mne_bem_surface.cpp

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//=============================================================================================================
 
//=============================================================================================================
// INCLUDES
//=============================================================================================================
 
#include "mne_bem_surface.h"
 
//=============================================================================================================
// USED NAMESPACES
//=============================================================================================================
 
using namespace MNELIB;
using namespace Eigen;
using namespace FIFFLIB;
 
//=============================================================================================================
// DEFINE MEMBER METHODS
//=============================================================================================================
 
MNEBemSurface::MNEBemSurface()
: MNESurface()
, tri_cent(MatrixX3d::Zero(0, 3))
, tri_nn(MatrixX3d::Zero(0, 3))
, tri_area(VectorXd::Zero(0))
{
    id = -1;
    np = -1;
    ntri = -1;
    coord_frame = -1;
    sigma = -1;
}
 
//=============================================================================================================
 
MNEBemSurface::MNEBemSurface(const MNEBemSurface& p_MNEBemSurface)
: MNESurface()
, tri_cent(p_MNEBemSurface.tri_cent)
, tri_nn(p_MNEBemSurface.tri_nn)
, tri_area(p_MNEBemSurface.tri_area)
{
    id = p_MNEBemSurface.id;
    np = p_MNEBemSurface.np;
    ntri = p_MNEBemSurface.ntri;
    coord_frame = p_MNEBemSurface.coord_frame;
    sigma = p_MNEBemSurface.sigma;
    rr = p_MNEBemSurface.rr;
    nn = p_MNEBemSurface.nn;
    itris = p_MNEBemSurface.itris;
    neighbor_tri = p_MNEBemSurface.neighbor_tri;
    neighbor_vert = p_MNEBemSurface.neighbor_vert;
}
 
//=============================================================================================================
 
MNEBemSurface::~MNEBemSurface()
{
}
 
//=============================================================================================================
 
void MNEBemSurface::clear()
{
    id = -1;
    np = -1;
    ntri = -1;
    coord_frame = -1;
    sigma = -1;
    rr.resize(0, 3);
    nn.resize(0, 3);
    itris.resize(0, 3);
    tri_cent = MatrixX3d::Zero(0, 3);
    tri_nn = MatrixX3d::Zero(0, 3);
    tri_area = VectorXd::Zero(0);
    neighbor_tri.clear();
    neighbor_vert.clear();
}
 
//=============================================================================================================
 
bool MNEBemSurface::addTriangleData()
{
    //
    //   Main triangulation
    //
    qInfo("\tCompleting triangulation info...");
    this->tri_cent = MatrixX3d::Zero(this->ntri, 3);
    this->tri_nn = MatrixX3d::Zero(this->ntri, 3);
    this->tri_area = VectorXd::Zero(this->ntri);
 
    Matrix3d r;
    Vector3d a, b;
    int k = 0;
    float size = 0;
    for (qint32 i = 0; i < this->ntri; ++i) {
        for (qint32 j = 0; j < 3; ++j) {
            k = this->itris(i, j);
 
            r(j, 0) = this->rr(k, 0);
            r(j, 1) = this->rr(k, 1);
            r(j, 2) = this->rr(k, 2);
 
            this->tri_cent(i, 0) += this->rr(k, 0);
            this->tri_cent(i, 1) += this->rr(k, 1);
            this->tri_cent(i, 2) += this->rr(k, 2);
        }
        this->tri_cent.row(i) /= 3.0f;
 
        //cross product {cross((r2-r1),(r3-r1))}
        a = (r.row(1) - r.row(0)).transpose();
        b = (r.row(2) - r.row(0)).transpose();
        this->tri_nn(i, 0) = a(1) * b(2) - a(2) * b(1);
        this->tri_nn(i, 1) = a(2) * b(0) - a(0) * b(2);
        this->tri_nn(i, 2) = a(0) * b(1) - a(1) * b(0);
 
        //area
        size = this->tri_nn.row(i) * this->tri_nn.row(i).transpose();
        size = std::pow(size, 0.5f);
 
        this->tri_area(i) = size / 2.0f;
        this->tri_nn.row(i) /= size;
    }
 
    qInfo("Adding additional geometry info\n");
    add_geometry_info();
 
    qInfo("[done]\n");
 
    return true;
}
 
//=============================================================================================================
 
bool MNEBemSurface::add_geometry_info()
{
    int k, c, p, q;
    bool found;
 
