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?

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Answer 1

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

Flip normals, flip UVs, Inverse Face winding

Version for 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();
}