The volume gradient is calculated based on local variations in volume density values, determined using finite differences between neighboring voxels. This technique is particularly valuable for tasks such as highlighting boundaries, generating shading effects, or driving procedural workflows using volume data.
Render engines like Houdini’s Mantra and 3Delight leverage the volume gradient to incorporate a specular layer directly on the volume, creating a “wet look.”
As a workaround in Blender, the volume can be duplicated and converted into a mesh. A glass shader with slight roughness can then be applied to the mesh, and it can be composited as an additional layer in Blender’s compositor.
Below are two examples rendered with Blender Cycles: one without the specular component and one with it added.
Cast your vote here: https://blender.community/c/rightclickselect/g7D9/
