Core: glDrawArraysInstanced
It's different from the past. It's changed to this API when drawing

Add: glVertexAttribDivisor
This function tells OpenGL when to update the content of vertex attributes to a new set of data. Its first parameter is the required vertex attribute, and its second parameter is the attribute divisor. By default, the attribute divisor is 0, which tells OpenGL that we need to update the vertex attributes every iteration of the vertex shader. When it is set to 1, we tell OpenGL that we want to update vertex properties when rendering a new instance. When set to 2, we want to update the properties every 2 instances, and so on.

Code

   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);
        }
 }