mesh_export.cpp

#include "mesh_export.h"

#include "print_remesh.h"

#include "xatlas.h"

#define STB_IMAGE_WRITE_IMPLEMENTATION
#include "stb_image_write.h"

#include <algorithm>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <functional>
#include <mutex>
#include <unordered_map>

namespace t2glb {
namespace {

// Stage logging to stderr, opt-in via T2GLB_VERBOSE (keeps the library quiet by
// default while giving the CLI / debugging a progress trace).
bool verbose() { static bool v = std::getenv("T2GLB_VERBOSE") != nullptr; return v; }
#define GLBLOG(...) do { if (verbose()) { std::fprintf(stderr, "[glb] " __VA_ARGS__); std::fprintf(stderr, "\n"); } } while (0)

// ─────────────────────────────────────────────────────────────────────────
// Copy the original topology while dropping only invalid/degenerate faces and
// unreferenced vertices. Export and showcase deliberately retain full polygon
// density; component cleanup below is the only operation allowed to delete
// valid faces.
// ─────────────────────────────────────────────────────────────────────────
bool copy_mesh(const float * verts, int nv, const int32_t * tris, int nt,
               const float * pbr, std::vector<float> & dverts,
               std::vector<int32_t> & dtris, std::vector<float> & dpbr,
               std::string & err) {
    std::vector<int> remap((size_t) nv, -1);
    dverts.clear(); dverts.reserve((size_t) nv * 3);
    dtris.clear(); dtris.reserve((size_t) nt * 3);
    dpbr.clear(); if (pbr) dpbr.reserve((size_t) nv * 6);
    for (int t = 0; t < nt; ++t) {
        const int32_t a = tris[3*t], b = tris[3*t+1], c = tris[3*t+2];
        if (a < 0 || b < 0 || c < 0 || a >= nv || b >= nv || c >= nv) {
            err = "triangle index out of range";
            return false;
        }
        if (a == b || b == c || a == c) continue;
        for (int32_t v : {a, b, c}) {
            if (remap[v] < 0) {
                remap[v] = (int)(dverts.size() / 3);
                dverts.push_back(verts[3*v+0]); dverts.push_back(verts[3*v+1]); dverts.push_back(verts[3*v+2]);
                if (pbr) dpbr.insert(dpbr.end(), pbr + (size_t) v * 6, pbr + (size_t) v * 6 + 6);
            }
            dtris.push_back(remap[v]);
        }
    }
    return true;
}

// Remove triangles belonging to small connected components (islands). The
// dual-grid surface can contain tiny disconnected bits that each force a
// separate UV chart and may correspond to unwanted background geometry.
// Mirrors the reference's remove_small_connected_components. Keeps components
// with at least `min_frac` of the triangles; recompacts verts/tris in place.
void filter_components(std::vector<float> & v, std::vector<int32_t> & f,
                       std::vector<float> & pbr,
                       float min_frac, ComponentFilter mode) {
    if (mode == ComponentFilter::KeepAll) return;
    const int nv = (int)(v.size() / 3), nt = (int)(f.size() / 3);
    if (nv == 0 || nt == 0) return;
    std::vector<int> parent((size_t) nv), sz((size_t) nv, 1);
    for (int i = 0; i < nv; ++i) parent[i] = i;
    std::function<int(int)> find = [&](int x) { while (parent[x] != x) { parent[x] = parent[parent[x]]; x = parent[x]; } return x; };
    auto uni = [&](int a, int b) { a = find(a); b = find(b); if (a==b) return; if (sz[a]<sz[b]) std::swap(a,b); parent[b]=a; sz[a]+=sz[b]; };
    for (int t = 0; t < nt; ++t) { uni(f[3*t+0], f[3*t+1]); uni(f[3*t+1], f[3*t+2]); }

    std::unordered_map<int, int> tris_in;   // component root -> triangle count
    for (int t = 0; t < nt; ++t) tris_in[find(f[3*t+0])]++;
    const int keep_min = std::max(1, (int)(nt * min_frac));
    int largest = -1, largest_n = -1;
    for (const auto & it : tris_in) {
        if (it.second > largest_n) { largest = it.first; largest_n = it.second; }
    }

    std::vector<int> vremap((size_t) nv, -1);
    std::vector<float> nvts; nvts.reserve(v.size());
    std::vector<float> npbr; if (!pbr.empty()) npbr.reserve(pbr.size());
    std::vector<int32_t> nfs; nfs.reserve(f.size());
    for (int t = 0; t < nt; ++t) {
        const int root = find(f[3*t+0]);
        if (mode == ComponentFilter::KeepLargest ? root != largest
                                                  : tris_in[root] < keep_min) continue;
        for (int k = 0; k < 3; ++k) {
            int vi = f[3*t+k];
            if (vremap[vi] < 0) {
                vremap[vi] = (int)(nvts.size()/3);
                nvts.push_back(v[3*vi+0]); nvts.push_back(v[3*vi+1]); nvts.push_back(v[3*vi+2]);
                if (!pbr.empty())
                    npbr.insert(npbr.end(), pbr.begin() + (size_t) vi * 6,
                                pbr.begin() + (size_t) vi * 6 + 6);
            }
            nfs.push_back(vremap[vi]);
        }
    }
    if (!nfs.empty()) {
        v.swap(nvts); f.swap(nfs);
        if (!pbr.empty()) pbr.swap(npbr);
    }
}

