OpenGL-Learning-2

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Loading model

介绍

Assimp 一个模型导入库的介绍，通常OpenGL所能用到的最小的渲染单元应该是mesh，一个model由多个mesh组成。
A single mesh is the minimal representation of what we need to draw an object in OpenGL (vertex data, indices and material properties). A model (usually) consists of several meshes.

mesh

初始化

核心是mesh类和几个数据结构体
注意，结构体在cpp内存中是连续的，这是个很重要的点。
比如，这个是等价的：
Vertex vertex;
vertex.Position = glm::vec3(0.2f, 0.4f, 0.6f);
vertex.Normal = glm::vec3(0.0f, 1.0f, 0.0f);
vertex.TexCoords = glm::vec2(1.0f, 0.0f);
// = [0.2f, 0.4f, 0.6f, 0.0f, 1.0f, 0.0f, 1.0f, 0.0f];

渲染

核心问题，不知道有多少个texture，我们在mesh.h中实现绘制mesh方法的时候，可以根据texture数组的size和type来确定有多少个什么样子的shader，然后经过字符处理将它传给shader。注意shader中我们可以定义很多texture，但是不一定都用到。。占存储空间吗？

代码

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struct Vertex {
    glm::vec3 Position;
    glm::vec3 Normal;
    glm::vec2 TexCoords;
    glm::vec3 Tangent;
    glm::vec3 Bitangent;};
struct Texture {
    unsigned int id;
    string type;
    string path;};

class Mesh {
public:
    /*  Mesh Data  */
    vector<Vertex> vertices;
    vector<unsigned int> indices;
    vector<Texture> textures;
    unsigned int VAO;

    /*  Functions  */
    // constructor
    Mesh(vector<Vertex> vertices, vector<unsigned int> indices, vector<Texture> textures){
        this->vertices = vertices;
        this->indices = indices;
        this->textures = textures;
        setupMesh();}

    // render the mesh
    void Draw(Shader shader) 
    {
        // bind appropriate textures
        unsigned int diffuseNr  = 1;
        unsigned int specularNr = 1;
        unsigned int normalNr   = 1;
        unsigned int heightNr   = 1;
        for(unsigned int i = 0; i < textures.size(); i++)
        {
            glActiveTexture(GL_TEXTURE0 + i); // active proper texture unit before binding
            // retrieve texture number (the N in diffuse_textureN)
            string number;
            string name = textures[i].type;
            if(name == "texture_diffuse")
				number = std::to_string(diffuseNr++);
			else if(name == "texture_specular")
				number = std::to_string(specularNr++); // transfer unsigned int to stream
            else if(name == "texture_normal")
				number = std::to_string(normalNr++); // transfer unsigned int to stream
             else if(name == "texture_height")
			    number = std::to_string(heightNr++); // transfer unsigned int to stream

													 // now set the sampler to the correct texture unit
            glUniform1i(glGetUniformLocation(shader.ID, (name + number).c_str()), i);
            // and finally bind the texture
            glBindTexture(GL_TEXTURE_2D, textures[i].id);
        }
        
        // draw mesh
        glBindVertexArray(VAO);
        glDrawElements(GL_TRIANGLES, indices.size(), GL_UNSIGNED_INT, 0);
        glBindVertexArray(0);

        // always good practice to set everything back to defaults once configured.
        glActiveTexture(GL_TEXTURE0);
    }

private:
    /*  Render data  */
    unsigned int VBO, EBO;
    /*  Functions    */
    // initializes all the buffer objects/arrays
    void setupMesh()
    {
        // create buffers/arrays
        glGenVertexArrays(1, &VAO);
        glGenBuffers(1, &VBO);
        glGenBuffers(1, &EBO);

        glBindVertexArray(VAO);
        // load data into vertex buffers
        glBindBuffer(GL_ARRAY_BUFFER, VBO);
        // A great thing about structs is that their memory layout is sequential for all its items.
        // The effect is that we can simply pass a pointer to the struct and it translates perfectly to a glm::vec3/2 array which
        // again translates to 3/2 floats which translates to a byte array.
        glBufferData(GL_ARRAY_BUFFER, vertices.size() * sizeof(Vertex), &vertices[0], GL_STATIC_DRAW);  

        glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, EBO);
        glBufferData(GL_ELEMENT_ARRAY_BUFFER, indices.size() * sizeof(unsigned int), &indices[0], GL_STATIC_DRAW);

        // set the vertex attribute pointers
        // vertex Positions
        glEnableVertexAttribArray(0);	
        glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, sizeof(Vertex), (void*)0);
        // vertex normals
        glEnableVertexAttribArray(1);	
        glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, sizeof(Vertex), (void*)offsetof(Vertex, Normal));
        // vertex texture coords
        glEnableVertexAttribArray(2);	
        glVertexAttribPointer(2, 2, GL_FLOAT, GL_FALSE, sizeof(Vertex), (void*)offsetof(Vertex, TexCoords));
        // vertex tangent
        glEnableVertexAttribArray(3);
        glVertexAttribPointer(3, 3, GL_FLOAT, GL_FALSE, sizeof(Vertex), (void*)offsetof(Vertex, Tangent));
        // vertex bitangent
        glEnableVertexAttribArray(4);
        glVertexAttribPointer(4, 3, GL_FLOAT, GL_FALSE, sizeof(Vertex), (void*)offsetof(Vertex, Bitangent));

        glBindVertexArray(0);
    }
};
#endif

Model

这两天过OpenGL的这个过程中才发现，一个好的设计思路和清晰的思维比代码能力（我指的是编码能力）重要多了，这个的能力基础就是对IO、数据结构、算法、内存、显存、整个架构的正确理解。比如，我们这里要加载模型，根据模型文件的定义，我们核心要处理的是mesh。上一节，mesh搞定了，基于此我们来想model应该做什么：

