Trimming surfaces

For some reason the surface won’t trim! It just seems for some obscure reason, some times the surfaces I create trim and sometimes they don’t.
Also how do i rebuild a surface without the surface changing shape? I’ve been doing a tutorial which says you should rebuild your surfaces with 5 degree and 6 points, but now that I’ve started building my own vehicle, all the surfaces change shape when I rebuild them.

Hi Josiah- the first thing to check is whether Rhino finds an intersection between the objects an if so, what that looks like - Run the Intersect command - does that return the expected curves?

Rebuild always changes the shape, a lot or a little. Rebuilding to five by six sounds like maybe the next operations will involve matching surfaces for curvature to clean up, something like that?


Hi, I tried the intersect command and it looks like the trimming curve is intersecting with the surface fine, but it’s still not trimming.
Yes the next step was to match the surfaces. Is it possible to reduce the amount the surface changes when you rebuild? or if it changes is there anyway to fix it? It’s just for some reason when I was doing a tutorial, rebuilding the surfaces seemed to not change the surfaces very much at all, or at least when I matched the surface with the guide curve (selecting position in the match option) after I rebuilt it, it seemed to fix the surface back in position.
Also when creating a control point curve with five degrees does that mean you should only create 6 points? likewise if you create a control point curve with 3 degrees does that mean you only create 4 points? Should the number of points always be one greater than the degree of the curve? and when do you use a 3 degree curve and when do you use a 5 degree curve.

Hi Josiah -

I guess you should post the objects in a3dm file and we’ll take a look.

Generally with a higher point count- but the degree five, six point that you mention makes me think the tutorial is being pretty specific about that - that is of course not always the right combination - it really depends upon what you need to do.

Again, that depends on what you need to do - the degree + 1 point count is a minimum, that is, if you place degree number of points, you’ll end up with a curve that is actually points- 1 degree.
Working with this sort of minimal ‘single span’ or Bezier curves and surfaces is a common, but tricky way to build high quality models, but Rhino is not particularly designed to model in that way - there is no need to restrict yourself in this way unless you have a particular goal in mind.

I’d look at curve degree this way - up to degree 5, maybe in a pinch, 7 but that is really pretty rare, make the shape you want with as few points as possible and make the degree accordingly. Rhino kind of takes care of this for you in a way if you use the Curve command (control point curve)- if you keep the thing set to degree 5, and place only some number less than that, then the degree will be reduced accordingly. I generally try to place the right number of points in about the right places, and then adjust. Keep in mind you can ChangeDegree to add points without changing the shape of the curve (if you go up in degree) So for single span curves, that is a very convenient and nice way to get more control if you need it. See also InsertKnot, and InsertControlPoint. I tend to use ChangeDegree until maybe 5 and beyond that Insert*


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Hi thanks for the help. Here’s the 3dm file. I’m trying to create the base of the car. I’ve already created the curve, now I’m trying to trim the outside of the surface. r8.3dm (3.0 MB)

Hi Josiah - if you are looking to trim the surface that is under the nose, then the curve that is sitting there will not do it- the curve is not closed. Run CrvStart and CrvEnd to see the two points near each other that need to be closed up to trim.

BTW, I don’t know what the tutorial says here but just looking at your fender surface, I see the surface points are kind of all over the place and and Zebra on this surface is pretty wobbly. Just in case it helps, I’ve attached a rearranged surface - see how the points are more orderly and the Zebra looks a lot cleaner? I am not saying this is all done or anything, and I’m not convinced that this surface is really a single five by six surface - it might need a little more control than that, but just by way of showing you how keeping your points orderly can help keep the flow of the surface nice and clean and Zebra looking good.

