[Bug] Opacity texture depends on sample distance but is not updated

[bruyeret]

Bug description

In Sources\Rendering\OpenGL\VolumeMapper\index.js, the opacity texture is created by the function buildBufferObjects.
This function is only called when getNeedToRebuildBufferObjects returns true.
But opacity texture values depend on publicAPI.getCurrentSampleDistance but the change of sample distance is not taken into account in getNeedToRebuildBufferObjects.

Steps to reproduce

• Create a simple scene with a volume, don't use high level objects like proxies and vtkVolumeController, as they can change the sample distance
• Set the sample distance and make the very first render of the mapper
• Set the sample distance to something different and rerender

Detailed Behavior

The opacity texture is rebuilt.

Expected Behavior

The opacity texture should be rebuilt.
This can be fixed by rebuilding the opacity texture when sample distance changes, but it doesn't look like a good idea.
A better solution would be to make the opacity texture independant of sample distance and use the sample distance in the volume mapper fragment shader instead. It could be less performant though.

Environment

• vtk.js version: master
• Browsers: Chrome
• OS: Windows