OpenCV for secret agents - cat recogntion

The cat recognition chapter in the book reminds me of the recent 007 movie, Specter. The interesting things to note in this chapter is that the training cat images have to be rotated to upright position. The function getRotationMatrix2D is used to define the rotation and warpAffine is used to rotate the image. Obviously some black regions will be introduced in the image, and thus we can fill them by a specific color. The author uses 50% gray, as it does not tend to introduce bias into the image histogram equalization.