b_engine/src/ModelLoader.cpp

//
// Created by lbmas on 4/29/2026.
//

#include <spdlog/spdlog.h>

#include "ModelLoader.h"

#include <assimp/Importer.hpp>
#include <assimp/postprocess.h>
#include <assimp/scene.h>

#include <cstring>

#include "TextureLoader.h"
#include "glm/ext/matrix_transform.hpp"

std::unordered_map<std::string, std::shared_ptr<Model>> ModelLoader::models;

void process_ai_mesh(aiMesh* aiMesh, const aiScene* scene, glm::mat4 transform, Mesh& mesh);
void process_ai_node(aiNode* node, const aiScene* scene, glm::mat4 transform, Model* model);

void process_ai_material(aiMaterial* aiMat, const aiScene* scene, Material& mat);

auto zUpMatrix = glm::mat4(
    1.0f,  0.0f,  0.0f,  0.0f,  // Column 0
    0.0f,  0.0f,  1.0f,  0.0f,  // Column 1
    0.0f, -1.0f,  0.0f,  0.0f,  // Column 2
    0.0f,  0.0f,  0.0f,  1.0f   // Column 3
);

std::shared_ptr<Model> ModelLoader::load_from_file(std::string_view _path, bool zUp)
{
    Assimp::Importer importer;

    const aiScene* scene = importer.ReadFile(_path.data(),
                                             aiProcess_Triangulate |
                                             aiProcess_GenSmoothNormals |
                                             aiProcess_FlipUVs);

    if (!scene || scene->mFlags & AI_SCENE_FLAGS_INCOMPLETE || !scene->mRootNode)
    {
        spdlog::error("failed to load model {}: {}", _path, importer.GetErrorString());
        return nullptr;
    }

    auto model = std::make_shared<Model>();

    auto transform = glm::identity<glm::mat4>();
    if (zUp) {
        transform = zUpMatrix;
    }

    process_ai_node(scene->mRootNode, scene, transform, model.get());

    return model;
}

void process_ai_node(aiNode* node, const aiScene* scene, glm::mat4 transform, Model* model)
{
    auto m = node->mTransformation;
    transform = transform * glm::mat4(m.a1, m.a2, m.a3, m.a4, m.b1, m.b2, m.b3, m.b4, m.c1, m.c2, m.c3, m.c4, m.d1, m.d2, m.d3, m.d4);

    for (unsigned int i = 0; i < node->mNumMeshes; i++)
    {
        aiMesh* aiMesh = scene->mMeshes[node->mMeshes[i]];
        Mesh mesh{};
        process_ai_mesh(aiMesh, scene, transform, mesh);
        model->meshes.push_back(std::make_shared<Mesh>(mesh));

        aiMaterial* aiMat =  scene->mMaterials[aiMesh->mMaterialIndex];
        Material mat{};
        process_ai_material(aiMat, scene, mat);
        model->materials.push_back(std::make_shared<Material>(mat));
    }

    for (unsigned int i = 0; i < node->mNumChildren; i++)
    {
        process_ai_node(node->mChildren[i], scene, transform, model);
    }
}

void process_ai_mesh(aiMesh* aiMesh, const aiScene* scene, glm::mat4 transform, Mesh& mesh)
{
    auto positions = std::vector<float>(aiMesh->mNumVertices * 3);
    auto uvs = std::vector<float>(aiMesh->mNumVertices * 2);
    auto normals = std::vector<float>(aiMesh->mNumVertices * 3);
    auto indices = std::vector<unsigned int>();

    for (unsigned int i = 0; i < aiMesh->mNumVertices; i++)
    {
        auto position = transform * glm::vec4(aiMesh->mVertices[i].x, aiMesh->mVertices[i].y, aiMesh->mVertices[i].z, 1.0f);
        positions[3 * i] = position.x;
        positions[3 * i + 1] = position.y;
        positions[3 * i + 2] = position.z;

        if (aiMesh->HasNormals())
        {
            aiVector3D norm = aiMesh->mNormals[i];
            normals[3 * i]     = norm.x;
            normals[3 * i + 1] = norm.y;
            normals[3 * i + 2] = norm.z;
        }
        else
        {
            normals[3 * i] = 0;
            normals[3 * i + 1] = 0;
            normals[3 * i + 2] = 0;
        }

        if (aiMesh->HasTextureCoords(0))
        {
            uvs[2 * i] = aiMesh->mTextureCoords[0][i].x;
            uvs[2 * i + 1] = aiMesh->mTextureCoords[0][i].y;
        }
        else
        {
            uvs[2 * i] = 0;
            uvs[2 * i + 1] = 0;
        }
    }

    for(unsigned int i = 0; i < aiMesh->mNumFaces; i++)
    {
        const aiFace face = aiMesh->mFaces[i];
        for(unsigned int j = 0; j < face.mNumIndices; j++)
            indices.push_back(face.mIndices[j]);
    }

    mesh = {positions, uvs, normals, indices};
}

void process_ai_material(aiMaterial* aiMat, const aiScene* scene, Material& mat) {
    aiString matName;
    aiReturn ret = aiMat->Get(AI_MATKEY_NAME, matName);
    if (ret != AI_SUCCESS) {
        spdlog::error("Could not find material name");
    }

    // grab the phong stuff
    aiColor3D ambientColor {0.f, 0.f, 0.f};
    ret = aiMat->Get(AI_MATKEY_COLOR_DIFFUSE, ambientColor);
    if (ret != AI_SUCCESS) {
        spdlog::error("Could not find ambient color for material: {}", matName.C_Str());
        ambientColor = {.3f, .3f, .3f};
    }

    aiColor3D diffuseColor {1.f, 1.f, 1.f};
    ret = aiMat->Get(AI_MATKEY_COLOR_DIFFUSE, diffuseColor);
    if (ret != AI_SUCCESS) {
        spdlog::error("Could not find diffuse color for material: {}", matName.C_Str());
        diffuseColor = {1.f, 1.f, 1.f};
    }

    aiColor3D specularColor {1.f, 1.f, 1.f};
    ret = aiMat->Get(AI_MATKEY_COLOR_SPECULAR, specularColor);
    if (ret != AI_SUCCESS) {
        spdlog::error("Could not find specular color for material: {}", matName.C_Str());
        specularColor = {1.f, 1.f, 1.f};
    }

    float shininess = 32.f;
    ret = aiMat->Get(AI_MATKEY_SHININESS, shininess);
    if (ret != AI_SUCCESS) {
        spdlog::error("Could not find shininess for material: {}", matName.C_Str());
        shininess = 32.f;
    }

    // now grab the textures, first diffuse
    for (unsigned int i = 0; i < aiMat->GetTextureCount(aiTextureType_DIFFUSE); i++) {
        aiString textureName;
        aiMat->GetTexture(aiTextureType_DIFFUSE, i, &textureName);

        if (!TextureLoader::textures.contains(textureName.C_Str())) {
            TextureLoader::textures[textureName.C_Str()] = TextureLoader::load_from_file(textureName.C_Str());
        }
    }

    mat.name = matName.C_Str();
    mat.phong.ambient = glm::vec3{ambientColor.r, ambientColor.g, ambientColor.b};
    mat.phong.diffuse = glm::vec3{diffuseColor.r, diffuseColor.g, diffuseColor.b};
    mat.phong.specular = glm::vec3{specularColor.r, specularColor.g, specularColor.b};
    mat.phong.shininess = shininess;
}