OpenGL: dynamically modify vbo/ebo

A recent one Demo, It needs to be in operation , Support switching models , Just need to change the data

own OpenGL I haven't learned systematically , A batch of water , After a long time, I just can't

class GlWidget : public QOpenGLWidget, protected QOpenGLFunctions_3_3_Core
{
    Q_OBJECT
public:
    GlWidget(QWidget *parent);
    ~GlWidget()  Q_DECL_OVERRIDE;

    void LoadModel();

protected:
    // On the first call resizeGL() or paintGL() Before calling once .
    void initializeGL() Q_DECL_OVERRIDE;
    // Whenever adjusted Widget The size of （ Show the window for the first time Widget It will be called when , Because all new creation Widget Will automatically get resizing events ）.
    void resizeGL(int width, int height) Q_DECL_OVERRIDE;
    void paintGL() Q_DECL_OVERRIDE;// Rendering OpenGL scene , You need to update Widget Is called .
    void mousePressEvent(QMouseEvent *event) Q_DECL_OVERRIDE;
    void mouseMoveEvent(QMouseEvent *event) Q_DECL_OVERRIDE;
    void mouseReleaseEvent(QMouseEvent *event) Q_DECL_OVERRIDE;
    void wheelEvent(QWheelEvent *event) Q_DECL_OVERRIDE;

Top loaded initializeGL Function is mainly initialization , Bind some model data that does not need to be changed VAO

overloaded function paintGL It mainly refreshes constantly , You can also modify the data here , But the loading model is not suitable here , Because this is a place that keeps refreshing , And the frequency of model loading will be very small , And there must be a key trigger

So the function to load the model is LoadModel, It is necessary to modify the data here , The modified data will be synchronized to paintGL,paintGL Real time refresh

I started LoadModel The code is like this ：

//------------- Read the model data from the file and store it in the dynamic array -----------
    float* screen = new float[N];
    unsigned int* screen_idx = new unsigned int[M];

    //----------------------  screen  VAO ----------------------
    screenProgram.bind();// Bind shader 
    glGenVertexArrays(1, &screenVao);// Generate VAO
    glBindVertexArray(screenVao);// binding VAO

    glGenBuffers(1, &screenVbo);// Generate VBO
    glBindBuffer(GL_ARRAY_BUFFER, screenVbo);/// binding VBO
    glBufferData(GL_ARRAY_BUFFER, N * sizeof(float), screen, GL_STATIC_DRAW);// Go to VBO Fill in the data 

    glGenBuffers(1, &screenEbo);// Generate EBO
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, screenEbo);// binding EBO
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, M * sizeof(unsigned int), screen_idx, GL_STATIC_DRAW);// Go to EBO Fill in the data 

    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 8 * sizeof(float), reinterpret_cast<void*>(0));
    glEnableVertexAttribArray(0);

    glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, 8 * sizeof(float), (void*)(3 * sizeof(float)));
    glEnableVertexAttribArray(1);

    glBindVertexArray(screenVao);
    glDrawElements(GL_TRIANGLES, vbo_num, GL_UNSIGNED_INT, 0);
    screenProgram.release();

    delete[]screen;
    delete[]screen_idx;

It just doesn't work , You can't even try , It was once suspected that there was something wrong with dynamic arrays , Later, it was replaced by a vector

    //...
    QVector<float> screen;
    QVector<unsigned int> screen_idx;
    //...
    glBufferData(GL_ARRAY_BUFFER, N * sizeof(float), &screen[0], GL_STATIC_DRAW);
    //...
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, M * sizeof(unsigned int), &screen_idx[0], GL_STATIC_DRAW);

no way .

Later, a big man's words inspired me opengl VAO VBO Correct misunderstandings _ The squalid -CSDN Blog

Mistake 1 ：

to VBO When giving data , namely call glBindBuffer(GL_ARRAY_BUFFER,vbo)
glBufferData(GL_ARRAY_BUFFER,sizeof(data),GL_STATIC_DRAW)
These two functions , It needs to be done in advance adopt glBindVertexArray() Bind to a VAO On .

Corrigendum ：

actually ,glBufferData(), hold Array data write in One vbo when , There is no need to bind glBindVertexArray() One VAO.
It's also very simple ： I want to give one vbo Write data , I don't really care about vao Which is , It has nothing to do with the fact that I want to write data ！
Only in hold vbo The data of , Really pass to shader When , That is to say call glVertexAttribPointer() Such as for shader Operation of ( such as glUseProgram,glEnableVertexAttribArray ) when , You must clearly specify to which VAO On .
stay glBufferData() when , Just give a brief answer vbo The data transfer , Follow shader irrelevant , No need at all bind whatever VAO.

Then modify the main code to the following ：

    glBindBuffer(GL_ARRAY_BUFFER, screenVbo);
    glBufferData(GL_ARRAY_BUFFER, N * sizeof(float), screen, GL_STATIC_DRAW);

    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, screenEbo);
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, M * sizeof(unsigned int), screen_idx, GL_STATIC_DRAW);

    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 8 * sizeof(float), reinterpret_cast<void*>(0));
    glEnableVertexAttribArray(0);
    glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, 8 * sizeof(float), (void*)(3 * sizeof(float)));
    glEnableVertexAttribArray(1);

  Is to remove the rebinding VAO, Because of this VAO In fact, it has been bound in initialization

There is no need to regenerate VAOVBOEBO And so on. , Because there are also , Just bind VBO/EBO, Then fill in the data .