[vtk-developers] ReadOpenGLExtensions() should not call Render()

[Clinton Stimpson]

There is already a vtkRenderWindow::Start() which creates a context before 
rendering, so after that, one can query extensions.

Clint

On Tuesday, September 07, 2010 11:23:44 am Moreland, Kenneth wrote:
> I think we can all agree that having vtkOpenGLExtensionManager call Render
> is not a great idea.  The problem is that there is a chicken-and-egg
> problem where vtkOpenGLExtensionManager needs the OpenGL context already
> created to accurately report the extensions supported, but you often want
> query available extensions early in initialization before rendering
> occurs.
> 
> So, how should ReadOpenGLExtensions behave if the context has not yet been
> created?  Perhaps vtkRenderWindow could expose a method that forces the
> context to be created.  But does that actually solve the problem?  Could
> this still cause problems if the user has not set up size, position,
> offscreen rendering, alpha channel, etc?  What if the render window is
> later used with a QVTKWidget?
> 
> Perhaps ReadOpenGLExtensions should just error out if the OpenGL context is
> not yet created.  This might "break" a lot of existing code, but should we
> consider that code already broken?  If this is the right solution, then
> vtkRenderWindow should probably still get a method that forces the context
> to created.  That would allow users to initialize the parameters of the
> render window, create the OpenGL context, and then use
> vtkOpenGLExtensionManager to initialize the rest of the rendering.
> 
> -Ken
> 
> 
> On 9/7/10 7:30 AM, "David Gobbi" <david.gobbi at gmail.com> wrote:
> 
> Hi All,
> 
> I have a little complaint about this code in
> vtkOpenGLExtensionManager::ReadOpenGLExtensions()
> 
>     this->RenderWindow->MakeCurrent();
>     if (!this->RenderWindow->IsCurrent())
>       {
>       // Really should create a method in the render window to create
>       // the graphics context instead of forcing a full render.
>       this->RenderWindow->Render();
>       }
> 
> Calling Render() inside ReadOpenGLExtensions() is a bad thing.  It can
> cause the first render to occur as a side effect, and it might result
> in the window being rendered before it has been properly sized and
> positioned.  From what I've seen, this Render always gets called on
> Windows but never on X.  This render was also the cause of bug 11208
> (http://www.vtk.org/Bug/view.php?id=11208) which took me the better
> part of a day to track down.
> 
>   David
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> 
>    ****      Kenneth Moreland
>     ***      Sandia National Laboratories
> ***********
> *** *** ***  email: kmorel at sandia.gov
> **  ***  **  phone: (505) 844-8919
>     ***      web:   http://www.cs.unm.edu/~kmorel