Fender_PG.3dm (51.6 KB)


Thanks a lot! I managed to get that surface trimmed.
I couldn’t get that fender surface you uploaded because I think you are using a newer version of rhino.
OK, so I’m also modelling up a truck in rhino. I’m having some trouble with the front left edge just above where the wind screen meets the left side of the truck. I keep getting a sharp edge, where I need a smooth one. I’ve uploaded the model.
I’ve highlighted the part where I’m having problems in green. It’s the edge where the two green surfaces meet that I’m having a problem. MERC2.rar (2.0 MB)

Also on the same edge but lower, bellow the wind screen, I’m struggling to create a surface which will not look weird.
I’ve uploaded the model again. The gap is between the two red surfaces. MERC3.rar (2.0 MB)

Yeah… you need to decide what surfaces are meant to be continuous - I mean at least tangent, or in many cases curvature continuous- this should be set up to begin with when you build the input curves.For example, all, I would say, of the horizontal curves down the front that end on the centerline can be made much more simple and have much more consistent and controlled curvature. Keep in mind that just making a curve degree 5 does not make it a good or useful curve- you need to get control of these a little better.

Spend some time with the curves and the surfaces will come along mode easily. The area you are asking about will be difficult, it seems to me, unless you have a better idea of what the surrounding surfaces are supposed to be like - where are they smoothly continuous, for example, and make sure they are the way you want. You might also take a look at organizing surface points a little more thoroughly- believe me, it helps…

V4 fender from before attached-
Fender_PG_V4.3dm (36.7 KB)


Hi, so going back to the problem I mentioned before…
“I’m having some trouble with the front left edge just above where the wind screen meets the left side of the truck. I keep getting a sharp edge, where I need a smooth one. I’ve uploaded the model.
I’ve highlighted the part where I’m having problems in green. It’s the edge where the two green surfaces meet that I’m having a problem”
I tried matching the surfaces using tangency, which more or less worked, but it distorted the surface. The problem is, I can match the distorted surface with the guide curve because it was a trimmed surface. MERC2.rar (2.0 MB)

Hi Josiah- I’ll try to make some more specific suggestions a little later when I get a chance to poke at it a little more. But, for now, it comes back the same things I mentioned before - that area is made of surfaces that are made from curves that are, let’s say far from ideal.
I would try to think about this type of surfacing by making two major categories of surfaces, and analyzing the shape to see what is what

  • Main, or primary shape surfaces- these define the big shapes, and have relatively low and consistent curvature

  • Transition surfaces - these transition between primaries and have higher curvature - i.e. soft corners.

The two green surfaces are each trying to be both - you can do it that way, but it’s harder - instead think

  1. Slabby, flattish surface on the front,
  2. Slabby flattish surface on the side
  3. Transition surface, relatively curvey - say a blend, making the corner between the two.

Get these all clean and matched up THEN trim the window shapes etc.


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Hi, ok so I’m having a little trouble matching a surface to a guide curve. For some reason when I match the surface with the curve nothing happens. The surface is the top right one on the truck, highlighted in yellow and the guide curve is the one that runs along the bottom of the same surface, highlighted in blue. MERC2.rar (2.0 MB)

Hi Josiah - there is not enough control in that surface to hit the curve- using the Refine setting in MatchSrf will help.


I’m trying to trim the two surfaces in the top right part on the front of the truck. The surfaces are highlighted in yellow. The curve I am trying to use to trim the surfaces is blue.
MERC4.rar (2.1 MB)
You don’t happen to know if there are any rhino truck tutorials do you?

Hi Josiah - explode the polysurface and just trim the individual surfaces here. You might try, in the future, Exporting the objects to a small file to post here rather than compressing and attaching the whole huge file.


The surfaces are already separate. There are 2 yellow surfaces.

actually, I exploded the 2 individual surfaces, and it worked. thanks

I’m trying to trim the surface along the side of the truck, highlighted in yellow. The trimming curve is blue and part of the curve runs through the middle of the highlighted surface.
MERC02.3dm (1.9 MB)

Hi Josiah - your curve is short of the surface edge. As you can see in this very small file that contains only the objects you are asking about.

MERC02_TrimProblem.3dm (133.7 KB)


Hi, the blueprints I’ve been using to model up the truck don’t have the details of the headlights. Do you know how I might get detailed headlights?

Just a quick question, how do I make the isocurves invisible?