Fill the center of ROI

How can I fill ROI, so that the image is placed in the centre of the ROI?

The ROI is found with (x, y, w, h) = cv2.boundingRect(contour) and I insert the input image with

                       image = cv2.resize(input_image, (w, h))
                       img2gray = cv2.cvtColor(image, cv2.COLOR_BGR2GRAY)
                       _, image_mask = cv2.threshold(img2gray, 1, 255, cv2.THRESH_BINARY)
                       roi = frame[y:y+h, x:x+w]
                       roi[np.where(image_mask)] = 0
                       roi += image

The h and w are the dimensions of my input image.

How can I add the offsets so that the ROI += image will result like shown in the image?

As per my understanding, you want to place the resized image within the center of the ROI.

You have already obtained the ROI roi = frame[y:y+h, x:x+w].

# using this as background, hence making it white
roi[:] = (255, 255, 255)

# Obtain the spatial dimensions of ROI and image
roi_h, roi_w = roi.shape[:2]
image_h, image_w = image.shape[:2]

# Calculate height and width offsets
height_off = int((roi_h - image_h)/2)
width_off = int((roi_w - image_w)/2)

# With Numpy slicing, place the image in the center of the ROI
roi[height_off:height_off+image_h, width_off:width_off+image_w] = image

I hope this gives you an idea on how to proceed further