    //Create neighboring triangle vector using temporary std::vector for efficient appending
    {
        // One list per vertex, as mne-python's neighbor_tri
        std::vector<std::vector<int>> temp_ntri(this->np);
        for (p = 0; p < this->itris.rows(); p++) {
            for (k = 0; k < 3; k++) {
                temp_ntri[this->itris(p, k)].push_back(p);
            }
        }
        neighbor_tri.resize(this->np);
        for (k = 0; k < static_cast<int>(temp_ntri.size()); k++) {
            neighbor_tri[k] = Eigen::Map<Eigen::VectorXi>(temp_ntri[k].data(), temp_ntri[k].size());
        }
    }
 
    //Create the neighboring vertices vector using temporary std::vector
    {
        std::vector<std::vector<int>> temp_nvert(this->np);
        for (k = 0; k < this->np; k++) {
            for (p = 0; p < neighbor_tri[k].size(); p++) {
                //Fit in the other vertices of the neighboring triangle
                for (c = 0; c < 3; c++) {
                    int vert = this->itris(neighbor_tri[k][p], c);
 
                    if (vert != k) {
                        found = false;
 
                        for (q = 0; q < static_cast<int>(temp_nvert[k].size()); q++) {
                            if (temp_nvert[k][q] == vert) {
                                found = true;
                                break;
                            }
                        }
 
                        if (!found) {
                            temp_nvert[k].push_back(vert);
                        }
                    }
                }
            }
        }
        neighbor_vert.resize(this->np);
        for (k = 0; k < this->np; k++) {
            neighbor_vert[k] = Eigen::Map<Eigen::VectorXi>(temp_nvert[k].data(), temp_nvert[k].size());
        }
    }
 
    return true;
}
 
//=============================================================================================================
 
bool MNEBemSurface::addVertexNormals()
{
    //
    //   Accumulate the vertex normals from scratch (MNE-C mne_add_vertex_normals)
    //
    this->nn = NormalsT::Zero(this->np, 3);
 
    for (qint32 p = 0; p < this->ntri; ++p) //check each triangle
    {
        for (qint32 j = 0; j < 3; ++j) {
            int nodenr;
            nodenr = this->itris(p, j);                //find the corners(nodes) of the triangles
            this->nn(nodenr, 0) += this->tri_nn(p, 0); //add the triangle normal to the nodenormal
            this->nn(nodenr, 1) += this->tri_nn(p, 1);
            this->nn(nodenr, 2) += this->tri_nn(p, 2);
        }
    }
 
    // normalize
    for (qint32 p = 0; p < this->np; ++p) {
        const float size = this->nn.row(p).norm();
        if (size > 0.0f)
            this->nn.row(p) /= size;
    }
 
    return true;
}
 
 
//=============================================================================================================
 
QString MNEBemSurface::id_name(int id)
{
    switch (id) {
        case FIFFV_BEM_SURF_ID_BRAIN:
            return "Brain";
        case FIFFV_BEM_SURF_ID_SKULL:
            return "Skull";
        case FIFFV_BEM_SURF_ID_HEAD:
            return "Head";
        case FIFFV_BEM_SURF_ID_UNKNOWN:
            return "Unknown";
        default:
            return "Unknown";
    }
}
 
//=============================================================================================================

static double edgeLenSq(const Eigen::MatrixX3d& rr, int v0, int v1)
{
    return (rr.row(v0) - rr.row(v1)).squaredNorm();
}
 
//=============================================================================================================
 
QList<MNEBemSurface> MNEBemSurface::makeScalpSurfaces(
    const MNEBemSurface& outerSkin,
    const QList<int>& targetVertices)
{
    QList<MNEBemSurface> results;
 
    if (outerSkin.np <= 0 || outerSkin.ntri <= 0) {
        qWarning("MNEBemSurface::makeScalpSurfaces - Input surface is empty.");
        return results;
    }
 
    for (int target : targetVertices) {
        if (target >= outerSkin.np) {
            // No decimation needed, just copy
            results.append(MNEBemSurface(outerSkin));
            continue;
        }
 
        // Work on a copy
        MNEBemSurface surf(outerSkin);
 
        // Build vertex-referenced rr as doubles, triangles as ints
        // Use iterative edge collapse: find shortest edge, collapse to midpoint
        Eigen::MatrixX3d rr = surf.rr.cast<double>();
        Eigen::MatrixX3d nn = surf.nn.cast<double>();
 
