Three.js: How to flip normals after negative scale

Question

I have cloned and than flipped an object using negative scale, which causes my single sided faces to inverse. My question is, how can i flip the normals too?

I don't want to use material.side = THREE.DoubleSide, for reasons: 1) didn't work properly (some shades are drawn from inside) and 2) wanna keep as much performance as possible. So DoubleSide isn't an option for me.

Thats how my object if flipped.

mesh.scale.x = - scale_width;

Thanks in advance!

Answer 1

I would advise against negative scale for a whole host of reasons, as explained in this link: Transforming vertex normals in three.js

You can instead apply an inversion matrix to your geometry like so

geometry.scale( - 1, 1, 1 );

As explained in the link, a consequence to doing this, however, is the geometry faces will no longer have counterclockwise winding order, but clockwise.

You can manually traverse your geometry and flip the winding order of each face. This may work for you -- if you do not have a texture applied and are not using UVs. If your geometry is to be textured, the UVs will need to be corrected, too.

Actually, a geometry inversion utility would be a nice addition to three.js. Currently, what you want to do is not supported by the library.

three.js r.72

Answer 2

Just dumping this here. I found somewhere flipNormals and translated it for BufferGeometry

Flip normals, flip UVs, Inverse Face winding

Version for indexed BufferGeometry

function flipBufferGeometryNormalsIndexed(geometry) {
  const index = geometry.index.array
  for (let i = 0, il = index.length / 3; i < il; i++) {
    let x = index[i * 3]
    index[i * 3] = index[i * 3 + 2]
    index[i * 3 + 2] = x
  }
  geometry.index.needsUpdate = true
}

Version for non-indexed BufferGeometry

export function flipBufferGeometryNormals(geometry) {
  const tempXYZ = [0, 0, 0];

  // flip normals
  for (let i = 0; i < geometry.attributes.normal.array.length / 9; i++) {
    // cache a coordinates
    tempXYZ[0] = geometry.attributes.normal.array[i * 9];
    tempXYZ[1] = geometry.attributes.normal.array[i * 9 + 1];
    tempXYZ[2] = geometry.attributes.normal.array[i * 9 + 2];

    // overwrite a with c
    geometry.attributes.normal.array[i * 9] =
      geometry.attributes.normal.array[i * 9 + 6];
    geometry.attributes.normal.array[i * 9 + 1] =
      geometry.attributes.normal.array[i * 9 + 7];
    geometry.attributes.normal.array[i * 9 + 2] =
      geometry.attributes.normal.array[i * 9 + 8];

    // overwrite c with stored a values
    geometry.attributes.normal.array[i * 9 + 6] = tempXYZ[0];
    geometry.attributes.normal.array[i * 9 + 7] = tempXYZ[1];
    geometry.attributes.normal.array[i * 9 + 8] = tempXYZ[2];
  }

  // change face winding order
  for (let i = 0; i < geometry.attributes.position.array.length / 9; i++) {
    // cache a coordinates
    tempXYZ[0] = geometry.attributes.position.array[i * 9];
    tempXYZ[1] = geometry.attributes.position.array[i * 9 + 1];
    tempXYZ[2] = geometry.attributes.position.array[i * 9 + 2];

    // overwrite a with c
    geometry.attributes.position.array[i * 9] =
      geometry.attributes.position.array[i * 9 + 6];
    geometry.attributes.position.array[i * 9 + 1] =
      geometry.attributes.position.array[i * 9 + 7];
    geometry.attributes.position.array[i * 9 + 2] =
      geometry.attributes.position.array[i * 9 + 8];

    // overwrite c with stored a values
    geometry.attributes.position.array[i * 9 + 6] = tempXYZ[0];
    geometry.attributes.position.array[i * 9 + 7] = tempXYZ[1];
    geometry.attributes.position.array[i * 9 + 8] = tempXYZ[2];
  }

  // flip UV coordinates
  for (let i = 0; i < geometry.attributes.uv.array.length / 6; i++) {
    // cache a coordinates
    tempXYZ[0] = geometry.attributes.uv.array[i * 6];
    tempXYZ[1] = geometry.attributes.uv.array[i * 6 + 1];

    // overwrite a with c
    geometry.attributes.uv.array[i * 6] =
      geometry.attributes.uv.array[i * 6 + 4];
    geometry.attributes.uv.array[i * 6 + 1] =
      geometry.attributes.uv.array[i * 6 + 5];

    // overwrite c with stored a values
    geometry.attributes.uv.array[i * 6 + 4] = tempXYZ[0];
    geometry.attributes.uv.array[i * 6 + 5] = tempXYZ[1];
  }

  geometry.attributes.normal.needsUpdate = true;
  geometry.attributes.position.needsUpdate = true;
  geometry.attributes.uv.needsUpdate = true;
}

For old style Geometry

export function flipNormals (geometry) {
  let temp = 0;
  let face;

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

  // change face winding order
  for (let i = 0; i < geometry.faces.length; i++) {
    const face = geometry.faces[i];
    temp = face.a;
    face.a = face.c;
    face.c = temp;
  }

  // flip UV coordinates
  const faceVertexUvs = geometry.faceVertexUvs[0];
  for (let i = 0; i < faceVertexUvs.length; i++) {
    temp = faceVertexUvs[i][0];
    faceVertexUvs[i][0] = faceVertexUvs[i][2];
    faceVertexUvs[i][2] = temp;
  }

  geometry.verticesNeedUpdate = true;
  geometry.normalsNeedUpdate = true;

  geometry.computeFaceNormals();
  geometry.computeVertexNormals();
  geometry.computeBoundingSphere();
}

Answer 3

This question is two years old, but in case anyone passes by. Here is a non-destructive way of doing this:

You can enter the "dirty vertices/normals" mode, and flip the normals manually:

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();

+1 @WestLangley, I suggest you never use negative scale.