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();
}