[Interest] Using OpenGL with Qt

[Yves Bailly]

Le 17/03/2014 10:05, Agocs Laszlo a écrit :
> I must correct my previous statement about not resolving functions that are not called. This deferred behavior is only true for QOpenGLFunctions. The versioned variants will resolve all functions for the given version already when initializeOpenGLFunctions() is first called with a given context.

Here's what I tried.

First I define a default GL format:
   QGLFormat fmt;
   fmt.setVersion(3, 3);
   fmt.setProfile(QGLFormat::CoreProfile);
   fmt.setStencilBufferSize(8);
   QGLFormat::setDefaultFormat(fmt);


Now let's say I have two classes:

class Gl_Model: protected QOpenGLFunctions_3_3_Core
{
   public:
     Gl_Model();
     void create();
     void draw();
   private:
     GLuint buff_ids[3];
}; // class Gl_Model

class Gl_Widget:
     public QGLWidget,
     protected QOpenGLFunctions_3_3_Core
{
     Q_OBJECT
   public:
     explicit Gl_Widget(QWidget* const parent = nullptr);
   protected:
     virtual void initializeGL() override;
     virtual void paintGL() override;
     virtual void resizeGL(int width, int height) override;
   private:
     Gl_Model* model{nullptr};
}; // class Gl_Widget

inside Gl_Model::create():
void Gl_Model::create()
{
   this->initializeOpenGLFunctions();
   /* ... */
}

inside Gl_Widget::initializeGL():
{
   QOpenGLContext* ctx = QOpenGLContext::currentContext();
   ctx->versionFunctions<QOpenGLFunctions_3_3_Core>();
   this->initializeOpenGLFunctions();
   this->model = new Gl_Model;
   this->model->create();
}


Now if I follow the program step-by-step, indeed the second call to
initializeOpenGLFunctions (inside Gl_Model::create) it will reuse some
already allocated data... but it will perform 12 searchs in a QHash<>.
If I have to create, say, 100000 instances of Gl_Model, that will
imply 1200000 searchs. I know QHash<> is quite fast, but that seems
a bit overkill to me...

Or am I missing something again?

-- 
      /- Yves Bailly - Software developer   -\
      \- Sescoi R&D  - http://www.sescoi.fr -/
"The possible is done. The impossible is being done. For miracles,
thanks to allow a little delay."