// Taubin (λ|μ) smoothing optionally regularises zig-zag sliver triangles so
// per-face normals become coherent. Without this, xatlas may split a chart at
// every normal jump (tens of thousands of one-off charts).
// Taubin's alternating positive/negative passes smooth without the shrinkage of
// plain Laplacian. The texture is unaffected (it samples the dense source).
void taubin_smooth(std::vector<float> & v, const std::vector<int32_t> & f, int iters) {
    const int nv = (int)(v.size() / 3), nt = (int)(f.size() / 3);
    if (nv == 0 || nt == 0) return;
    std::vector<int> deg((size_t) nv, 0);
    // Accumulate neighbour sums per pass; build degree once.
    for (int t = 0; t < nt; ++t)
        for (int k = 0; k < 3; ++k) deg[f[3*t+k]] += 2; // each vertex in 2 edges of its triangle
    const float lambda = 0.5f, mu = -0.53f;
    std::vector<float> acc((size_t) nv * 3);
    auto pass = [&](float w) {
        std::fill(acc.begin(), acc.end(), 0.0f);
        for (int t = 0; t < nt; ++t) {
            int a = f[3*t+0], b = f[3*t+1], c = f[3*t+2];
            for (int e = 0; e < 3; ++e) {
                int i = (e==0?a:e==1?b:c), j = (e==0?b:e==1?c:a);
                acc[3*i+0]+=v[3*j+0]; acc[3*i+1]+=v[3*j+1]; acc[3*i+2]+=v[3*j+2];
                acc[3*j+0]+=v[3*i+0]; acc[3*j+1]+=v[3*i+1]; acc[3*j+2]+=v[3*i+2];
            }
        }
        for (int i = 0; i < nv; ++i) {
            if (deg[i] == 0) continue;
            float inv = 1.0f / deg[i];
            for (int k = 0; k < 3; ++k)
                v[3*i+k] += w * (acc[3*i+k]*inv - v[3*i+k]); // move toward neighbour centroid
        }
    };
    for (int it = 0; it < iters; ++it) { pass(lambda); pass(mu); }
}

// Area-weighted per-vertex normals over a mesh.
void vertex_normals(const std::vector<float> & v, const std::vector<int32_t> & f,
                    std::vector<float> & n) {
    const size_t nv = v.size() / 3;
    n.assign(nv * 3, 0.0f);
    for (size_t t = 0; t < f.size() / 3; ++t) {
        int a = f[3*t+0], b = f[3*t+1], c = f[3*t+2];
        float e1[3] = {v[3*b+0]-v[3*a+0], v[3*b+1]-v[3*a+1], v[3*b+2]-v[3*a+2]};
        float e2[3] = {v[3*c+0]-v[3*a+0], v[3*c+1]-v[3*a+1], v[3*c+2]-v[3*a+2]};
        float fn[3] = {e1[1]*e2[2]-e1[2]*e2[1], e1[2]*e2[0]-e1[0]*e2[2], e1[0]*e2[1]-e1[1]*e2[0]};
        for (int k : {a, b, c}) { n[3*k+0]+=fn[0]; n[3*k+1]+=fn[1]; n[3*k+2]+=fn[2]; }
    }
    for (size_t i = 0; i < nv; ++i) {
        float l = std::sqrt(n[3*i+0]*n[3*i+0]+n[3*i+1]*n[3*i+1]+n[3*i+2]*n[3*i+2]) + 1e-20f;
        n[3*i+0]/=l; n[3*i+1]/=l; n[3*i+2]/=l;
    }
}

// ─────────────────────────────────────────────────────────────────────────
// PNG encode to an in-memory byte vector.
// ─────────────────────────────────────────────────────────────────────────
void png_sink(void * ctx, void * data, int size) {
    auto * v = (std::vector<uint8_t> *) ctx;
    auto * p = (uint8_t *) data;
    v->insert(v->end(), p, p + size);
}
bool encode_png(int w, int h, int comp, const uint8_t * pix, std::vector<uint8_t> & out) {
    out.clear();
    return stbi_write_png_to_func(png_sink, &out, w, h, comp, pix, w * comp) != 0;
}

// ─────────────────────────────────────────────────────────────────────────
// glTF 2.0 binary (GLB) assembly. One buffer holding, in order: POSITION,
// NORMAL, TEXCOORD_0, indices, baseColor PNG, metallicRoughness PNG.
// ─────────────────────────────────────────────────────────────────────────
void put_u32(std::vector<uint8_t> & b, uint32_t v) {
    b.push_back(v & 0xFF); b.push_back((v>>8)&0xFF); b.push_back((v>>16)&0xFF); b.push_back((v>>24)&0xFF);
}
void pad4(std::vector<uint8_t> & b, uint8_t fill) { while (b.size() % 4) b.push_back(fill); }

void write_glb(const std::vector<float> & pos, const std::vector<float> & nrm,
               const std::vector<float> & uv, const std::vector<uint32_t> & idx,
               const std::vector<uint8_t> & basecolor_png,
               const std::vector<uint8_t> & metalrough_png,
               bool transparent,
               std::vector<uint8_t> & out) {
    const uint32_t nv = (uint32_t)(pos.size() / 3);
    const uint32_t ni = (uint32_t) idx.size();

