Computing heat diffusion creates weird results

Question

I am trying to model heat diffusion on the surface of a mesh. I annexed the most important bits of theory about this topic as screenshots on the question see the bottom.

The crux of the issue is we are solving a linear system $(M - tL_C)u = \delta_y$. In this case $M$ is a diagonal matrix, $L_C$ is the discrete Laplace operator, $t$ is a constant time step equal to some epsilon and $\delta_y$ is a sparse vector with only one non-zero entry which is equal to 1.

My expectation is that, upon solving this system and drawing the resulting vertex distances, I get a fading from white to black starting at the source vertex and slowly fading away as the vertices go away from the source. I am however seeing this:

Which looks like random noise to me, as if I messed up the calculation.

I implemented the solver using Eigen, this is the snippet:

Eigen::DiagonalMatrix<float, Eigen::Dynamic> areas(sources.size());
    Eigen::SparseMatrix<float> laplace_operator(sources.size(), sources.size());

    for(uint i = 0; i < sources.size(); i++)
        laplace_operator.insert(i, i) = 0;

    areas.setZero();

    for(uint e = 0; e < mesh.Edges().size(); e += 2)
    {
        const auto& edge = mesh.Edges()[e];
        const uint i = edge.Vert().ID();
        const uint j = edge.Pair().Vert().ID();

        const Eigen::Vector3f e1 = -edge.Next().Dir().normalized();
        const Eigen::Vector3f e2 = edge.Prev().Dir().normalized();

        const float alpha = acos(e1.dot(e2));

        const Eigen::Vector3f p1 = -edge.Pair().Next().Dir().normalized();
        const Eigen::Vector3f p2 = edge.Pair().Prev().Dir().normalized();

        const float beta = acos(p1.dot(p2));

        const float laplace_coeff = -0.5f * ((1.f / tan(alpha)) + (1.f / tan(beta)));

        laplace_operator.insert(i, j) = laplace_coeff;
        laplace_operator.insert(j, i) = laplace_coeff;

        laplace_operator.coeffRef(i, i) -= laplace_coeff;
        laplace_operator.coeffRef(j, j) -= laplace_coeff;
    }

    uint count = 0;
    for(auto& v: mesh.Verts())
    {
        std::vector<HMesh<VertexData>::MFace*> faces = v.ContainingFaces();
        for(auto f: faces)
        {
            areas.diagonal()[count] += f->Area();
        }

        areas.diagonal()[count] /= 3.f;

        count++;
    }

    SparseLU<SparseMatrix<float>, COLAMDOrdering<int> > solver;

    const Eigen::SparseMatrix<float> a =
        Eigen::SparseMatrix<float>(areas) - 0.01 * laplace_operator;
    solver.analyzePattern(a);
    solver.factorize(a);

    return solver.solve(sources);

I have checked and sources is indeed a sparse vector with a singular source vertex set to 1 so the input is correct. The mesh operations that fetch values are very tested and they are returning what is expected, so I know the error is in the logic of this function.

I suspect my mistake might be creating the laplace operator, but I am not sure.

Answer 1

I messed up I am doing a double negative and that is causing the problem:

const float laplace_coeff = -0.5f * ((1.f / tan(alpha)) + (1.f / tan(beta)));

This should be positive.