ORB features [closed]

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I'am using ORB feature detector and extractor to get features from a list of grayscale images. The problem is that I get different features from the same image if I try to detect\extract it more than once. So it is impossible to use them for detection later.

The code:

bmp=BitmapFactory.decodeResource(getResources(),R.drawable.t1);
Utils.bitmapToMat(bmp, mat);
FeatureDetector detector = FeatureDetector.create(FeatureDetector.ORB);
detector.detect(mat, keypoints);
DescriptorExtractor extractor = DescriptorExtractor.create(DescriptorExtractor.ORB);
extractor.compute(mat, keypoints, features);

If I input the same image more than once, features is different each time. Maybe someone have some insight into that?

(Here are few of the images I use: imgur.com/a/IfgcI#0)