Hi, I'm using SIFT algorithm for comparing the features between two images. I'm curious to know about the second argument of sift.detectAndCompute() method. I'm using none in my script. I know the method's syntax cv2.SIFT.detectAndCompute(image, mask[, descriptors[, useProvidedKeypoints]]) → keypoints, descriptors¶ But I couldnot understand how can I use this "mask"?