Center of a circular arc

Coding
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#1

anyone here can help me,
I am trying to get the centre point position of a 3-point arc in 3d space.
but I am really bad with vector math.
I google for a long time, but no luck.
Screenshot 2022-06-28 215644

#2

Are you trying to solve the math, or just find the point?

There is a built-in 3 point arc tool in Maya. Create arcs | Maya 2020 | Autodesk Knowledge Network

Create → Curve Tools → Three Point Circular Arc

#3

hi clesage, I am trying to solve the math,
I can get the point I want with Three-Point Circular Arc,
but I want to make a script without select points manually,
the problem I have currently I can create node and create arc without any problem,
but it just won’t give me the centre point of the circular arc.
the pivot point will only stay at 0,0,0

#4

Cool, just checking since you said you were bad at math and found nothing in google.

All the same though, when you create a makeThreePointCircularArc node, it certainly does have an output .center that you can query.

(Oddly, if you connect a transform to it, it doesn’t update automatically when the 3 points are modified. But you can force it to refresh by force reconnecting it.)

#6

thanks, because in my code I acutally freeze(kill) the hitsory, that’s why . you have my day :slight_smile:

#7

No problem.

Another obstacle to watch out for. When you are running your code, I’m noticing that the arc node won’t return a center until the UI is refreshed.

import maya.cmds as cmds
p1 = (0,1,1)
p2 = (0,2,3)
p3 = (0,4,5)
newNode = cmds.createNode('makeThreePointCircularArc')
cmds.setAttr(newNode + '.pt1', p1[0], p1[1], p1[2])
cmds.setAttr(newNode + '.pt2', p2[0], p2[1], p2[2])
cmds.setAttr(newNode + '.pt3', p3[0], p3[1], p3[2])
cmds.setAttr(newNode + '.d', 3)
cmds.setAttr(newNode + '.s', 4)
newCurve = cmds.createNode('nurbsCurve')
cmds.connectAttr(newNode + '.oc', newCurve + '.cr')
#cmds.delete(ch=1)
transformNode = cmds.listRelatives(newCurve, fullPath=True, parent=True)
cmds.rename(transformNode, 'arcCurve')


print(cmds.getAttr(newNode + '.center'))
# RESULT IS [(0,0,0)]

cmds.refresh()

print(cmds.getAttr(newNode + '.center'))
# RESULT IS [(0.0, 8.499999999999979, -1.499999999999985)]
#8

yp, refresh UI is a bit annoying but will do the job for now. thank you so much

#9

you could do it completely through vector math like this

def rayPlaneIntersection(rayOrigin: Vector, rayDirection: Vector, planeOrigin: Vector, planeNormal: Vector) -> Vector:

    distance = ((rayOrigin - planeOrigin) * planeNormal.normal())
    return rayOrigin - rayDirection * (distance / (rayDirection * planeNormal))

def centerOfCircle(point1: Vector, point2: Vector, point3: Vector) -> Vector:

    vec1 = point1 - point2
    vec2 = point3 - point2
    cross1 = vec1.normal() ^ vec2.normal()
    aim1 = vec1.normal() ^ cross1

    center1 = (vec1 * .5) + point2
    center2 = (vec2 * .5) + point2

    centerPoint = rayPlaneIntersection(center1, aim1, center2, vec2.normal())
    return centerPoint
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