Add KHR_materials_scatter for discussion#2579
Conversation
|
|
…nisotropy semantics - Add scatterFactor (default 0) and scatterTexture (alpha channel) properties to control scattering weight; default-off follows glTF extension convention - scatterTexture uses the alpha channel so it can share a texture with multiscatterColorTexture - In thin-walled mode, scatterFactor lerps between specular BTDF and scatter BSDF, and lerps the scattered color between baseColor and multiscatterColor - In volumetric mode, scatterFactor scales the effective multi-scatter albedo - Correct scatterAnisotropy thin-walled semantics: the parameter controls the hemisphere split (forward fraction = (1+g)/2, backward = (1-g)/2), not a bias on a pure BTDF; rename scatter_btdf -> scatter_bsdf throughout Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
emackey
left a comment
emackey
left a comment
There was a problem hiding this comment.
Notes from today's PBR TSG meeting.
|
|
||
| Light that enters a material can be scattered before it exits, producing effects from the bright subsurface glow of skin and wax to the hazy translucency of frosted glass. The character of this scattering depends on whether the object is modeled as a thin surface or as a solid volume. | ||
|
|
||
| `KHR_materials_scatter` is a unified scattering extension that works in both contexts. When applied to a thin-walled material (no `KHR_materials_volume`, or `thicknessFactor = 0`), it converts the specular transmission lobe defined by `KHR_materials_transmission` into a diffuse scattered transmission lobe. When applied to a volumetric material (`KHR_materials_volume` with `thicknessFactor > 0`), it adds volumetric scattering to the medium, augmenting the pure-absorption model of `KHR_materials_volume`. |
There was a problem hiding this comment.
We should acknowledge strength and anisotropy parameters in the second sentence here.
There was a problem hiding this comment.
I'm not clear what is meant by this comment. Let's discuss on Monday's call.
|
|
||
| - `scatterStrengthFactor` scales the effective multi-scatter albedo. At 0, there is no scattering (the medium is purely absorbing, as in `KHR_materials_volume` alone); at 1, the full `multiscatterColor` albedo is applied. | ||
| - `multiscatterColor` defines the multi-scatter albedo $\rho_{ms}$, representing the perceived color of the scattering medium after many internal bounces. | ||
| - `scatterAnisotropy` controls the Henyey-Greenstein phase function for individual scattering events. |
There was a problem hiding this comment.
Replace this description of scatterAnisotropy with the same one from the properties table above.
There was a problem hiding this comment.
The "Henyey-Greenstein" phase function seems to be the standard model used in most (all?) renderers. I've added a note about what it is:
"The Henyey-Greenstein phase function is the standard model for anisotropic volumetric scattering in production renderers, parameterized by a single asymmetry parameter between -1 and 1"
3 participants
Based on some discussions in the glTF PBR group this morning, I've had Claude throw together this proposed extension.
The purpose is to simplify our current approach to supporting diffuse transmission and scattering and to bring it more into line with material models like OpenPBR and Standard Surface which don't support both volumetric materials and diffusely-transmitting surfaces at the same time.
This is one extension that replaces both
KHR_materials_volume_scatterandKHR_materials_diffuse_transmission:KHR_materials_volume_scatter.scatterStrengthproperty is used to scale the affect of the extension and defaults to 0.0 (i.e. "off").Some notes:
KHR_materials_transmissionextension. That's probably okay because the OpenPBR slabs both implement the volume coefficients (including scattering) leading to somewhat redundant behaviour. In our glTF proposal, there is a single way to define coefficients, surface transmission, etc.