Hi everyone,

I usually try to ask questions regarding knowledge about grasshopper. This time I thought I might ask some help in solving a geometric question.

I am trying to calculate trigonometrically the projected arc from a circle in the xy plane to another plane with an angle to it. It’s been a long day and my mind is jammed so if anyone is interested and would like to have a look at it I would appreciate it.

arcEllipse.gh (18.7 KB)

# Calculate Projected Arc

**anikolo**(Aris Nikolopoulos) #1

**anikolo**(Aris Nikolopoulos) #3

No, the opposite, I noticed it now.

It should start from x and measure counter clockwise from 0 to 360.

the vector angle always choses the smallest of the two arcs. we don’t want that

**anikolo**(Aris Nikolopoulos) #4

Now I genuinely don’t know if it is a GH thing or a geometry thing.

the expression that I came to is *atan(tan(x)*cos(y))*

(no cotans btw )

It should work for all quarters, but for some reason it jumps back and forth between 0 and 90…

I even tried using absolute values for sines and cosines but still no good.

arcEllipse.gh (16.7 KB)

**andy.vanmater**(Andy Vanmater) #5

You need to specify a plane (likely the XY plane) in order to get the full 0 to 2pi (360) angle in your top “angle” component. I don’t know how to fix your exact issue of your numbers not agreeing but I do know that in order to get your full angle you need a plane.

**anikolo**(Aris Nikolopoulos) #6

hi andy,

you are right but what you say applies to the ‘mechanical’ way of finding the angle.

My concern is primarily the trigonometric formula.

**andy.vanmater**(Andy Vanmater) #7

Trig it is… GH doesn’t like giving the negative but it’s ultimately the same. Modulus forever!

ArcAngle.gh (12.4 KB)

**andy.vanmater**(Andy Vanmater) #8

And if you don’t want the reflex angle you can always do ( 2*pi - yourangle)

**DavidRutten**(David Rutten) #9

If it’s vectors in 3D space, vector arithmetic is usually a lot more elegant than trigonometry. But that would involve you inputting the planes into the expression as well.