Joining surfaces along two edges

I’m sorry for the very elementary question, but I am just trying to test out Rhino and am new to the whole affair.

I have three surfaces below, a left, middle, and right portion. The three surfaces were created outside of Rhino using 3d coordinates, and then brought into Rhino using the AddSrfPtGrid command. All three surface should fit together perfectly.

You can see the album of images here:

or just this single image:

Basically, I can merge the left and middle surface together using MergSrf, but I can’t merge the middle and right surface together (“Edges too far apart”). I think this is because the right surface has a longer edge, so I need to tell Rhino somehow to only merge part of the edge.

Does anybody know how this is done?


Joining and merging are different. In the example you’ve shown, the surfaces cannot be merged into one untrimmed surface but they could be joined into a polysurface as long as the edges are within the unit tolerance of the file. So the question is if a polysurface is sufficient for your goal.

Think of NURBS surfaces as pieces of four sided graph paper. Even if they get trimmed, like a circular profile for instance, the control point structure is still four sided. If you ultimately need the form to be one single surface you could create it in other ways but your requirements will determine what to suggest.

Hi Brian,

Thank you for the correction about joining vs. merging. Yes, I did notice that I could join the surfaces together using the Join command.

Think of NURBS surfaces as pieces of four sided graph paper.

So basically anything that can’t be uniquely projected onto the plane cannot be merged into a single surface? What would be the advantages of working with a joined surface vs. a single merged surface?

In terms of my requirements, I don’t have many. I primarily want both the option to render and not have any seams appearing (that seems to be the case), but also I would prefer not to have seams appearing in the GUI view as well.

The reason why is that sometimes I’d like to export the GUI view as an image using one of the view modes (shaded, wireframe, technical, pen, etc.). Noticed for instance that using the pen view mode, if the surfaces are separate like this, you see the seams between them.

If you click the above album picture, you will see that when the edges are not highlighted, the seams have darker contour lines. My intention in merging the surfaces was to get rid of that.

Does that make sense?

heh… I was asking about a display mode last night to do what you want.

as far as I can gather, it’s not possible in rhino (or, definitely not possible in mac rhino)


oh… just found a similar request from a while back.


Thanks, this indeed helps (to at least know it does not seem to be possible to hide the tangent edges in pen mode. Since we seem to be looking at the same thing, have you managed to find a workaround (not using the pen display) for displaying technical drawings with a similar visual?

If anybody else has any advice on what can be done to join/merge the surfaces together to remove the tangential edges, I’d very much like to hear.

So the question is if a polysurface is sufficient for your goal.

@BrianJ: actually with further testing, seams (tangential edges) do show up in renders, so I suppose I am indeed in need of some way to join these surfaces together. What do you suggest?

the workaround (for me at least) is to set up a display mode that doesn’t show edges but it shows curves… then using things like _DupEdge and _DupBorder, make the edges you want to be visible as curves…

same model, same join_age, tangent seams:

when you say ‘renders’, what do you mean- exactly? do you have an example render which shows the seam problem?
(like, is there literally a line being drawn at the seam or do you mean you can see a hard(er) transition where the seam is?)

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@jeff_hammond: Your advice to use _DupEdge looks to be very useful.

when you say ‘renders’, what do you mean- exactly? do you have an example render which shows the seam problem?
(like, is there literally a line being drawn at the seam or do you mean you can see a hard(er) transition where the seam is?)

I had the opportunity to play with Penguin render, which I tried to create the same effect as Pen mode. You can see the seams.

I’m not sure what the workaround here, because Penguin does not seem to have a way to hide edges but show curves (they treat edges of surfaces and curves as the same thing). I assume that you can do a really manual workflow by creating one layer with nothing but the shaded surface, then create a layer with the outlines, which you’ve manually created using _DupeEdge, then combine the two images later.

However, with all this in mind, I think the best workflow for my purposes is none of the above. I had a quick look at the videos suggested in this post:

and I think for what I want (technical drawings with crisp lines but with a little shading), the Make2D option is the best way to go. The curves are brought into Illustrator, cleaned up, and we can use any renderer (with no edges) and overlay the vector image with the shading from the renderer.

Either way, it looks like there’s some manual work involved, but that’s probably fine given that you’d like to clean up the lines in any case, and there’s not enough control with any of the default renderers.

It’s not a matter of planarity or a projected profile but rather if the resulting surface is trimmed or untrimmed. A seemingly six sided surface can be represented as a trimmed surface. Turing on the control points for the surface will show the four sided rectangular nature of the underlying structure. In your case, the surfaces appear to be untrimmed so joining is the best option versus reconstruction of the entire form in a way that would create one single trimmed surface.

It depends on what you what to do with the geometry. For instance, if 3D printing is the goal, a joined polysurface that is closed can be exported as a polygonal mesh. Or if you have continuity across a seam, reflections will not be interrupted in a rendering (raytraced or OpenGL). On the other hand, a single surface can be control point edited whereas a polysurface cannot be edited in that specific way.

Like @jeff_hammond mentioned, there are some options here relating to display mode settings. In shaded mode the ‘show edges’ option in the Display panel is the fast way to remove all edges but I see you actually want something else. In the case of Technical based modes like Artistic and Pen, the Silhouette display option will get you close but using DupEdge with Curve display for the active mode allows for full customization.

So in short, I don’t think you need to merge the surfaces for this display/render/screenshot goal.

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ah… ok.
i’ve never used penguin before… all of my rendering solutions (combo of mac rhino, indigo, & sketchup) require any desired linework to be exported separately then overlayed on the render… i thought that was normal.
pretty neat that penguin can render lines (in the right situations)

(unless i’m doing a rhino ViewCapture… then the lines will come along in the file.)

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