Hi folks,

I have two lines: Line1 and Line2. Each line is defined by two points (P1L1(x1, y1), P2L1(x2, y2) and P1L1(x1, y1), P2L3(x2, y3)). I want to know the inner angle defined by these two lines.

For do it I calculate the angle of each line with the abscissa:

double theta1 = atan(m1) * (180.0 / PI);
double theta2 = atan(m2) * (180.0 / PI);

After to know the angle I calculate the following:

double angle = abs(theta2 - theta1);

The problem or doubt that I have is: sometimes I get the correct angle but sometimes I get the complementary angle (for me outer). How can I know when subtract 180º to know the inner angle? There is any algorithm better to do that? Because I tried some methods: dot product, following formula:

result = (m1 - m2) / (1.0 + (m1 * m2));

But always I have the same problem; I never known when I have the outer angle or the inner angle!

Thanks in advance for reading my trouble and for your time!

Oscar.