# vtk.js picking when VolumeRendering

**URL:** https://discourse.vtk.org/t/vtk-js-picking-when-volumerendering/12109
**Category:** vtk.js
**Created:** [August 7, 2023, 6:05am UTC](https://discourse.vtk.org/t/vtk-js-picking-when-volumerendering/12109 "2023-08-07T06:05:54Z")
**Posts on this page:** 1
**Page:** 1

<div class="post-metadata">

### Author: ![mazurkin.daniel](https://discourse.vtk.org/user_avatar/discourse.vtk.org/mazurkin.daniel/32/7277_2.png) [@mazurkin.daniel](https://discourse.vtk.org/u/mazurkin.daniel)
#### Post date: [August 7, 2023, 6:05am UTC](https://discourse.vtk.org/t/vtk-js-picking-when-volumerendering/12109/1 "2023-08-07T06:05:54Z")

</div>

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](https://github.com/Kitware/vtk-js/pull/2885/files). 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.