核心：glDrawArraysInstanced
和以往的不同就是這句Draw的時候改成這個API

補充：glVertexAttribDivisor
這個函式告訴了OpenGL該什麼時候更新頂點屬性的內容至新一組資料。它的第一個引數是需要的頂點屬性，第二個引數是屬性除數(Attribute Divisor)。預設情況下，屬性除數是0，告訴OpenGL我們需要在頂點著色器的每次迭代時更新頂點屬性。將它設定為1時，我們告訴OpenGL我們希望在渲染一個新例項的時候更新頂點屬性。而設定為2時，我們希望每2個例項更新一次屬性，以此類推。

程式碼

   unsigned int amount = 10000
 
;
    glm::mat4* modelMatrices;
    modelMatrices = new glm::mat4[amount];
    srand(glfwGetTime()); // initialize random seed
    float radius = 150.0;
    float offset = 25.0f;
    for (unsigned int i = 0; i < amount; i++)
    {
        glm::mat4 model;
        // 1. translation: displace along circle with 'radius' in range [-offset, offset]
 

        float angle = (float)i / (float)amount * 360.0f;
        float displacement = (rand() % (int)(2 * offset * 100)) / 100.0f - offset;
        float x = sin(angle) * radius + displacement;
        displacement = (rand() % (int)(2 * offset * 100)) / 100.0f - offset;
        float y = displacement * 0.4f; // keep height of asteroid field smaller compared to width of x and z
 

        displacement = (rand() % (int)(2 * offset * 100)) / 100.0f - offset;
        float z = cos(angle) * radius + displacement;
        model = glm::translate(model, glm::vec3(x, y, z));

        // 2. scale: Scale between 0.05 and 0.25f
        float scale = (rand() % 20) / 100.0f + 0.05;
        model = glm::scale(model, glm::vec3(scale));

        // 3. rotation: add random rotation around a (semi)randomly picked rotation axis vector
        float rotAngle = (rand() % 360);
        model = glm::rotate(model, rotAngle, glm::vec3(0.4f, 0.6f, 0.8f));

        // 4. now add to list of matrices
        modelMatrices[i] = model;
    }

    // configure instanced array
    // -------------------------
    unsigned int buffer;
    glGenBuffers(1, &buffer);
    glBindBuffer(GL_ARRAY_BUFFER, buffer);
    glBufferData(GL_ARRAY_BUFFER, amount * sizeof(glm::mat4), &modelMatrices[0], GL_STATIC_DRAW);

    // set transformation matrices as an instance vertex attribute (with divisor 1)
    // note: we're cheating a little by taking the, now publicly declared, VAO of the model's mesh(es) and adding new vertexAttribPointers
    // normally you'd want to do this in a more organized fashion, but for learning purposes this will do.
    // -----------------------------------------------------------------------------------------------------------------------------------
    for (unsigned int i = 0; i < rock.meshes.size(); i++)
    {
        unsigned int VAO = rock.meshes[i].VAO;
        glBindVertexArray(VAO);
        // set attribute pointers for matrix (4 times vec4)
        glEnableVertexAttribArray(3);
        glVertexAttribPointer(3, 4, GL_FLOAT, GL_FALSE, sizeof(glm::mat4), (void*)0);
        glEnableVertexAttribArray(4);
        glVertexAttribPointer(4, 4, GL_FLOAT, GL_FALSE, sizeof(glm::mat4), (void*)(sizeof(glm::vec4)));
        glEnableVertexAttribArray(5);
        glVertexAttribPointer(5, 4, GL_FLOAT, GL_FALSE, sizeof(glm::mat4), (void*)(2 * sizeof(glm::vec4)));
        glEnableVertexAttribArray(6);
        glVertexAttribPointer(6, 4, GL_FLOAT, GL_FALSE, sizeof(glm::mat4), (void*)(3 * sizeof(glm::vec4)));

        glVertexAttribDivisor(3, 1);
        glVertexAttribDivisor(4, 1);
        glVertexAttribDivisor(5, 1);
        glVertexAttribDivisor(6, 1);

        glBindVertexArray(0);
    }


 while (!glfwWindowShouldClose(window))
 {
 [...]
  for (unsigned int i = 0; i < rock.meshes.size(); i++)
        {
            glBindVertexArray(rock.meshes[i].VAO);
            glDrawElementsInstanced(GL_TRIANGLES, rock.meshes[i].indices.size(), GL_UNSIGNED_INT, 0, amount);
            glBindVertexArray(0);
        }
 }