# How do I draw and orient polylines/curves onto a curved surface?

Hi everyone, new to Rhino, looked everywhere…tutorials/vids for a couple hrs to find an answer for this…how do I orient/align to a curved surface in order to draw a set of simple curves/lines to use as a template for trimming that curved surface? Apolgize in advance, I dont have an Internet connection where I live so can’t post an image. Essentially I’m trying to draw a 3D tail-section of a spaceship. I started with a rectangular flat surface that is bent in half @ center like an arc, on either side I wanna simply draw the lines that will be used to trim it. I figured curve to surface would be the correct command but it won’t work, says it requires points(??). I saw a command-createUVcurve that may work, idk, several others like unroll surface/follow along surface…idk, just seems there should be a much simpler way of aligning/orienting on a curved surface, a sphere for example, and ur cursor follows that curvature around the sphere. Plz any help/advice would be greatly appreciated, thx.

Use the InterpcrvOnSrf command to draw curves on a surface. You can also get curves on a surface with Project command, Pull command, or FlowAlongSrf command.

Another alternative is to draw a curve off the surface and use Trim with ApparentIntersections=Yes. The results depend on the active viewport.

Thx, I got it to work by extruding the curves thru it and trimming, pretty obvious way to do it, ha. However I gotta ask: using the interpolate crv to surface command, it only aligns to the outer edges of the surface, is there a way to get it to snap/align to the mesh vertices (wrong name for 'em I’m sure) that span is across the surface? Thx

You can usually obtain the same result using Trim with ApparentIntersections=Yes

Are you talking about the control points which are not on the surface, or the isocurves, or did you start with a mesh rather than a surface?

Some time going through the Level I and II training manuals would probably result in better models faster. https://wiki.mcneel.com/rhino/6/trainingguides/en

Just in case there is confusion - ApparentIntersections=Yes/No only applies to trimming curves.

-Pascal

It works for trimming surfaces with curves.

The CurveOnSurface plugin is now available for Rhino 6 onFood4Rhino.com16. This plugin requires at least version 6.3 of Rhino.
The CurveOnSurface plug-in for Rhino provides two commands. The CurveOnSurface command is an alternative for the InterpCrvOnSrf command, while the CurveOnSurfacePtOn command turns on control points to edit this curve. The CurveOnSurface plugin is for Rhino for Windows only. There are different versions of the plugin for Rhino 6 and Rhino 5 (both 32-bit and 64-bit).
CurveOnSurfacePtOn will turn on control points at the interpolation points that defined the curve. These can then be edited individually, or in larger selection sets, to directly edit the curve. The points will be constrained to stay on the surface. Control point editing commands such as Nudge may also be used. The PtOff command turns off all control points.
The Rhino 6 version of CurveOnSurface will allow one to draw over the seam of a smooth closed surface. source: CurveOnSurface plugin for Rhino 6

Unlike InterpCrvOnSrf, the curve created with CurveOnSurface is independent of the parametrization of the surface. The curve will tend to go in a straight or direct path instead of following the surface isocurves. In fact, when only two points are specified the curve is the shortest possible path (a geodesic) – the same curve made by the ShortPath command. CurveOnSurfacePtOn will turn on control points at the interpolation points that defined the curve. These can then be edited individually, or in larger selection sets, to directly edit the curve. The points will be constrained to stay on the surface. Control point editing commands such as Nudge may also be used. The PtOff command turns off all control points. source: http://www.food4rhino.com/app/curveonsurface-0

HI David - what I mean is, the setting does not affect trimming surfaces - it can be set to Yes or No and `Trim` does not care, if the target is not a curve.

-Pascal

Sorry so late to respond, what I meant regarding the Interpolate curve on surface command, it snaps to the edges of the surface fine but I was hoping there’s a way/snap setting that’ll allow ya to snap to the surface’s individual isocurve “corners/intersections” not control points, somehow I doubt that’s possible, idk. Btw I tried Downloading the Curve On Surface plugin, every time I tried to open/save the download file, for some reason it would just disappear off the screen when I selected either option…don’t get what happened there. I found that Unroll command worked great for what I needed, only thing is, there’s no way to roll it back up!! Ha. And the CreateUVcurve command…well when the surface gets flattened out, it seems oriented backwards from the way you’d expect it to flatten, and it’s very tricky to figure out where to draw ur curves so they are oriented correctly and not on the wrong end/etc…is there a setting/invert option to reorient the flattened surface in a more accurate direction/alignment to the original surface? Sorry if I’m using the wrong names for things here…the frustrating part of Rhino, to me, is learning the massive amount of commands/command variations available to use. Btw yeah I def looked all through the Rhino level 1 & 2 training manuals, don’t seem to remember much regarding these commands in there, but I’ll have to take another look. I certainly like written lessons/tutorials to learn from, and they’re not many I’ve found online other than random college course lessons/etc, if y’all know a great site to learn Rhino, plz let me know, thx

There is no way to directly snap to the isocurves displayed on surfaces. You can use ExtractIsocurve to create a curve from any isocurve.

You can use Mirror to turn over the unrolled surface. Also Unroll with Labels=Yes will put labels near the original and unrolled surfaces which aid in orienting the unrolled surface.

Ok, great info, I really appreciate it everyone, thx.