv python. The version of opencv used is 2.4.3. However when I try to call the API

cv2.SetImageROI


        

Answer 1

Here's a visualization for selecting a ROI from an image

-------------------------------------------
|                                         | 
|    (x1, y1)      w                      |
|      ------------------------           |
|      |                      |           |
|      |                      |           | 
|      |         ROI          | h         |  
|      |                      |           |   
|      |                      |           |   
|      |                      |           |       
|      ------------------------           |   
|                           (x2, y2)      |    
|                                         |             
|                                         |             
|                                         |             
-------------------------------------------

Consider (0,0) as the top-left corner of the image with left-to-right as the x-direction and top-to-bottom as the y-direction. If we have (x1,y1) as the top-left and (x2,y2) as the bottom-right vertex of a ROI, we can use Numpy slicing to crop the image with:

ROI = image[y1:y2, x1:x2]

But normally we will not have the bottom-right vertex. In typical cases we will most likely have the ROI's bounding box (x,y,w,h) coordinates obtained from cv2.boundingRect() when iterating through contours

cnts = cv2.findContours(grayscale_image, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
cnts = cnts[0] if len(cnts) == 2 else cnts[1]

for c in cnts:
    x,y,w,h = cv2.boundingRect(c)
    ROI = image[y:y+h, x:x+w]

Since OpenCV v2.2, Numpy arrays are naively used to display images. This Numpy slicing method to extract the ROI may not work with older versions

Answer 2

As mentioned in documentation, and regarding the error message you got, you rather need to import the appropriate module and then call SetImageROI() method:

import cv
cv.SetImageROI(imag, rect)

Answer 3

Okay, On further analysis realized that the cv2 since it has been supporting numpy array structure, there is no longer any need for a API, the entire image can be manipulated in the array itself. eg:

img = cv2.imread('image.png')
img = img[c1:c1+25,r1:r1+25]

Here c1 is the left side column pixel location, and r1 is the corresponding row location. And img now has the image specified within the pixels as the ROI.

EDIT: Very nicely explained here, How to copy a image region using opencv in python?