    // Assemble the BIN buffer, tracking 4-aligned bufferView offsets.
    std::vector<uint8_t> bin;
    auto view = [&](const void * data, size_t bytes, uint32_t & off, uint32_t & len) {
        pad4(bin, 0);
        off = (uint32_t) bin.size();
        len = (uint32_t) bytes;
        const uint8_t * p = (const uint8_t *) data;
        bin.insert(bin.end(), p, p + bytes);
    };
    uint32_t off_pos, len_pos, off_nrm, len_nrm, off_uv, len_uv, off_idx, len_idx;
    uint32_t off_bc, len_bc, off_mr, len_mr;
    view(pos.data(), pos.size()*4, off_pos, len_pos);
    view(nrm.data(), nrm.size()*4, off_nrm, len_nrm);
    view(uv.data(),  uv.size()*4,  off_uv,  len_uv);
    view(idx.data(), idx.size()*4, off_idx, len_idx);
    view(basecolor_png.data(), basecolor_png.size(), off_bc, len_bc);
    view(metalrough_png.data(), metalrough_png.size(), off_mr, len_mr);
    pad4(bin, 0);

    float pmin[3] = {1e30f,1e30f,1e30f}, pmax[3] = {-1e30f,-1e30f,-1e30f};
    for (uint32_t i = 0; i < nv; ++i)
        for (int k = 0; k < 3; ++k) { pmin[k]=std::min(pmin[k],pos[3*i+k]); pmax[k]=std::max(pmax[k],pos[3*i+k]); }

    char buf[2048];
    std::string j = "{\"asset\":{\"version\":\"2.0\",\"generator\":\"trellis2cpp\"},";
    j += "\"scene\":0,\"scenes\":[{\"nodes\":[0]}],\"nodes\":[{\"mesh\":0}],";
    j += "\"meshes\":[{\"primitives\":[{\"attributes\":{\"POSITION\":0,\"NORMAL\":1,\"TEXCOORD_0\":2},\"indices\":3,\"material\":0}]}],";
    j += "\"materials\":[{\"pbrMetallicRoughness\":{\"baseColorTexture\":{\"index\":0},\"metallicRoughnessTexture\":{\"index\":1},\"metallicFactor\":1.0,\"roughnessFactor\":1.0},";
    if (transparent) j += "\"alphaMode\":\"BLEND\",";
    j += "\"doubleSided\":true}],";
    j += "\"textures\":[{\"source\":0,\"sampler\":0},{\"source\":1,\"sampler\":0}],";
    j += "\"images\":[{\"bufferView\":4,\"mimeType\":\"image/png\"},{\"bufferView\":5,\"mimeType\":\"image/png\"}],";
    // No mipmaps: opt-in xatlas charts use explicit dilation around their seams.
    j += "\"samplers\":[{\"magFilter\":9729,\"minFilter\":9729,\"wrapS\":33071,\"wrapT\":33071}],";
    snprintf(buf, sizeof buf,
        "\"accessors\":[{\"bufferView\":0,\"componentType\":5126,\"count\":%u,\"type\":\"VEC3\",\"min\":[%.8g,%.8g,%.8g],\"max\":[%.8g,%.8g,%.8g]},"
        "{\"bufferView\":1,\"componentType\":5126,\"count\":%u,\"type\":\"VEC3\"},"
        "{\"bufferView\":2,\"componentType\":5126,\"count\":%u,\"type\":\"VEC2\"},"
        "{\"bufferView\":3,\"componentType\":5125,\"count\":%u,\"type\":\"SCALAR\"}],",
        nv, pmin[0],pmin[1],pmin[2], pmax[0],pmax[1],pmax[2], nv, nv, ni);
    j += buf;
    snprintf(buf, sizeof buf,
        "\"bufferViews\":[{\"buffer\":0,\"byteOffset\":%u,\"byteLength\":%u,\"target\":34962},"
        "{\"buffer\":0,\"byteOffset\":%u,\"byteLength\":%u,\"target\":34962},"
        "{\"buffer\":0,\"byteOffset\":%u,\"byteLength\":%u,\"target\":34962},"
        "{\"buffer\":0,\"byteOffset\":%u,\"byteLength\":%u,\"target\":34963},"
        "{\"buffer\":0,\"byteOffset\":%u,\"byteLength\":%u},"
        "{\"buffer\":0,\"byteOffset\":%u,\"byteLength\":%u}],",
        off_pos,len_pos, off_nrm,len_nrm, off_uv,len_uv, off_idx,len_idx, off_bc,len_bc, off_mr,len_mr);
    j += buf;
    snprintf(buf, sizeof buf, "\"buffers\":[{\"byteLength\":%u}]}", (uint32_t) bin.size());
    j += buf;

    std::vector<uint8_t> json(j.begin(), j.end());
    while (json.size() % 4) json.push_back(0x20); // pad JSON chunk with spaces

    const uint32_t total = 12 + 8 + (uint32_t) json.size() + 8 + (uint32_t) bin.size();
    out.clear();
    out.reserve(total);
    put_u32(out, 0x46546C67); put_u32(out, 2); put_u32(out, total);          // header
    put_u32(out, (uint32_t) json.size()); put_u32(out, 0x4E4F534A);          // JSON chunk
    out.insert(out.end(), json.begin(), json.end());
    put_u32(out, (uint32_t) bin.size()); put_u32(out, 0x004E4942);           // BIN chunk
    out.insert(out.end(), bin.begin(), bin.end());
}

// Portable full-density export. glTF natively supports interpolated RGBA
// vertex colours, which are a much better representation for TRELLIS' dense
// per-vertex material field than assigning millions of triangles to a finite
// 2D atlas. Metallic/roughness have no standard per-vertex glTF semantics, so
// their averages drive the standard material while the original quantised
// values are retained in the application attribute _METALLIC_ROUGHNESS.
void write_vertex_glb(const PreparedMesh & mesh, std::vector<uint8_t> & out) {
    const uint32_t nv = (uint32_t)(mesh.verts.size() / 3);
    const uint32_t ni = (uint32_t) mesh.tris.size();
    const bool textured = mesh.pbr.size() == (size_t) nv * 6;
    auto to8 = [](float f) {
        int v = (int) std::lround(f * 255.0f);
        return (uint8_t) std::min(255, std::max(0, v));
    };
    auto clamp01 = [](float f) { return std::min(1.0f, std::max(0.0f, f)); };
    auto srgb_to_linear = [&](float f) {
        f = clamp01(f);
        return f <= 0.04045f ? f / 12.92f
                             : std::pow((f + 0.055f) / 1.055f, 2.4f);
    };
    auto to16 = [](float f) {
        int v = (int) std::lround(f * 65535.0f);
        return (uint16_t) std::min(65535, std::max(0, v));
    };

