Flip normals in three.js on sphere

Question

I have been searching around and haven't found any really good answer to my question yet.. The thing is that I have this sphere.. just a basic sphere, and I want to flip the normals so the sphere gets the sort of "hollow/carved effect" and then plater on apply my textures to the "inside" of the sphere. any ideas of how to flip the normals?

Also.. if its not possible to do this in three.js.. would it be possible to import a model where the normals are already flipped and get the effect I'm looking for?

Expectoration

Try material.side = THREE.BackSide instead and see if the result is acceptable to you. – Cha

That could work I guess... any other ideas are most welcome as well :) I will try the above and ill get back with how it went – Expectoration

Tanks for The hela.. seems till have the desiered effect :-) – Expectoration

Answer 1

This answer only applies to versions of three.js prior to r.125.

The Legacy Geometry class has been removed.

---

You can flip the normals in your geometry by reversing the winding order of your faces. You then have to fix UVs.

for ( let i = 0; i < geometry.faces.length; i ++ ) {

    const face = geometry.faces[ i ];
    const temp = face.a;
    face.a = face.c;
    face.c = temp;
    
}

geometry.computeFaceNormals();
geometry.computeVertexNormals();

const faceVertexUvs = geometry.faceVertexUvs[ 0 ];
for ( let i = 0; i < faceVertexUvs.length; i ++ ) {

    const temp = faceVertexUvs[ i ][ 0 ];
    faceVertexUvs[ i ][ 0 ] = faceVertexUvs[ i ][ 2 ];
    faceVertexUvs[ i ][ 2 ] = temp;
    
}

However, you can get the same effect by simply setting Material.side = THREE.BackSide, or Material.side = THREE.DoubleSide.

In either case, your texture will be rendered flipped. You can either flip your texture before-hand, or build a model outside of three.js and import it.

three.js r.124

Cha

Well,it confuse me,using code above in a skybox,I have to set side:THREE.FrontSide,but the texture is still rendered flipped.With fliping the texture ,I get alias.So i have to build another BoxGeometry?Why BoxGeometry built from vertex will render it without flipping texture? – Saphena

@Saphena If you have a question, you need to ask it in a new post. – Cha

This solution is the way to get the ray caster to work. Ray caster only sees on side, doubleSide doesn't work. – Educt

@Educt Raycaster supports THREE.DoubleSide for meshes. – Cha

@westlangley Strange, I posted this a while ago, and I haven't got it working, have to try again. #40864644 – Educt

@Cha for the hundredth time your help save my life! Thank you! – Malodorous

It's not worth it to make your mesh/CAD exporter to export with the "right" normals. Instead use the Material.side = THREE.DoubleSide as @Cha pointed out. This saved me a lot of trouble!! – Neese

Here's the code to unwind vertices with modern versions of THREE.js. Seems like unwinding vertices still has some niche use-cases like export to GLTF/GLB where BackSide is unsupported. for ( var i = 0; i < geometry.index.array.length-1; i=i +3 ) { var temp = skyGeo.index.array[i]; geometry.index.array[i] = skyGeo.index.array[i+2]; geometry.index.array[i+2] = temp; } Longer explanation avail here: discourse.threejs.org/t/… This info should be added to answer (too many pending edits rn). – Helices

Answer 2

When you are creating material for your sphere, specify {side:THREE.DoubleSide}. This will make faces visible from both sides.

You can also change it anytime after your material is created.

Allista

Answer 3

It is fixed !!

The flip of an object with a negative scale object.scale.x = -1 also reverse the normals since three.js r89 (see: Support reflection matrices. #12787).

(But I have to upgrade to r91 to solve my normal issue.)

Recto

This did not work for me in r91 using shadermaterial. Setting side: THREE.BackSide worked though. – Streamlined

Answer 4

Another way is to simply flip the normals manually by making your object's geometry dynamic.

mesh.geometry.dynamic = true
mesh.geometry.__dirtyVertices = true;
mesh.geometry.__dirtyNormals = true;

mesh.flipSided = true;

//flip every vertex normal in mesh by multiplying normal by -1
for(var i = 0; i<mesh.geometry.faces.length; i++) {
    mesh.geometry.faces[i].normal.x = -1*mesh.geometry.faces[i].normal.x;
    mesh.geometry.faces[i].normal.y = -1*mesh.geometry.faces[i].normal.y;
    mesh.geometry.faces[i].normal.z = -1*mesh.geometry.faces[i].normal.z;
}

mesh.geometry.computeVertexNormals();
mesh.geometry.computeFaceNormals();

Oxazine

__dirtyVertices was removed from the library 3 years ago. You are using an old version of the library. – Cha