OpenCV via python: Is there a fast way to zero pixels outside a set of rectangles?

Question

I have an image of a face and I have used haar cascades to detect the locations (x,y,width,height) of the mouth, nose and each eye. I would like to set all pixels outside these regions to zero. What would be the fastest (computationally) way to do this? I'll eventually be doing it to video frames in real time.

Answer 1

I don't know whether it is the fastest way, but It is a way to do it.

Create a mask image with region of face as white, then apply bitwise_and function with original image and mask image.

x = y = 30
w = h = 100

mask = np.zeros(img.shape[:2],np.uint8)
mask[y:y+h,x:x+w] = 255
res = cv2.bitwise_and(img,img,mask = mask)

It takes 0.16 ms in my system (core i5,4GB RAM) for an image of size 400x300

EDIT - BETTER METHOD: You need not do as above. Simply create a zero image and then copy ROI from original image to zero image. that's all.

mask = np.zeros(img.shape,np.uint8)
mask[y:y+h,x:x+w] = img[y:y+h,x:x+w]

It takes only 0.032 ms in my system for above parameters, 5 times faster than above.

Results :

Input Image :

Output :

Answer 2

If a polygon ROI is to be made. Create the polygon and make a mask for it. Multiply the image with the created frame.

    ret,frame = cv2.imread()
    xr=1
    yr=1
    # y,x
    pts = np.array([[int(112*yr),int(32*xr)],[int(0*yr),int(623*xr)],[int(789*yr),int(628*xr)],[int(381*yr),int(4*xr)]], np.int32)
    pts = pts.reshape((-1,1,2))
    cv2.polylines(frame,[pts],True,(0,255,255))

    mask = np.zeros(frame.shape[:2],np.uint8)
    cv2.fillPoly(mask,[pts],(255,255,255))
    frame = cv2.bitwise_and(frame,frame,mask = mask)
    cv2.imshow("masked frame", frame)