    std::vector<float> pos((size_t) nv * 3), nrm((size_t) nv * 3);
    // glTF COLOR_0 is linear rather than sRGB. Sixteen-bit UNORM keeps dark
    // generated colours precise after converting from the texture model's
    // sRGB-like output.
    std::vector<uint16_t> color((size_t) nv * 4);
    std::vector<uint8_t> metalrough((size_t) nv * 2);
    std::vector<uint32_t> idx(ni);
    double metal_sum = 0.0, rough_sum = 0.0, weight_sum = 0.0;
    uint32_t translucent = 0;
    for (uint32_t i = 0; i < nv; ++i) {
        // Trellis -> glTF Y-up, preserving handedness.
        pos[3*i+0]= mesh.verts[3*i+0]; pos[3*i+1]= mesh.verts[3*i+2]; pos[3*i+2]=-mesh.verts[3*i+1];
        nrm[3*i+0]= mesh.normals[3*i+0]; nrm[3*i+1]= mesh.normals[3*i+2]; nrm[3*i+2]=-mesh.normals[3*i+1];
        const float * p = textured ? mesh.pbr.data() + (size_t) i * 6 : nullptr;
        const float r = p ? p[0] : 0.7f, g = p ? p[1] : 0.7f, b = p ? p[2] : 0.7f;
        const float m = p ? clamp01(p[3]) : 0.0f;
        const float ro = p ? clamp01(p[4]) : 0.6f;
        const float a = p ? clamp01(p[5]) : 1.0f;
        color[4*i+0]=to16(srgb_to_linear(r)); color[4*i+1]=to16(srgb_to_linear(g));
        color[4*i+2]=to16(srgb_to_linear(b)); color[4*i+3]=to16(a);
        metalrough[2*i+0]=to8(m); metalrough[2*i+1]=to8(ro);
        // Near-transparent samples should not skew the visible material's
        // standard fallback factors.
        const double w = std::max(0.001f, a);
        metal_sum += m * w; rough_sum += ro * w; weight_sum += w;
        if (a < 0.95f) ++translucent;
    }
    for (uint32_t i = 0; i < ni; ++i) idx[i] = (uint32_t) mesh.tris[i];
    const float metallic = weight_sum ? (float)(metal_sum / weight_sum) : 0.0f;
    const float roughness = weight_sum ? (float)(rough_sum / weight_sum) : 0.6f;
    // BLEND disables normal opaque depth writes in many viewers. Do not switch
    // an entire multi-million-face primitive to blending because of a handful
    // of near-one decoder outliers; upstream likewise exports ordinary surfaces
    // as opaque. Keep blending for a material with meaningful transparency.
    const bool transparent = translucent > 0 &&
                             (uint64_t) translucent * 1000 >= (uint64_t) nv;

    std::vector<uint8_t> bin;
    auto view = [&](const void * data, size_t bytes, uint32_t & off, uint32_t & len) {
        pad4(bin, 0);
        off = (uint32_t) bin.size(); len = (uint32_t) bytes;
        const uint8_t * p = (const uint8_t *) data;
        bin.insert(bin.end(), p, p + bytes);
    };
    uint32_t off_pos, len_pos, off_nrm, len_nrm, off_col, len_col;
    uint32_t off_mr, len_mr, off_idx, len_idx;
    view(pos.data(), pos.size()*4, off_pos, len_pos);
    view(nrm.data(), nrm.size()*4, off_nrm, len_nrm);
    view(color.data(), color.size()*sizeof(uint16_t), off_col, len_col);
    view(metalrough.data(), metalrough.size(), off_mr, len_mr);
    view(idx.data(), idx.size()*4, off_idx, len_idx);
    pad4(bin, 0);

    float pmin[3] = {1e30f,1e30f,1e30f}, pmax[3] = {-1e30f,-1e30f,-1e30f};
    for (uint32_t i = 0; i < nv; ++i)
        for (int k = 0; k < 3; ++k) {
            pmin[k]=std::min(pmin[k],pos[3*i+k]); pmax[k]=std::max(pmax[k],pos[3*i+k]);
        }

    char buf[3072];
    std::string j = "{\"asset\":{\"version\":\"2.0\",\"generator\":\"trellis2cpp\"},";
    j += "\"scene\":0,\"scenes\":[{\"nodes\":[0]}],\"nodes\":[{\"mesh\":0}],";
    j += "\"meshes\":[{\"primitives\":[{\"attributes\":{\"POSITION\":0,\"NORMAL\":1,\"COLOR_0\":2,\"_METALLIC_ROUGHNESS\":3},\"indices\":4,\"material\":0}]}],";
    std::snprintf(buf, sizeof buf,
        "\"materials\":[{\"pbrMetallicRoughness\":{\"baseColorFactor\":[1,1,1,1],\"metallicFactor\":%.8g,\"roughnessFactor\":%.8g},",
        metallic, roughness);
    j += buf;
    if (transparent) j += "\"alphaMode\":\"BLEND\",";
    j += "\"doubleSided\":true}],";
    std::snprintf(buf, sizeof buf,
        "\"accessors\":[{\"bufferView\":0,\"componentType\":5126,\"count\":%u,\"type\":\"VEC3\",\"min\":[%.8g,%.8g,%.8g],\"max\":[%.8g,%.8g,%.8g]},"
        "{\"bufferView\":1,\"componentType\":5126,\"count\":%u,\"type\":\"VEC3\"},"
        "{\"bufferView\":2,\"componentType\":5123,\"normalized\":true,\"count\":%u,\"type\":\"VEC4\"},"
        "{\"bufferView\":3,\"componentType\":5121,\"normalized\":true,\"count\":%u,\"type\":\"VEC2\"},"
        "{\"bufferView\":4,\"componentType\":5125,\"count\":%u,\"type\":\"SCALAR\"}],",
        nv, pmin[0],pmin[1],pmin[2], pmax[0],pmax[1],pmax[2], nv, nv, nv, ni);
    j += buf;
    std::snprintf(buf, sizeof buf,
        "\"bufferViews\":[{\"buffer\":0,\"byteOffset\":%u,\"byteLength\":%u,\"target\":34962},"
        "{\"buffer\":0,\"byteOffset\":%u,\"byteLength\":%u,\"target\":34962},"
        "{\"buffer\":0,\"byteOffset\":%u,\"byteLength\":%u,\"target\":34962},"
        "{\"buffer\":0,\"byteOffset\":%u,\"byteLength\":%u,\"target\":34962},"
        "{\"buffer\":0,\"byteOffset\":%u,\"byteLength\":%u,\"target\":34963}],",
        off_pos,len_pos, off_nrm,len_nrm, off_col,len_col, off_mr,len_mr, off_idx,len_idx);
    j += buf;
    std::snprintf(buf, sizeof buf, "\"buffers\":[{\"byteLength\":%u}]}", (uint32_t) bin.size());
    j += buf;

