flat shading ue4

if (smooth)
{
    UKismetProceduralMeshLibrary::CalculateTangentsForMesh(data.VertexArray, data.IndexArray, data.UVArray, data.NormalArray, data.TangentArray);
}
else
{
    for (int32 Index = 0; Index < IndexCount; Index += 3)
    {
        const FVector Edge21 = data.VertexArray[data.IndexArray[Index + 1]] - data.VertexArray[data.IndexArray[Index + 2]];
        const FVector Edge20 = data.VertexArray[data.IndexArray[Index + 0]] - data.VertexArray[data.IndexArray[Index + 2]];
        FVector TriNormal = (Edge21 ^ Edge20).GetSafeNormal();

        FVector FaceTangentX = Edge20.GetSafeNormal();
        FVector FaceTangentY = (FaceTangentX ^ TriNormal).GetSafeNormal();
        FVector FaceTangentZ = TriNormal;

        // Use Gram-Schmidt orthogonalization to make sure X is orth with Z
        FaceTangentX -= FaceTangentZ * (FaceTangentZ | FaceTangentX);
        FaceTangentX.Normalize();

        // See if we need to flip TangentY when generating from cross product
        const bool bFlipBitangent = ((FaceTangentZ ^ FaceTangentX) | FaceTangentY) < 0.f;
        TriNormal.Normalize();
        FaceTangentX.Normalize();
        FProcMeshTangent tangent = FProcMeshTangent(FaceTangentX, bFlipBitangent);
        for (int32 i = 0; i < 3; i++)
        {
            data.NormalArray[data.IndexArray[Index + i]] = TriNormal;
            data.TangentArray[data.IndexArray[Index + i]] = tangent;
        }

    }
}