Matrix multiplication fine solution! (A picture is worth a thousand words)

A picture is worth a foreword to continue! ! Matrix multiplication explanation

 

The coordinates of each pixel according to a rotation angle, converted into pixel coordinates after the rotation angle.

int angle = 45;

int n = 0; n is a vector.

newX [n] = x [n] * cos (angle) - y [n] * sin (angle)

OF NEW [n] = x [n] * sin (angle) + y [n] * cos (angle)