Official Compute Thickness pass Feedbacks / Support

Hi !
In 2023.1, part of the focus on HDRP was the improvement of transparents in general.

Among other tasks, HDRP now provides a fullscreen pass to compute the accumulated thickness for objects on a given **LayerMask**.

HDRP computes optical path and the overlap count (i.e the number of triangles traversed), which can be useful for instance for SubSurface Scattering or Refraction.

The overlap count can be used for flat or non-closed objects like vegetation.

There’s a few limitations when mixing Transparent and Opaque objects but it should work with any type of material.

This post is to centralize feedback and support for this specific features and transparency questions in general.

Documentation is coming but in the meantime, here’s a few tips to use this if someone is interested:

• Compute Thickness needs to be enabled on the Default Frame Settings under Rendering foldout
  
• Compute Thickness needs to be enabled on the current HDRP Asset
  
• One or more layer needs to be checked under the Compute Thickness properties in the HDRP asset
  
• One or more object has to be in the one of those selected layer for the pass to be filled with any meaningful data
  
• Lastly, the result of the thickness pass is accessible via the HD Sample Buffer node in the Shader graph. The node requires the layer index as input

Default Frame Settings



HDRP Asset



Layer index



HD Sample Buffer node



The use cases:

• More accurate Refraction and Absorption without having to author a thickness map.

• SubSurfaceScattering/Translucency with no thickness map needed

• For Vegetation and non closed objects, an overlap count can be used to multiply the thickness of leaves to have a fake accumulation for dense alpha clipped vegetation for exemple.

Varying the thickness of the leaves (see attached gif)

• This can be used as well in more creative ways like sampling the thickness of objects in other layers than the one the object is to create X-Rays effects like.

This will be demonstrated further in an upcoming sample on transparency as well.

Limitations :

• Does not support Tessellation
• Mixing open and closed mesh on the same layer can cause negative values. For better results, use separate layers.
  
• Mixing transparent and opaque objects can creates unexpected results due to when the transparent object are rendered in HDRP

Cheers!

 

It add a simple fullscreen pass to compute the thickness of multiple object at the same time for each given layer. That sample the existing depth buffer to have the thickness interacting with the environement cf. the image of the green glass dragon interacting with the plane.

You have a control on the performance by controlling the resolution of this layer it can be full screen, half resolution or quarter resolution.

 

For the buffer to be filled with the thickness of the object, the object needs to be in a layer and that layer needs to be selected in the HDRP asset list.
Then the only way to sample this buffer is via shader graph by using HDSampleBuffer node.
So, you don't have to use shader graph for object to write their thickness into the buffer, but you have to use shader graph to read that texture and do something with it.

And yes, it's compatible with transparent and opaque objects.

The best way to setup this would be to have a specific layer for object that needs to write their thicknesses and only use that layer for that and nothing else.

 

That would be possible, but that will block us to split object on different layer. For instance we can split transparent and opaque on different thickness layers to avoid interaction issues.
We'll think about it as an improvement to have an index reserved for material-based selection if possible.