1. Only the part of the image inside the global
bounding box is untilted;
2. Functions “sin” and “cos” are called only once;
3. Floating point multiplication/division has been
reduced to minimum;
4. The lower limit of the tilt angle is estimated before
untilting, the image is untilted only when it is
necessary (i.e. the tilt angle is above the lower
limit).