[vtkusers] Maximum and minimum intensity blend in vtkVolumeTextureMapper3D

[T.R.Shashwath]

Hello everyone,

A slightly unrelated line of inquiry led me to a feature request on the 
tracker about using GL_EXT_blend_minmax in the opengl 3D texture mapper (bug 
id 4824). I took the file posted in that report and compiled VTK with it. It 
appears to work after a fashion (I had to change two calls to 
vtkgl::BlendEquation to vtkgl::BlendEquationEXT). Obviously, this still needs 
work, but I was wondering if anyone else had tried this out.

Right now, we are using the raycast mapper's MIP function, but with hardware 
available, I'd like to use that too.

I've tried this out on Windows, and I'm in the process of compiling and 
creating some tests in Linux also.

Regards,
Shashwath