[Development] The Dynamic OpenGL on Windows Change

[Agocs Laszlo]

You cannot include multiple GL headers. In a dynamic build you have the desktop GL header ( + qopenglext.h), just like in a 'desktop' build. This does not change at all.

Runtime errors are not a problem since all ES-specific paths are guarded by the appropriate condition.

I realize however that there are genuine concerns around exporting all the symbols from QtGui, and that the approach will most likely not scale to other platforms. A compromise could be to revert the change for 5.3 (since it is anyway behind an experimental configuration option) and instead try gradually introducing an alternative approach in future releases:

1. Leave code that directly calls OpenGL functions alone. Instead, extend QOpenGLFunctions with the common subset of OpenGL 1.x and ES2 functions which are missing today. This would allow writing context->functions()->glClear() etc. which is not possible today.

2. Once this is in place, all of qtbase and qtdeclarative (and later all other modules) have to be changed to stop directly calling OpenGL functions. Instead, they must rely on QOpenGLFunctions (which they most likely do already for GL 2 specific calls anyhow). 

3. QOpenGLFunctions gets platform specific backends that perform dynamic loading of the OpenGL implementation. For example, the Windows backend would do the tests for choosing between desktop - Angle and then dynamically load the selected implementation. QtGui stops linking to opengl32/libegl/libglesv2.

4. The windowing system interfacing code, for example all the usage of WGL and EGL in the windows platform plugin, has to be placed behind a new private wrapper similar to QOpenGLFunctions. This could expose an EGL-style API, with platform specific backends  that provide an implementation using the dynamically resolved EGL/WGL/GLX functions.
 
Another alternative would be to continue betting on Angle as the primary OpenGL enabler on Windows, and investigate how well the software rasterizer fallback in D3D11-based Angle works. As I understand this would be available on Win7+ in VS2012+ builds. If this could solve the issues we see today with virtual machines and machines without decent drivers, it might reduce the urgency for needing a true dynamic GL solution. 

Cheers,
Laszlo

-----Original Message-----
From: Sean Harmer [mailto:sean.harmer at kdab.com] 
Sent: 28. februar 2014 11:41
To: development at qt-project.org
Cc: Gunnar Sletta; Agocs Laszlo
Subject: Re: Re: [Development] The Dynamic OpenGL on Windows Change

Hi Gunnar,

On Friday 28 February 2014 07:14:24 Gunnar Sletta wrote:
> On 27 Feb 2014, at 22:17, Agocs Laszlo <Laszlo.Agocs at digia.com> wrote:
> > On Thu, Feb 27, 2014 at 02:28:26PM +0000, Sean Harmer wrote:
> >> The apparent problem that this is attempting to address is the need for
> >> both ANGLE and desktop OpenGL builds of Qt on windows. As you know Qt
> > 
> > As pointed out by Friedmann earlier, the big picture is a somewhat more
> > complicated. This is just the beginning.
> ...
> 
> And I agree to all this. It is a good change, and I'm happy to see it.
> 
> Also, I would be very surprised if the added if-statements inside Qt makes
> much of a difference to performance. As long as that logic stays primarily
> inside Qt, it also doesn't hurt application complexity.

What I don't understand, and which is quite likely entirely my fault as I'm 
rather sleep-deprived recently (new baby), is that to me it seems like 
situations such as trying to use desktop GL with an ES 2 build or vice versa 
would have until now resulted in compile time errors. Now they will become 
potential runtime errors (e.g. GL_INVALID_ENUM). Also, is it safe in #include 
the ES2 headers on top of the desktop ones now and in the future? That's a 
genuine question, I honestly don't know.

If everybody else is happy to live with this then I'll shut up. We should 
still consider if it makes sense for QtGui to export these symbols or whether 
they should be moved to a new library though.

Cheers,

Sean
--
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