Problem with backside

Questions
1

Hello,

I have done a surface with three.js : Togliatti surface (the code is available by looking at the source).

This surface is two-sided. There’s a problem: the front side is nice, but there are some gaps in the back side:

Capture%20du%202018-11-21%2010-04-00

How can I get rid of these gaps? I have tried THREE.MeshNormalMaterial({ side: THREE.DoubleSide })) and mesh.doubleSided = true.

2

Hi!
It doesn’t look like back sides. It looks like front sides with wrong normals.

3

Hi,

Thank you. But I’m sure the normals are correct. I have done the same surface with R, in the same way, and the result is nice: gfycat

At least the gradient (which defines the normals) is correct, I have compared with a numerical derivation of the gradient in R. I have tried to negate the normals and to change the order of the vertices of the triangles, but no way, the gaps are always there.

4

Try to use THREE.FaceNormalHelper() and/or THREE.VertexNormalHelper() to visualize normals.

5

Thanks, I didn’t know that. I have tried VertexNormalHelper but there are >300000 triangles, so we don’t clearly see. I will try to reduce the resolution of the marching cubes…

6

Here is the figure with the normals, with less triangles: https://laustep.github.io/stlahblog/frames/threejs_temp.html
Do you see something bad?

7

Yeah. Looks like there are no faces at some spots:
Togliatti_emptiness

1 Like
8

Well seen, thanks. I have realized something. The role of the NaN’s in the function f is to clip the surface by a ball. If I don’t put these NaN, I get no issue, there’s no gap. So these NaN cause a problem in the marching cubes algorithm. Oddly, the NaN’s does not cause any problem in R and Haskell, with the same algorithm. I have also compared with R the number of triangles, and there are less triangles in the Javascript version, when the NaN’s are here (same number when there are no NaN).
So I have to investigate the algorithm when it deals with some NaN’s.
Thanks for your help!

9

Yeah it works now! But now the forum prevents me to insert any link to my blog :frowning:

But not a tiny url ahah: Togliatti

There were some vertices [NaN,NaN,NaN] in the output of the marching cubes algorithm, and three.js does not like that. Now the algorithm deletes all triangles containing such a vertex.

1 Like