    std::vector<uint8_t> json(j.begin(), j.end());
    while (json.size() % 4) json.push_back(0x20);
    const uint32_t total = 12 + 8 + (uint32_t) json.size() + 8 + (uint32_t) bin.size();
    out.clear(); out.reserve(total);
    put_u32(out, 0x46546C67); put_u32(out, 2); put_u32(out, total);
    put_u32(out, (uint32_t) json.size()); put_u32(out, 0x4E4F534A);
    out.insert(out.end(), json.begin(), json.end());
    put_u32(out, (uint32_t) bin.size()); put_u32(out, 0x004E4942);
    out.insert(out.end(), bin.begin(), bin.end());
}

// Chart-based unwrap via xatlas → per-output-vertex position/normal/uv (texel
// space) + index buffer + atlas dims. Proper editable charts, but only fast on
// clean, manifold meshes. Opt-in with T2GLB_XATLAS.
bool xatlas_unwrap(const std::vector<float> & dverts, const std::vector<float> & dnrm,
                   const std::vector<float> & dpbr, int dnv,
                   const std::vector<int32_t> & dtris, int dnt, const MeshExportOptions & opt, int TS,
                   std::vector<float> & opos, std::vector<float> & onrm, std::vector<float> & ouv,
                   std::vector<float> & opbr, std::vector<uint32_t> & oidx,
                   int & AW, int & AH, std::string & err) {
    xatlas::Atlas * atlas = xatlas::Create();
    xatlas::MeshDecl md{};
    md.vertexCount = (uint32_t) dnv;
    md.vertexPositionData = dverts.data();
    md.vertexPositionStride = sizeof(float) * 3;
    md.indexCount = (uint32_t)(dnt * 3);
    md.indexData = dtris.data();
    md.indexFormat = xatlas::IndexFormat::UInt32;
    if (xatlas::AddMesh(atlas, md) != xatlas::AddMeshError::Success) {
        xatlas::Destroy(atlas); err = "xatlas AddMesh failed"; return false;
    }
    GLBLOG("xatlas computing charts ...");
    xatlas::ChartOptions co{};
    co.normalDeviationWeight = 0.5f; co.roundnessWeight = 0.0f; co.straightnessWeight = 1.0f;
    co.normalSeamWeight = 1.0f; co.textureSeamWeight = 0.0f; co.maxCost = 8.0f; co.maxIterations = 1;
    xatlas::ComputeCharts(atlas, co);

    xatlas::PackOptions po{};
    po.padding = (uint32_t) opt.padding;
    po.bilinear = true;
    po.createImage = false;
    uint32_t res = (uint32_t) TS;
    for (int attempt = 0; attempt < 4; ++attempt) {
        po.resolution = res;
        xatlas::PackCharts(atlas, po);
        if (atlas->atlasCount <= 1) break;
        res = (uint32_t)(res * 1.5f);
    }
    GLBLOG("atlas %ux%u, %u pages, %u charts", atlas->width, atlas->height, atlas->atlasCount, atlas->chartCount);
    if (atlas->meshCount != 1 || atlas->atlasCount != 1 || atlas->width == 0 || atlas->height == 0) {
        xatlas::Destroy(atlas); err = "xatlas packing failed (charts did not fit one atlas)"; return false;
    }
    AW = (int) atlas->width; AH = (int) atlas->height;
    const xatlas::Mesh & om = atlas->meshes[0];
    const uint32_t onv = om.vertexCount;
    opos.resize(onv*3); onrm.resize(onv*3); ouv.resize(onv*2);
    if (!dpbr.empty()) opbr.resize(onv*6);
    for (uint32_t i = 0; i < onv; ++i) {
        uint32_t xr = om.vertexArray[i].xref;
        if (xr >= (uint32_t) dnv) xr = 0;
        opos[3*i+0]=dverts[3*xr+0]; opos[3*i+1]=dverts[3*xr+1]; opos[3*i+2]=dverts[3*xr+2];
        onrm[3*i+0]=dnrm[3*xr+0];   onrm[3*i+1]=dnrm[3*xr+1];   onrm[3*i+2]=dnrm[3*xr+2];
        ouv[2*i+0]=om.vertexArray[i].uv[0]; ouv[2*i+1]=om.vertexArray[i].uv[1];
        if (!dpbr.empty())
            std::memcpy(opbr.data() + (size_t) i * 6,
                        dpbr.data() + (size_t) xr * 6, 6 * sizeof(float));
    }
    oidx.assign(om.indexArray, om.indexArray + om.indexCount);
    xatlas::Destroy(atlas);
    return true;
}

