[vtkusers] Bare OpenGL 2.1 support

[marcinadamczyk at ortosoft.eu]

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