# How to model a double curve surface without joint line

Hi, All
I would like to ask a question abt how to model a perfect Polysurface like the image below.
I tried many times from curves, but it always has joint line on the surface.
Could someone tell me a good way to do this?
Many thanks!

Please post the curves and surface in a .3dm file. You can drag the file to where you type a post, or you can use the vertical arrow icon above where you type a post.

it looks to me as this was modelled as a continuos part and split / trimmed later. rebuilding it from scratch as a polysurface might not make much sense as far as i understand.

at least if you take the middle part you can see it seems to have 5 boundaries which exceeds a Nurb surface and indicates that it is trimmed at this part. While It could be 4 boundaries under special conditions posting some geometry to work with would definitely help.

1 Like

Hi, David.
Thanks for ur reply. I attached my file here. I would like to know the steps your experienced guys to model this kind of complex curved surface . For me, this surface really hard.
Thanks!
polysurface.3dm (1.3 MB)

Hi, @encephalon
thanks for the method provided. I will try to model it as your way. It seems to me my model logic need to be improved for this kind of surface. also I tried a bit, as I could not find my geometry boundary accurately, so rebuild a 4 boundary polysurface first seems hard. Could u show me what command I need to take for it?
Cheers!

@encephalon
This is the step I use. Is it the only way to do it? Thanks

That can be a good method if a trimmed surface is okay. How close does the surface need to match the edges? Patch can be used to improve the match of the surface to the input curves. Create the untrimmed surface as in your post. Divide the input curves with closely spaced points. Delete the points in the area to be trimmed. Use Patch with the untrimmed surface as the starting surface and preserve edges, and the points as the input data. You may need to experiment with adding control points using InsertKnot for a better match with the input curves.

Hi, David
The curves I use are in the model. I did this quickly, so the untrimed surface just similar to but not totally with same curvature as the target polysurface. I will test follow your step to add some points or move some of them to match .
Thanks!
I used your method and succeed. thanks!

@Flora Are you trying to create a single surface which matches a polysurface? If so try this proceedure.

Create a four-sided surface similar to the grey one in your post above. This will be the “`Start surface`” for `Patch`.

Mesh the polysurface. This will be input to `Patch`.

`Patch` with the mesh as the input and use the four-sided surface from above as the `Start surface` and the `Preserve edges` option checked.

You may need to experiment with using `InsertKnot` to add control points to the surface for a better match with the input curves. `InsertKnot` does not change the shape unlike `InsertControlPoint` or `Rebuild`.Then use `Patch` again but with the surface with increased control points as the `Start surface`

Iterate until the single surface is close enough to the polysurface.

`Trim` the single surface with the edge of the polysurface. You may need to `Pull` the edge to the single surface before `Trim`

Hi, Davide
Thanks again for ur reply. Yes, I need to model a single surface which is easy to give material for rendering afterwards.
I will test it.
Cheers!

Hi Flora - keep in mind that you can use an approximate surface as a custom mapping object for the polysurface.

-Pascal

Hi, Pascal.
Thanks for the summary . It seems I need to test more to be professional , really easier said than done. I tried many times before I get an ideal surface
Like this image
And really thanks ur guys for answering my questions patiently!