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As you can see in the picture, I have a line and two points(p1 and p4). what I need to do is to get snapped point of p1/p4 on the line and then use atan2 to calculate the angle between (p1 and p2) and (p3 and p4). Now, I have two formulas:

var anglep1p2 = Math.atan2(p2[1] - p1[1], p2[0] - p1[0]) * 180 / Math.PI;
var anglep4p3 = Math.atan2(p4[1] - p3[1], p4[0] - p3[0]) * 180 / Math.PI;

anglep1p2 is calculated 103.66797855556482 anglep4p3 is calculated -76.74971541138642

I wonder how does atan2 calculate those values? thanks for any help

enter image description here

Maryam Koulaei
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1 Answers1

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These answers do make sense. You are sort of calculating a the angle of a single line, starting from the positive x-axis. The way you calculate anglep1p2, it corresponds to the line drawn from p1 to p2.

If you plunk the origin of a coordinate system at the starting point p1 (you put it at p2 in your diagram), then the number you get should be the rotation from the positive x-axis to the line you drew - a bit over 90 degrees makes intuitive sense.

Your second result is flipped from your first (notice you used p4/p3 in the same order as your variable name, whereas you reversed this order in the p1/p2 case). To avoid confusion, I'd use the p1/p2 case to gain understanding, then apply it the same way to the other case once you know what you want.

If you have a specific geometry/relationship problem you need to figure out, you can provide the details and I might be able to help more specifically.

arbuthnott
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