Determine whether the triangle is clockwise or clockwise.

Use the vector cross product to determine whether it is clockwise or clockwise. If the vector p = (x1, Y1), q = (X2, Y2) is set, the vector cross product is defined as (0, 0) the signed area of the parallelogram consisting of P1, P2, and P1 + P2, that is, p × q = x1 * Y2-X2 * Y1. The result is a scalar. Obviously, the properties include P x q =-(q x P) and P x (-q) =-(P x q ). A very important property of the cross product is that it can be used to determine the clockwise relationship between two vectors:

If p × q> 0, P is clockwise in Q.
If p × q <0, P is in the counterclockwise direction of Q.
If p × q = 0, p and q are both in the same direction, but they may be reversed.
Explanation: A × B = (Ay * BZ-by * AZ, AZ * Bx-Ax * BZ, ax * by-ay * BX) and because AZ BZ is 0, so a × B = (, ax * by-ay * BX) according to the right hand system (the Right Hand System is satisfied by the cross multiplication), if p × q> 0, ax * by-ay * BX> 0, that is, the thumb points up, so P is clockwise in Q.