[vtkusers] Bare OpenGL 2.1 support

[Ken Martin]

I believe standard OpenGL 2.1 includes gl_FrontFacing so I'm not sure why
your system failed to link on that, maybe a driver bug?

Having OpenGL 2.1 without gpu_shader4 means a fair bit of the new
rendering engine will not work. I have no plans to support OpenGL 2.1
without gpu_shader4 as that combination just lacks too many key features
that have been introduced in the past nine years. I would like to drop 2.1
entirely as OpenGL 3.2 has been out for five or so years and there are
features in 3.2 I would like to rely on. But I do understand that
sometimes you do not have a choice and are stuck with an old driver or
system so I am trying to keep an eye out for how often this happens. The
old backend is still there and working so for old systems the old backend
should work as one option.

And yes, I am certainly open to any changes that enable better 2.1 support
without adding much code complexity as that is a win-win for everyone.

Thanks
Ken

Ken Martin PhD
Chairman & CFO
Kitware Inc.
28 Corporate Drive
Clifton Park NY 12065
ken.martin at kitware.com
919 869-8871 (w)


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-----Original Message-----
From: vtkusers [mailto:vtkusers-bounces at vtk.org] On Behalf Of
marcinadamczyk at ortosoft.eu
Sent: Monday, November 16, 2015 9:25 AM
To: vtkusers at vtk.org
Subject: [vtkusers] Bare OpenGL 2.1 support

Hello,

Our company develops an application which uses VTK. We recently upgraded
to VTK 6.3 and wanted to use the new OpenGL2 renderer. We tried running it
on a machine with OGL 2.1 support, but without EXT_gpu_shader4 extension
(no hardware OGL3.x support) - and we (unsurprisingly) failed, as VTK's
shader code has hard-coded requirement for that extension.
Scenes rendered by our software use simple one-light lighting and
transculency with depth peeling enabled. According to our understanding,
it should be able to run on OpenGL 2.1 hardware (as it ran fine on VTK's
OGL 1.1 renderer). We tried removing the "#extension GL_EXT_gpu_shader4
: require\n" line from 1.2 shader source. Shader compilation succedeed on
test mashine after that, but shader linking failed with an following
error: "Fragment shader uses varying gl_FrontFacing but vertex shader does
not write to it.".

I have the following questions related to our problems:

Can the lack of OpenGL2 renderer support for pure 2.1 hardware be
considered as a bug, or is it by design?

Is OpenGL2 renderer support for OpenGL 2.1 hardware (without
EXT_gpu_shader4 extension) planned for future VTK releases?

If not, in the case we wanted to introduce slight modifications into VTK's
code to run the aforementioned software built with OpenGL2 renderer and on
2.1 hardware, would it be possible?

Best regards,
Marcin Adamczyk
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