I can't seem to get my uv mapping to work for my icosphere using CreateMeshSection

Development Programming & Scripting C++
1

Hey guys, hope you’re all doing well! I’ve recently been working on a project in an attempt to create some procedurally generated planets using a subdivided icosahedron. I’ve managed to get the subdivision working fine and I’ve calculated my normals but I’m trying to get a naive render of a texture onto my sphere it comes out like the sphere on the left here -

SphereUV.jpg
SphereUV.jpg1928×1056 287 KB

On the left is the result as mentioned above while the right is a default sphere from the engine with the desired texture. I ended up calculating my normals with a formula I found researching and ended up with something like this -


 for (int i = 0; i < m_triangles.Num() ; i++)
{
  CreateUV(m_triangles*, m_uvmapping*);
}

void Icosphere::CreateUV(const Triangle& tri,FVector2D& UV)
{
  FVector x = m_vertices[tri.vert[0]];
  x.Normalize();
  FVector y = m_vertices[tri.vert[1]];
  y.Normalize();
  FVector z = m_vertices[tri.vert[2]];
  z.Normalize();

  UV.X = (FMath::Atan2(x.X, z.Z) / (PI * 2)) + .5f;
  UV.Y = (FMath::Asin(y.Y) / PI) - .5f;
}

  MeshComponent->CreateMeshSection(0, m_vertices, m_triangles, m_normals, m_uvmapping, dummy_color, TArray<FProcMeshTangent>(), true);

I still sort of new to the concept of UV mapping so I’m unsure of exactly what’s going wrong I was wondering if anybody had a similar experience. If any more code is required feel free to let me know and thanks any help or time in advance!

2

With UV mapping, you want to project the surface of your 3D-object onto a 2D-plane so you can use a texture (since they are a 2D-image). UV-maps contain 2D-coordinates, ranging from 0 to 1, for each vertex.
Therefore you preferably would take a point like Vector(sphere_radius, 0, 0) as the center of the UV-map (for a sphere). This means, the UV.X coordinates correspond to the rotation around the z-axis (viewing from above, a clockwise rotation from 0…PI being on the left half of the UV.map [UV.X <= 0.5], a counterclockwise rotation from 0…PI being on the right half [UV.X >= 0.5]). This rotation angle is calculated by atan2(y, x). The UV.Y coordinates correspond to the elevation angle = asin(z).

Your code is iterating over the triangles, not the vertices. I suggest the following:


for (int i = 0; i < m_vertices.Num() ; i++)
{
    m_uvmapping*.X = .5f - FMath::Atan2(m_vertices*.Y, m_vertices*.X) / (2 * PI);
    m_uvmapping*.Y = .5f - FMath::Asin(m_vertices*.Z / m_vertices*.Size()) / PI;
}

Normalization is only required for asin(z). The center point / offset Vector2D is (0.5f, 0.5f). Subtract atan2(y, x) and asin(z) from the offset so that your texture is not mirrored.

With this code, you will get a vertical line of blurred texturing. Depending on your requirements, the solution for this may vary. You can read the following for two suggestions.