[vtkusers] Volume rendering bug for large volumes

[Jim Geach]

I am using vtkFixedPointVolumeRayCastMapper to render very large volumes with two data components. So far, I have been unsuccessful in properly rendering volumes when the number of elements >MAX_INT. I'm using v5.11.0.

Independent checks show that the problem is not with the data, but in the rendering itself.

I have USE_64BIT indexing ON, and the volume rending actually completes and dumps an image without complaint. However, but the image is garbled: it looks broadly correct, but some chunks are missing, there are large discontinuities, etc. 

Is this a bug? e.g. is there some int casting within a function of vtkFixedPointVolumeRayCastMapper or associated volume rendering code that prevents VTK from properly volume rendering of 64-bit class data arrays?

Any help on this issue would be very gratefully received!

Thanks, Jim