        // Build triangle list as std::vector for easy manipulation
        int nTri = surf.ntri;
        std::vector<std::array<int, 3>> tris(nTri);
        for (int i = 0; i < nTri; ++i) {
            tris[i] = {surf.itris(i, 0), surf.itris(i, 1), surf.itris(i, 2)};
        }
 
        // Track which vertices are alive
        int currentNp = surf.np;
        std::vector<bool> vertAlive(currentNp, true);
        // Redirect map: when a vertex is collapsed, redirect to its replacement
        std::vector<int> redirect(currentNp);
        for (int i = 0; i < currentNp; ++i)
            redirect[i] = i;
 
        auto resolve = [&](int v) -> int {
            while (redirect[v] != v)
                v = redirect[v];
            return v;
        };
 
        while (currentNp > target) {
            // Find shortest edge in the mesh
            double bestLen = std::numeric_limits<double>::max();
            int bestTri = -1;
            int bestEdge = -1;
 
            for (int t = 0; t < static_cast<int>(tris.size()); ++t) {
                int v0 = resolve(tris[t][0]);
                int v1 = resolve(tris[t][1]);
                int v2 = resolve(tris[t][2]);
                // Skip degenerate triangles
                if (v0 == v1 || v1 == v2 || v0 == v2)
                    continue;
 
                double d01 = edgeLenSq(rr, v0, v1);
                double d12 = edgeLenSq(rr, v1, v2);
                double d20 = edgeLenSq(rr, v2, v0);
 
                if (d01 < bestLen) {
                    bestLen = d01;
                    bestTri = t;
                    bestEdge = 0;
                }
                if (d12 < bestLen) {
                    bestLen = d12;
                    bestTri = t;
                    bestEdge = 1;
                }
                if (d20 < bestLen) {
                    bestLen = d20;
                    bestTri = t;
                    bestEdge = 2;
                }
            }
 
            if (bestTri < 0)
                break;
 
            // Collapse edge: merge the two vertices into the midpoint
            int va = resolve(tris[bestTri][bestEdge]);
            int vb = resolve(tris[bestTri][(bestEdge + 1) % 3]);
 
            // Place midpoint at va
            rr.row(va) = (rr.row(va) + rr.row(vb)) * 0.5;
            nn.row(va) = (nn.row(va) + nn.row(vb)).normalized();
 
            // Redirect vb -> va
            redirect[vb] = va;
            vertAlive[vb] = false;
            currentNp--;
        }
 
        // Rebuild compact vertex array and triangle array
        std::vector<int> oldToNew(surf.np, -1);
        int newNp = 0;
        for (int i = 0; i < surf.np; ++i) {
            if (vertAlive[i] && resolve(i) == i) {
                oldToNew[i] = newNp++;
            }
        }
 
        MNEBemSurface decimated;
        decimated.id = surf.id;
        decimated.coord_frame = surf.coord_frame;
        decimated.sigma = surf.sigma;
        decimated.np = newNp;
 
        // Use Eigen types matching MNESurfaceOrVolume
        MNESurfaceOrVolume::PointsT newRr(newNp, 3);
        Eigen::MatrixX3f newNn(newNp, 3);
        for (int i = 0; i < surf.np; ++i) {
            if (oldToNew[i] >= 0) {
                newRr.row(oldToNew[i]) = rr.row(i).cast<float>();
                newNn.row(oldToNew[i]) = nn.row(i).cast<float>();
            }
        }
        decimated.rr = newRr;
        decimated.nn = newNn;
 
        // Rebuild triangles
        std::vector<std::array<int, 3>> newTris;
        for (const auto& tri : tris) {
            int a = oldToNew[resolve(tri[0])];
            int b = oldToNew[resolve(tri[1])];
            int c = oldToNew[resolve(tri[2])];
            if (a >= 0 && b >= 0 && c >= 0 && a != b && b != c && a != c) {
                newTris.push_back({a, b, c});
            }
        }
 
        decimated.ntri = static_cast<int>(newTris.size());
        MNESurfaceOrVolume::TrianglesT newItris(decimated.ntri, 3);
        for (int i = 0; i < decimated.ntri; ++i) {
            newItris(i, 0) = newTris[i][0];
            newItris(i, 1) = newTris[i][1];
            newItris(i, 2) = newTris[i][2];
        }
        decimated.itris = newItris;
 
        // Recompute triangle data
        decimated.addTriangleData();
 
        results.append(decimated);
    }
 
    return results;
}