// UV unwrap and raster bake.  In the ordinary xatlas mode, PBR is interpolated
// from the atlas mesh's own vertices.  For print wrapping, projection_source is
// the dense pre-wrap mesh: every covered atlas texel is mapped to a 3D point on
// the wrapped target, projected to the closest source triangle, and sampled
// barycentrically.  That mirrors upstream's remesh -> UV raster -> BVH ->
// attribute-field path without requiring CUDA.
bool bake_atlas_locked(const PreparedMesh & mesh,
                       const PreparedMesh * projection_source,
                       const MeshExportOptions & opt,
                       std::vector<uint8_t> & out,
                       std::string & err) {
    const int TS = opt.texture_size;
    const int dnv = (int) (mesh.verts.size() / 3);
    const int dnt = (int) (mesh.tris.size() / 3);
    const bool vertex_pbr = mesh.pbr.size() == (size_t) dnv * 6;
    const bool projected_pbr = projection_source != nullptr;
    if (projected_pbr &&
        projection_source->pbr.size() != projection_source->verts.size() / 3 * 6) {
        err = "PBR projection source has no six-channel material";
        return false;
    }

    std::vector<float> opos, onrm, ouv, opbr;
    std::vector<uint32_t> oidx;
    int AW = TS, AH = TS;
    if (!xatlas_unwrap(mesh.verts, mesh.normals, mesh.pbr, dnv,
                       mesh.tris, dnt, opt, TS,
                       opos, onrm, ouv, opbr, oidx, AW, AH, err))
        return false;
    const uint32_t onv = (uint32_t) (opos.size() / 3);
    const uint32_t ntri = (uint32_t) (oidx.size() / 3);

    GLBLOG("rasterizing %u tris%s ...", ntri,
           projected_pbr ? " for source-surface PBR projection" : "");
    const int NP = AW * AH;
    std::vector<float> bc((size_t) NP * 3, 0.0f), met(NP, 0.0f), rou(NP, 0.0f), alp(NP, 0.0f);
    std::vector<uint8_t> mask(NP, 0);
    std::vector<float> query_points;
    std::vector<int32_t> query_pixels;
    if (projected_pbr) {
        query_points.reserve((size_t) NP * 2); // atlases are normally 60-80% occupied
        query_pixels.reserve((size_t) NP * 2 / 3);
    }
    auto set_default = [&](int pix) {
        bc[3*pix+0]=bc[3*pix+1]=bc[3*pix+2]=0.7f;
        met[pix]=0.0f; rou[pix]=0.6f; alp[pix]=1.0f;
    };

    for (uint32_t t = 0; t < ntri; ++t) {
        const uint32_t ia = oidx[3*t+0], ib = oidx[3*t+1], ic = oidx[3*t+2];
        const float ax=ouv[2*ia+0], ay=ouv[2*ia+1];
        const float bx=ouv[2*ib+0], by=ouv[2*ib+1];
        const float cx=ouv[2*ic+0], cy=ouv[2*ic+1];
        const float area = (bx-ax)*(cy-ay) - (by-ay)*(cx-ax);
        if (std::fabs(area) < 1e-9f) continue;
        const float inv_area = 1.0f / area;
        const int x0 = std::max(0, (int)std::floor(std::min({ax,bx,cx}) - 1));
        const int x1 = std::min(AW-1, (int)std::ceil (std::max({ax,bx,cx}) + 1));
        const int y0 = std::max(0, (int)std::floor(std::min({ay,by,cy}) - 1));
        const int y1 = std::min(AH-1, (int)std::ceil (std::max({ay,by,cy}) + 1));
        for (int py = y0; py <= y1; ++py) {
            const float sy = py + 0.5f;
            for (int px = x0; px <= x1; ++px) {
                const float sx = px + 0.5f;
                const float w0 = ((bx-sx)*(cy-sy) - (by-sy)*(cx-sx)) * inv_area;
                const float w1 = ((cx-sx)*(ay-sy) - (cy-sy)*(ax-sx)) * inv_area;
                const float w2 = 1.0f - w0 - w1;
                const float e = -0.001f;
                if (w0 < e || w1 < e || w2 < e) continue;
                const int pix = py*AW + px;
                if (projected_pbr) {
                    // Shared triangle edges can cover the same pixel twice.
                    // The positions agree, so retain the first and issue one
                    // expensive closest-surface query per atlas texel.
                    if (mask[pix]) continue;
                    for (int k = 0; k < 3; ++k)
                        query_points.push_back(w0*opos[3*ia+k] +
                                               w1*opos[3*ib+k] +
                                               w2*opos[3*ic+k]);
                    query_pixels.push_back(pix);
                    mask[pix] = 1;
                    continue;
                }
                if (!vertex_pbr) {
                    set_default(pix); mask[pix]=1; continue;
                }
                const float * a = opbr.data() + (size_t) ia * 6;
                const float * b = opbr.data() + (size_t) ib * 6;
                const float * c = opbr.data() + (size_t) ic * 6;
                float val[6];
                for (int ch = 0; ch < 6; ++ch) val[ch] = w0*a[ch] + w1*b[ch] + w2*c[ch];
                bc[3*pix+0]=val[0]; bc[3*pix+1]=val[1]; bc[3*pix+2]=val[2];
                met[pix]=val[3]; rou[pix]=val[4]; alp[pix]=val[5];
                mask[pix]=1;
            }
        }
    }

