Relative angle in degrees between 2 lines

Question

Could anybody help to solve this? Relative angle between 2 rays(lines) which starts from same point. They could point anywhere 360 clockwise, but angle should be always relative (just inside angle).

Answer 1

Did you try even a cursory search for this? It's pretty standard vector math:

v1 = normalize(end1X - startX, end1Y - startY);
v2 = normalize(end2X - startX, end2Y - startY);

angle = acos(dot(v1, v2)) * 180.0/pi;

This will always give a value from 0 to 180, giving you the smallest positive angle clockwise or counter-clockwise.

In 2D, you can fix a rotation direction like so:

v1perp = (-v1.y, v1.x);

if(dot(v2, v1perp) > 0)
    angle = 360.0 - angle;

This will give a clockwise angle in the range 0...180 if v2 points to the right of v1, or 180...360 if v2 is pointing to the left of v1.

Answer 2

The angle between two lines is the angle between direction vectors of the lines.

Here's a formula:

You need to remove the Z param from the equation and then calculate your angle from the cosine.