# Symbol on a NURBS surface

## Recommended Posts

How do I place a symbol on a NURBS surface normal to the surface.

Eg. A post on a arched bridge.

I can find the normal vector at the point with vs.EvaluateNurbsSurfacePointAndNormal(surfaceH, u, v)

I assume I can somehow locate the symbol with vs.SetEntityMatrixN( objectHandle, u , v , w,  offset)

How do I get the u,v and w vectors? or calculate the angels from the normal vector to use vs.SetEntityMatrix(vs.LNewObj(), *wp)

I can get it to work with:

vs.SetWorkingPlaneN(*pt, *norm, *uVec)
wp = vs.GetWorkingPlane()

vs.Symbol("post", (0,0), 0)
vs.SetEntityMatrix(vs.LNewObj(), *wp)

The symbol is placed in document space not the working plane but the GetWorkingPlane function returns the angle in degrees needed for SetEntityMatrix

There must be a better way I am missing...  I am not sure why vs.SetEntityMatrixN needs different inputs to vs.SetWorkingPlaneN

Any pointers appreciated. Thanks

To follow up the following works:

```def placeSymbolOnSurfaceAlignedWithUVDelta(u,v, surfaceHandel, symbolName, du,dv):
# get the 3d point and vector normal to the surface
ptExists, pt, normal = vs.EvaluateNurbsSurfacePointAndNormal(surfaceHandel, u, v)
if ptExists:
# place the symbol at (0,0,0) then move it. Assumes symbol is defined to be aligned by its insertion point
vs.Symbol(symbolName, (0,0), 0)
symbolHandel = vs.LNewObj()
# create an orientation vector using a point either side of u,v (u-du,v-dv), (u+du,v+dv) where du,dv are small compared to the surface curvature and their relative magnatudes define the orientation in the u,v plane eg u=0,v=0.0001 will result in a vector in the direction of v at the point
orentation = tuple(map(sub, vs.EvaluateNurbsSurfacePointAndNormal(surfaceHandel, u+du, v+dv)[1], vs.EvaluateNurbsSurfacePointAndNormal(surfaceHandel, u-du, v-dv)[1]))
# this is a good estiment but it fails because it is not exactly orthogonal to the normal vector. Correct by projecting onto the plane tangent to the surface at u,v
uVec = tuple(map(sub,orentation, [vs.DotProduct(orentation,normal)*i for i in normal]))
# w-vector is the normal vector
wVec = normal
# we know the other 2 and they are all orthogonal so v-vector is the cross product of u x w
vVec = vs.CrossProduct(uVec,wVec)
# offset is the 3d point because the symbol is currently at 0,0,0
offsetVec = pt
vs.SetEntityMatrixN(symbolHandel, uVec, vVec, wVec, offsetVec)
return symbolHandel```

## Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

×   Pasted as rich text.   Restore formatting

Only 75 emoji are allowed.

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×