    if (projected_pbr) {
        GLBLOG("projecting %zu covered texels to %zu source tris ...",
               query_pixels.size(), projection_source->tris.size() / 3);
        std::vector<float> query_pbr;
        if (!t2print::project_pbr(projection_source->verts,
                                  projection_source->tris,
                                  projection_source->pbr,
                                  query_points, query_pbr, err))
            return false;
        if (query_pbr.size() != query_pixels.size() * 6) {
            err = "PBR projection returned an invalid sample count";
            return false;
        }
        for (size_t i = 0; i < query_pixels.size(); ++i) {
            const int pix = query_pixels[i];
            const float * val = query_pbr.data() + i * 6;
            bc[3*pix+0]=val[0]; bc[3*pix+1]=val[1]; bc[3*pix+2]=val[2];
            met[pix]=val[3]; rou[pix]=val[4]; alp[pix]=val[5];
        }
        query_points.clear(); query_points.shrink_to_fit();
        query_pbr.clear(); query_pbr.shrink_to_fit();
    }

    // Edge-pad the gutter so bilinear sampling cannot bleed empty texels
    // across chart seams.
    {
        std::vector<uint8_t> m = mask;
        for (int pass = 0; pass < opt.dilate; ++pass) {
            std::vector<uint8_t> nm = m;
            for (int py = 0; py < AH; ++py)
            for (int px = 0; px < AW; ++px) {
                const int pix = py*AW+px;
                if (m[pix]) continue;
                float acc[6]={0,0,0,0,0,0}; int cnt=0;
                for (int dy=-1; dy<=1; ++dy)
                for (int dx=-1; dx<=1; ++dx) {
                    const int qx=px+dx, qy=py+dy;
                    if (qx<0||qx>=AW||qy<0||qy>=AH) continue;
                    const int q=qy*AW+qx;
                    if (!m[q]) continue;
                    acc[0]+=bc[3*q+0]; acc[1]+=bc[3*q+1]; acc[2]+=bc[3*q+2];
                    acc[3]+=met[q]; acc[4]+=rou[q]; acc[5]+=alp[q]; ++cnt;
                }
                if (cnt) {
                    bc[3*pix+0]=acc[0]/cnt; bc[3*pix+1]=acc[1]/cnt; bc[3*pix+2]=acc[2]/cnt;
                    met[pix]=acc[3]/cnt; rou[pix]=acc[4]/cnt; alp[pix]=acc[5]/cnt; nm[pix]=1;
                }
            }
            m.swap(nm);
        }
    }

    // glTF packs metallic in B and roughness in G. Base color is stored as
    // generated (sRGB-like), matching upstream's base_color*255 texture path.
    auto to8 = [](float f){ int v=(int)std::lround(f*255.0f); return (uint8_t) std::min(255,std::max(0,v)); };
    std::vector<uint8_t> bc8((size_t) NP*4), mr8((size_t) NP*3);
    for (int i = 0; i < NP; ++i) {
        bc8[4*i+0]=to8(bc[3*i+0]); bc8[4*i+1]=to8(bc[3*i+1]);
        bc8[4*i+2]=to8(bc[3*i+2]); bc8[4*i+3]=to8(alp[i]);
        mr8[3*i+0]=0; mr8[3*i+1]=to8(rou[i]); mr8[3*i+2]=to8(met[i]);
    }
    int covered = 0, translucent = 0;
    for (int i = 0; i < NP; ++i) if (mask[i]) {
        ++covered;
        if (alp[i] < 0.95f) ++translucent;
    }
    const bool transparent = translucent > 0 &&
                             (int64_t) translucent * 1000 >= (int64_t) covered;
    GLBLOG("inpaint + PNG encode ...");
    std::vector<uint8_t> bc_png, mr_png;
    if (!encode_png(AW, AH, 4, bc8.data(), bc_png) ||
        !encode_png(AW, AH, 3, mr8.data(), mr_png)) {
        err = "PNG encode failed"; return false;
    }

    std::vector<float> gpos((size_t)onv*3), gnrm((size_t)onv*3), guv((size_t)onv*2);
    for (uint32_t i = 0; i < onv; ++i) {
        gpos[3*i+0]= opos[3*i+0]; gpos[3*i+1]= opos[3*i+2]; gpos[3*i+2]=-opos[3*i+1];
        gnrm[3*i+0]= onrm[3*i+0]; gnrm[3*i+1]= onrm[3*i+2]; gnrm[3*i+2]=-onrm[3*i+1];
        guv[2*i+0]= ouv[2*i+0]/(float)AW; guv[2*i+1]= ouv[2*i+1]/(float)AH;
    }
    write_glb(gpos, gnrm, guv, oidx, bc_png, mr_png, transparent, out);
    GLBLOG("GLB %zu bytes (%u verts, %u tris; %s PBR atlas)",
           out.size(), onv, ntri, projected_pbr ? "projected" : "vertex");
    return true;
}

bool prepare_mesh_locked(const float * verts, int nv,
                         const int32_t * tris, int nt,
                         const float * pbr,
                         const MeshExportOptions & opt,
                         PreparedMesh & out,
                         std::string & err,
                         bool allow_atlas_smoothing = true) {
    if (!verts || !tris || nv <= 0 || nt <= 0) { err = "empty mesh"; return false; }
    out = PreparedMesh{};

    GLBLOG("input %d verts %d tris; preserving original topology", nv, nt);
    if (!copy_mesh(verts, nv, tris, nt, pbr, out.verts, out.tris, out.pbr, err)) return false;

    filter_components(out.verts, out.tris, out.pbr, 0.0005f, opt.components);
    const int dnv = (int)(out.verts.size()/3), dnt = (int)(out.tris.size()/3);
    if (dnv < 3 || dnt < 1) { err = "component cleanup produced an empty mesh"; return false; }
    GLBLOG("after component filter -> %d verts %d tris", dnv, dnt);

