cv2.flip

Flips a 2D array around vertical, horizontal, or both axes.

C++: void flip(InputArray src, OutputArray dst, int flipCode)

Python: cv2.flip(src, flipCode[, dst]) → dst
C: void cvFlip(const CvArr src, CvArr dst=NULL, int flip_mode=0 )
Parameters:
src – input array.
dst – output array of the same size and type as src.
flipCode – a flag to specify how to flip the array; 0 means flipping around the x-axis and positive value (for example, 1) means flipping around y-axis. Negative value (for example, -1) means flipping around both axes (see the discussion below for the formulas).
The function flip flips the array in one of three different ways (row and column indices are 0-based):

The example scenarios of using the function are the following:

Vertical flipping of the image (flipCode == 0) to switch between top-left and bottom-left image origin. This is a typical operation in video processing on Microsoft Windows* OS.
Horizontal flipping of the image with the subsequent horizontal shift and absolute difference calculation to check for a vertical-axis symmetry (flipCode > 0).
Simultaneous horizontal and vertical flipping of the image with the subsequent shift and absolute difference calculation to check for a central symmetry (flipCode < 0).
Reversing the order of point arrays (flipCode > 0 or flipCode == 0).