Sphere Texture Mapping shows visible seams

Question

As you can see from the above picture there is a visible seam in the texture mapping. The underlying mesh is a geosphere based on octahedron subdivisions. On that particular latitude, vertices have been duplicated. However there still is a visible seam.

Here is how I calculate the UV coordinates:

float longitude = (float)Math.Atan2(normal.X, -normal.Z);
float latitude = (float)Math.Acos(normal.Y);

float u = (float)(longitude / (Math.PI * 2.0) + 0.5);
float v = (float)(latitude / Math.PI);

Is this a problem in the coordinates or a mipmapping issue?

Edit: the GeoSphere is generated using SharpDX's Toolkit code. My SamplerState is set to Clamp. Further, I am using a normal mapping shader.

Could it be that the tangent of the duplicated vertices needs to be altered? I am using a Vector4 element for it, where w is the handedness. If so, how should it be modified?

Edit2: this is the code used to duplicate the vertices along the seam

int preCount = vertices.Count;
var indicesArray = indexList.ToArray();
fixed (void* pIndices = indicesArray)
{
    indices = (int*)pIndices;

    for (int i = 0; i < preCount; ++i)
    {
        // This vertex is on the prime meridian if position.x and texcoord.u are both zero (allowing for small epsilon).
        bool isOnPrimeMeridian = MathUtil.WithinEpsilon(vertices[i].Position.X, 0, XMVectorSplatEpsilon)
                                 && MathUtil.WithinEpsilon(vertices[i].TextureCoordinate.X, 0, XMVectorSplatEpsilon);

        if (isOnPrimeMeridian)
        {
            int newIndex = vertices.Count;

            // copy this vertex, correct the texture coordinate, and add the vertex
            VertexPositionNormalTexture v = vertices[i];
            v.TextureCoordinate.X = 1.0f;
            vertices.Add(v);

            // Now find all the triangles which contain this vertex and update them if necessary
            for (int j = 0; j < indexList.Count; j += 3)
            {
                var triIndex0 = &indices[j + 0];
                var triIndex1 = &indices[j + 1];
                var triIndex2 = &indices[j + 2];

                if (*triIndex0 == i)
                {
                    // nothing; just keep going
                }
                else if (*triIndex1 == i)
                {
                    Utilities.Swap(ref *triIndex0, ref *triIndex1);
                    Utilities.Swap(ref *triIndex1, ref *triIndex2);
                }
                else if (*triIndex2 == i)
                {
                    Utilities.Swap(ref *triIndex0, ref *triIndex2);
                    Utilities.Swap(ref *triIndex1, ref *triIndex2);
                }
                else
                {
                    // this triangle doesn't use the vertex we're interested in
                    continue;
                }

                // check the other two vertices to see if we might need to fix this triangle
                if (Math.Abs(vertices[*triIndex0].TextureCoordinate.X - vertices[*triIndex1].TextureCoordinate.X) > 0.5f ||
                    Math.Abs(vertices[*triIndex0].TextureCoordinate.X - vertices[*triIndex2].TextureCoordinate.X) > 0.5f)
                {
                    // yep; replace the specified index to point to the new, corrected vertex
                    indices[j + 0] = newIndex;
                }
            }
        }
    }

    FixPole(northPoleIndex);
    FixPole(southPoleIndex);

    // Clear indices as it will not be accessible outside the fixed statement
    indices = (int*)0;
}

Answer 1

This problem may be related to mipmapping. The pixels along the edge are being mapped to a much more copressed LOD than the rest of the sphere, so the entire map is actually being compressed into that line.

You may try putting in a tex2Dlod call in the shader and forcing the LOD to 0:

return tex2Dlod(TextureSampler, float4(u, v, 0, 0));

Explanation and possible solution for this over was found on stackoverflow: https://stackoverflow.com/questions/19723698/sphere-texture-mapping-error