    // xatlas optionally regularises the geometry; do it here so the preview is
    // the geometry that will actually be written to the GLB.
    if (allow_atlas_smoothing && std::getenv("T2GLB_XATLAS") &&
        !std::getenv("T2GLB_NOSMOOTH"))
        taubin_smooth(out.verts, out.tris, 3);
    vertex_normals(out.verts, out.tris, out.normals);
    return true;
}

std::mutex g_bake_mu; // serialize bakes (bounds peak RAM; keeps stb/xatlas tidy)

} // namespace

bool prepare_mesh(const float * verts, int nv,
                  const int32_t * tris, int nt,
                  const float * pbr,
                  const MeshExportOptions & opt,
                  PreparedMesh & out,
                  std::string & err) {
    std::lock_guard<std::mutex> lock(g_bake_mu);
    return prepare_mesh_locked(verts, nv, tris, nt, pbr, opt, out, err);
}

bool print_remesh_available() { return t2print::available(); }

bool prepare_print_mesh(const float * verts, int nv,
                        const int32_t * tris, int nt,
                        const float * pbr,
                        const MeshExportOptions & opt,
                        float alpha_ratio, float offset_ratio,
                        PreparedMesh & out,
                        std::string & err) {
    // Carry the source material through component filtering so it can be sampled
    // onto the wrap for a textured preview (source.pbr stays aligned with
    // source.verts/source.tris; empty when the caller passed no material).
    PreparedMesh source;
    if (!prepare_mesh(verts, nv, tris, nt, pbr, opt, source, err)) return false;

    PreparedMesh wrapped;
    if (!t2print::alpha_wrap(source.verts, source.tris, alpha_ratio, offset_ratio,
                             wrapped.verts, wrapped.normals, wrapped.tris, err))
        return false;
    // Alpha Wrap constructs entirely new offset vertices, so source per-vertex
    // attributes cannot be carried directly. When the source is textured, sample
    // its material at each wrap vertex by closest-surface projection — the same
    // transfer the GLB download bakes per texel, here per vertex for a cheap,
    // approximate preview. Projection is best-effort: on failure the wrap still
    // previews untextured rather than aborting the print preview entirely.
    if (!source.pbr.empty() && source.pbr.size() == source.verts.size() / 3 * 6) {
        std::string perr;
        if (!t2print::project_pbr(source.verts, source.tris, source.pbr,
                                  wrapped.verts, wrapped.pbr, perr)) {
            wrapped.pbr.clear();
        }
    } else {
        wrapped.pbr.clear();
    }
    out = std::move(wrapped);
    return true;
}

bool mesh_to_glb(const float * verts, int nv,
                 const int32_t * tris, int nt,
                 const float * pbr,
                 const MeshExportOptions & opt,
                 std::vector<uint8_t> & out,
                 std::string & err) {
    if (nv <= 0 || nt <= 0) { err = "empty mesh"; return false; }
    const int TS = opt.texture_size;
    if (TS < 16 || TS > 8192) { err = "bad texture_size"; return false; }

    std::lock_guard<std::mutex> lock(g_bake_mu);

    // 1) preserve topology + optional component filter --------------------
    PreparedMesh prepared;
    if (!prepare_mesh_locked(verts, nv, tris, nt, pbr, opt, prepared, err)) return false;
    const int dnv = (int)(prepared.verts.size()/3), dnt = (int)(prepared.tris.size()/3);
    const bool use_xatlas = std::getenv("T2GLB_XATLAS") != nullptr;

    // A fixed-size per-triangle grid atlas cannot represent a full-density
    // TRELLIS mesh: at 3.1M triangles, even 2048² gives each face barely one
    // texel before gutters. The old fallback consequently emitted unpainted,
    // transparent cells and visible rectangular/triangular colour blocks.
    // Preserve the material field directly on the original vertices instead.
    if (!use_xatlas) {
        write_vertex_glb(prepared, out);
        GLBLOG("GLB %zu bytes (%d verts, %d tris; vertex PBR)", out.size(), dnv, dnt);
        return true;
    }

    return bake_atlas_locked(prepared, nullptr, opt, out, err);
}

bool mesh_to_projected_glb(const float * target_verts, int target_nv,
                           const int32_t * target_tris, int target_nt,
                           const float * source_verts, int source_nv,
                           const int32_t * source_tris, int source_nt,
                           const float * source_pbr,
                           const MeshExportOptions & opt,
                           std::vector<uint8_t> & out,
                           std::string & err) {
    if (!t2print::available()) {
        err = "PBR projection is unavailable (rebuild with CGAL >= 5.5)";
        return false;
    }
    if (!target_verts || !target_tris || target_nv <= 0 || target_nt <= 0 ||
        !source_verts || !source_tris || !source_pbr || source_nv <= 0 || source_nt <= 0) {
        err = "empty projected GLB mesh";
        return false;
    }
    if (opt.texture_size < 16 || opt.texture_size > 8192) {
        err = "bad texture_size";
        return false;
    }

    std::lock_guard<std::mutex> lock(g_bake_mu);
    PreparedMesh target, source;
    MeshExportOptions target_opt = opt;
    target_opt.components = ComponentFilter::KeepAll;
    // The target is the exact already-previewed Alpha Wrap.  Do not let the
    // legacy T2GLB_XATLAS smoothing switch move it during export.
    if (!prepare_mesh_locked(target_verts, target_nv, target_tris, target_nt,
                             nullptr, target_opt, target, err, false))
        return false;
    if (!prepare_mesh_locked(source_verts, source_nv, source_tris, source_nt,
                             source_pbr, opt, source, err, false))
        return false;
    return bake_atlas_locked(target, &source, opt, out, err);
}

} // namespace t2glb