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example

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).

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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.

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  • 1
    \$\begingroup\$ I know that Google would fill in the blanks, but you should probably at least mention the term "dot product" so that OP has a better idea of what to Google for ;-) Perhaps something like "taking the dot product of two normalized vectors gives the cosine of the angle between them" \$\endgroup\$ – A C Oct 18 '17 at 17:52
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The angle between two lines is the angle between direction vectors of the lines.

Here's a formula:

enter image description here

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

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