[Qt-interest] Where to initialize GLEW?

[Oliver Demetz]

That's correct.
You would have to explicitly initialize the two widgets to use context 
sharing.
This is not done automatically.

Joe Liu schrieb:
> Hi Samuel,
> 
> Are you sure 'QGLWidget use context sharing'? I have two GLWindow 
> classes: CGLWnd1, CGLWnd2 which are both derived from class QGLWidget.
> 
> In intializeGL() of each GLWindow class, I generate a texture by using 
> glGenTexture(), like this:
> 
> class CGLWnd1 : public QGLWidget
> {
>     void initializeGL()
>    {
>        int tex;
>        glGenTexture(1, &tex);
>        cout<<tex;
>    }
> };
> 
> class CGLWnd2 : public QGLWidget
> {
>     void initializeGL()
>    {
>        int tex;
>        glGenTexture(1, &tex);
>        cout<<tex;
>    }
> };
> 
> CGLWnd1 a;
> CGLWnd2 b;
> 
> a.initializeGL();
> b.initializeGL();
> 
> The result outputs are the same: 1;
> 
> If the QGLWidget use context sharing, I think the generated texture 
> won't have the same texture id;
> 
> What is your opinion?
> 
> Thanks~~
> 
> 
> 2009/6/29 Joe Liu <passer.by007gg at gmail.com 
> <mailto:passer.by007gg at gmail.com>>
> 
>     Thanks Samuel,
> 
>     I would like to implement my own FBO class.
> 
>     So the QGLWidget use the context sharing mechanism. I am going to
>     read more about it.
> 
>     Thanks again for your suggestions~~
> 
>     2009/6/29 Samuel Rødal <sroedal at trolltech.com
>     <mailto:sroedal at trolltech.com>>
> 
>         Joe Liu wrote:
> 
>             Hi,
> 
>             I would like to implement my FBO class with GLEW; As you
>             know, users should invoke glewInit() after an OpenGL
>             rendering context is created and before calling any glew
>             functions.
> 
>             Since I use QGLWidget as the base class of my opengl window
>             class, and my GLWindow might have more than one instances,
>             so if I put glewInit() in function initializeGL(), it will
>             be called for multiple times. I think that is ugly.
> 
>             What is the right manner to call glewInit() in a Qt OpenGL
>             program? Is there any way to check whether the GLEW is
>             initialized or not?
> 
>             BTW, what will glewInit() do exactly? Is it associated with
>             GL Rendering Context? And How does the QGLWidget manage the
>             Rendering Context?
> 
>             Thank for your time~~
> 
> 
>             Joe
> 
> 
>         Calling glewInit() will initialize function pointers related to
>         the available OpenGL extensions, and let you query which
>         extensions are available. See http://glew.sourceforge.net/basic.html
> 
>         If you want to use GLEW from multiple contexts you should have a
>         look at the Multiple Rendering Contexts section at
>         http://glew.sourceforge.net/advanced.html
> 
>         As long as your QGLWidgets use context sharing I don't think it
>         should be necessary to use GLEW MX, though I'm not certain on this.
> 
>         For the single context case you can call glewInit() either
>         initializeGL() or right after the QGLWidget has been constructed
>         (in the latter case you need to call QGLWidget::makeCurrent()
>         first, this happens automatically before initializeGL() is called).
> 
>         Btw, there's a QGLFramebufferObject class in Qt already, are you
>         sure you need to implement your own?
> 
>         --
>         Samuel
> 
> 
> 
> 
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