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class Model 
{
    public:
        /*  Functions   */
        Model(char *path)
        {
            loadModel(path);
        }
        void Draw(Shader shader);	
    private:
        /*  Model Data  */
        vector<Mesh> meshes;
        string directory;
        /*  Functions   */
        void loadModel(string path);
        void processNode(aiNode *node, const aiScene *scene);
        Mesh processMesh(aiMesh *mesh, const aiScene *scene);
        vector<Texture> loadMaterialTextures(aiMaterial *mat, aiTextureType type, 
                                             string typeName);
};

剩下的实现，就是经验的问题了。

加载贴图的优化

这里需要主义的问题是，一个贴图资源，很有可能很多的模型都在用，所以避免重复加载资源或者优化处理比较重要。这是模型加载的性能瓶颈。
怎么解决呢，我们把已经加载的texture在global空间存起来，每次加载前先看是不是已经加载过了。

代码

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unsigned int TextureFromFile(const char *path, const string &directory, bool gamma = false);
class Model 
{
public:
    /*  Model Data */
    vector<Texture> textures_loaded;
    vector<Mesh> meshes;
    string directory;
    bool gammaCorrection;
    /*  Functions   */
    Model(string const &path, bool gamma = false) : gammaCorrection(gamma)
    {
        loadModel(path);
    }
    void Draw(Shader shader)
    {
        for(unsigned int i = 0; i < meshes.size(); i++)  meshes[i].Draw(shader); 
    }
    
private:
    /*  Functions   */
    void loadModel(string const &path)
    {
        const aiScene* scene = importer.ReadFile(path, aiProcess_Triangulate | aiProcess_FlipUVs | aiProcess_CalcTangentSpace);
        directory = path.substr(0, path.find_last_of('/'));
        processNode(scene->mRootNode, scene);
    }

    // processes a node in a recursive fashion. Processes each individual mesh located at the node and repeats this process on its children nodes (if any).
    void processNode(aiNode *node, const aiScene *scene)
    {
        // process each mesh located at the current node
        for(unsigned int i = 0; i < node->mNumMeshes; i++)
        {
            // the node object only contains indices to index the actual objects in the scene. 
            // the scene contains all the data, node is just to keep stuff organized (like relations between nodes).
            aiMesh* mesh = scene->mMeshes[node->mMeshes[i]];
            meshes.push_back(processMesh(mesh, scene));
        }
        // after we've processed all of the meshes (if any) we then recursively process each of the children nodes
        for(unsigned int i = 0; i < node->mNumChildren; i++)
        {
            processNode(node->mChildren[i], scene);
        }
    }

    Mesh processMesh(aiMesh *mesh, const aiScene *scene)
    {
        // data to fill
        vector<Vertex> vertices;
        vector<unsigned int> indices;
        vector<Texture> textures;

        // Walk through each of the mesh's vertices
        for(unsigned int i = 0; i < mesh->mNumVertices; i++)
        {
            Vertex vertex;
			//。。。
            vertices.push_back(vertex);
        }
        // wak through each of the mesh's faces (a face is a mesh its triangle) and retrieve the corresponding vertex indices.
        for(unsigned int i = 0; i < mesh->mNumFaces; i++)
        {
            aiFace face = mesh->mFaces[i];
            // retrieve all indices of the face and store them in the indices vector
            for(unsigned int j = 0; j < face.mNumIndices; j++)
                indices.push_back(face.mIndices[j]);
        }
        // process materials
        aiMaterial* material = scene->mMaterials[mesh->mMaterialIndex];    
        // return a mesh object created from the extracted mesh data
        return Mesh(vertices, indices, textures);
    }

    // checks all material textures of a given type and loads the textures if they're not loaded yet.
    // the required info is returned as a Texture struct.
    vector<Texture> loadMaterialTextures(aiMaterial *mat, aiTextureType type, string typeName)
    {
        vector<Texture> textures;
        for(unsigned int i = 0; i < mat->GetTextureCount(type); i++)
        {
            aiString str;
            mat->GetTexture(type, i, &str);
            // check if texture was loaded before and if so, continue to next iteration: skip loading a new texture
            bool skip = false;
            for(unsigned int j = 0; j < textures_loaded.size(); j++)
            {
                if(std::strcmp(textures_loaded[j].path.data(), str.C_Str()) == 0)
                {
                    textures.push_back(textures_loaded[j]);
                    skip = true; // a texture with the same filepath has already been loaded, continue to next one. (optimization)
                    break;
                }
            }
            if(!skip)
            {   // if texture hasn't been loaded already, load it
                Texture texture;
                texture.id = TextureFromFile(str.C_Str(), this->directory);
                texture.type = typeName;
                texture.path = str.C_Str();
                textures.push_back(texture);
                textures_loaded.push_back(texture);  // store it as texture loaded for entire model, to ensure we won't unnecesery load duplicate textures.
            }
        }
        return textures;
    }
};

unsigned int TextureFromFile(const char *path, const string &directory, bool gamma)
{
    return textureID;
}

整体结构

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#include <learnopengl/shader_m.h>
#include <learnopengl/camera.h>
#include <learnopengl/model.h>

Shader ourShader("1.model_loading.vs", "1.model_loading.fs");
Model ourModel(FileSystem::getPath("nanosuit.obj"));
renderloop()
{
    ourShader.use();
    ourShader.setMat4("projection", projection);
    ourShader.setMat4("view", view);
    ourShader.setMat4("model", model);
    ourModel.Draw(ourShader);
}