mesh-functions.cpp

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/*
 * Copyright (C) 2023  Christopher J. Howard
 *
 * This file is part of Antkeeper source code.
 *
 * Antkeeper source code is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * Antkeeper source code is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with Antkeeper source code.  If not, see <http://www.gnu.org/licenses/>.
 */
 
#include <engine/geom/mesh-functions.hpp>
#include <engine/utility/fundamental-types.hpp>
#include <unordered_map>
#include <vector>
 
namespace geom {
 
struct edge_hasher
{
    std::size_t operator()(const std::array<std::size_t, 2>& v) const noexcept
    {
        std::size_t hash = std::hash<std::size_t>()(v[0]);
        return hash ^ (std::hash<std::size_t>()(v[1]) + 0x9e3779b9 + (hash << 6) + (hash >> 2));
    }
};
 
void create_triangle_mesh(mesh& mesh, const std::vector<float3>& vertices, const std::vector<std::array<std::uint_fast32_t, 3>>& triangles)
{
    for (const auto& vertex: vertices)
        mesh.add_vertex(vertex);
 
    std::unordered_map<std::array<std::size_t, 2>, geom::mesh::edge*, edge_hasher> edge_map;
    const std::vector<mesh::vertex*>& mesh_vertices = mesh.get_vertices();
    std::vector<geom::mesh::edge*> loop(3);
 
    for (const auto& triangle: triangles)
    {
        geom::mesh::vertex* triangle_vertices[3] =
        {
            mesh_vertices[triangle[0]],
            mesh_vertices[triangle[1]],
            mesh_vertices[triangle[2]]
        };
 
        for (int j = 0; j < 3; ++j)
        {
            geom::mesh::vertex* start = triangle_vertices[j];
            geom::mesh::vertex* end = triangle_vertices[(j + 1) % 3];
 
            if (auto it = edge_map.find({start->index, end->index}); it != edge_map.end())
            {
                loop[j] = it->second;
            }
            else
            {
                loop[j] = mesh.add_edge(start, end);
                edge_map[{start->index, end->index}] = loop[j];
                edge_map[{end->index, start->index}] = loop[j]->symmetric;
            }
 
        }
 
        mesh.add_face(loop);
    }
}
 
void calculate_face_normals(float3* normals, const mesh& mesh)
{
    const std::vector<mesh::face*>& faces = mesh.get_faces();
 
    for (std::size_t i = 0; i < faces.size(); ++i)
    {
        const mesh::face& face = *(faces[i]);
        const float3& a = face.edge->vertex->position;
        const float3& b = face.edge->next->vertex->position;
        const float3& c = face.edge->previous->vertex->position;
 
        normals[i] = math::normalize(math::cross(b - a, c - a));
    }
}
 
float3 calculate_face_normal(const mesh::face& face)
{
    const float3& a = face.edge->vertex->position;
    const float3& b = face.edge->next->vertex->position;
    const float3& c = face.edge->previous->vertex->position;
    return math::normalize(math::cross(b - a, c - a));
}
 
void calculate_vertex_tangents(float4* tangents, const float2* texcoords, const float3* normals, const mesh& mesh)
{
    const std::vector<mesh::face*>& faces = mesh.get_faces();
    const std::vector<mesh::vertex*>& vertices = mesh.get_vertices();
    
    // Allocate tangent and bitangent buffers
    std::vector<float3> tangent_buffer(vertices.size(), float3{0.0f, 0.0f, 0.0f});
    std::vector<float3> bitangent_buffer(vertices.size(), float3{0.0f, 0.0f, 0.0f});
    
    // Accumulate tangents and bitangents
    for (std::size_t i = 0; i < faces.size(); ++i)
    {
        const mesh::face& face = *(faces[i]);
        std::size_t ia = face.edge->vertex->index;
        std::size_t ib = face.edge->next->vertex->index;
        std::size_t ic = face.edge->previous->vertex->index;
        const float3& a = vertices[ia]->position;
        const float3& b = vertices[ib]->position;
        const float3& c = vertices[ic]->position;
        const float2& uva = texcoords[ia];
        const float2& uvb = texcoords[ib];
        const float2& uvc = texcoords[ic];
 
        float3 ba = b - a;
        float3 ca = c - a;
        float2 uvba = uvb - uva;
        float2 uvca = uvc - uva;
        
        float f = 1.0f / (uvba.x() * uvca.y() - uvca.x() * uvba.y());
        float3 tangent = (ba * uvca.y() - ca * uvba.y()) * f;
        float3 bitangent = (ba * -uvca.x() + ca * uvba.x()) * f;
        
        tangent_buffer[ia] += tangent;
        tangent_buffer[ib] += tangent;
        tangent_buffer[ic] += tangent;
        bitangent_buffer[ia] += bitangent;
        bitangent_buffer[ib] += bitangent;
        bitangent_buffer[ic] += bitangent;
    }
    
    // Orthogonalize tangents
    for (std::size_t i = 0; i < vertices.size(); ++i)
    {
        const float3& n = normals[i];
        const float3& t = tangent_buffer[i];
        const float3& b = bitangent_buffer[i];
        
        // Gram-Schmidt orthogonalize tangent
        float3 tangent = math::normalize(t - n * math::dot(n, t));
        
        // Calculate bitangent sign
        float bitangent_sign = (math::dot(math::cross(n, t), b) < 0.0f) ? -1.0f : 1.0f;
        
        tangents[i] = {tangent.x(), tangent.y(), tangent.z(), bitangent_sign};
    }
}
 
box<float> calculate_bounds(const mesh& mesh)
{
    box<float> bounds;
    for (int i = 0; i < 3; ++i)
    {
        bounds.min[i] = std::numeric_limits<float>::infinity();
        bounds.max[i] = -std::numeric_limits<float>::infinity();
    }
 
    for (const mesh::vertex* vertex: mesh.get_vertices())
    {
        const auto& position = vertex->position;
        bounds.extend(position);
    }
 
    return bounds;
}
 
mesh::vertex* poke_face(mesh& mesh, std::size_t index)
{
    mesh::face* face = mesh.get_faces()[index];
    
    // Collect face edges and sum edge vertex positions
    mesh::loop loop = {face->edge};
    float3 sum_positions = face->edge->vertex->position;
    for (mesh::edge* edge = face->edge->next; edge != face->edge; edge = edge->next)
    {
        loop.push_back(edge);
        sum_positions += edge->vertex->position;
    }
    
    if (loop.size() <= 2)
        return nullptr;
    
    // Remove face
    mesh.remove_face(face);
    
    // Add vertex in center
    mesh::vertex* center = mesh.add_vertex(sum_positions / static_cast<float>(loop.size()));
    
    // Create first triangle
    geom::mesh::edge* ab = loop[0];
    geom::mesh::edge* bc = mesh.add_edge(ab->next->vertex, center);
    geom::mesh::edge* ca = mesh.add_edge(center, ab->vertex);
    mesh.add_face({ab, bc, ca});
    
    // Save first triangle CA edge
    geom::mesh::edge* first_triangle_ca = ca;
    
    // Create remaining triangles
    for (std::size_t i = 1; i < loop.size(); ++i)
    {
        ab = loop[i];
        ca = bc->symmetric;
        
        if (i == loop.size() - 1)
            bc = first_triangle_ca->symmetric;
        else
            bc = mesh.add_edge(ab->next->vertex, center);
        
        mesh.add_face({ab, bc, ca});
    }
    
    return center;
}
 
} // namespace geom