vtk.js picking when VolumeRendering

Hello, I don’t understand how vtk.js stores information about voxel transparency and other data during volume rendering. This understanding is necessary in order to write a ray casting picking algorithm for models that have been generated using VolumeRendering. There are some developments on this in my personal repository PR here: Picking for volume rendering by danielmazurkin · Pull Request #2885 · Kitware/vtk-js · GitHub. And my misunderstanding is also based on the fact that there is no implementation of the GetVoxelGradient method in vtk.js as in native VTK. And I don’t understand why normals are computed when using picking.