User:Lukasstockner97/Principled v2

Issues with the current Principled BSDF

  • MultiGGX is a headache - slower, noisier, has bugs, hard to maintain
  • No "proper" metallic Fresnel, just basic F0 control
  • Sheen is outdated and designed to fix a problem that no longer exists
  • Tint options are limited - if we're giving non-physical control for artistic tweaks, might as well do it properly
  • Subsurface controls are not designed for "real" SSS
    • Subsurface blend is unintuitive
    • Subsurface color is unnecessary
    • Subsurface IOR is not really doing much
  • Random Walk SSS gives white edges at low albedo
  • Fresnel is implemented strangly
  • Three separate IOR inputs (two explicit, one implicit through Specular input)
  • Transmission roughness is strange

Components of the new model

  • Base layer
    • Metallic specular
      • Use artist-friendly Fresnel from Imageworks slides, provides edge color and falloff control
        • At falloff=0.2, matches classic Schlick-F0-based metallic, so use that as default
      • Doesn't support accurate behavior (base->tint->white), so also support Gulbrandsen model as an option?
        • Maybe at falloff=0? Would mean a jump once you start adjusting the slider, though.
        • Do the linearization (compute F at g=0 and g=1, then lerp per-channel based on edge color)?
      • Only difference to Dielectric specular is Fresnel term, so can be implemented as one Closure
    • Diffuse
      • Do we want Oren-Nayar roughness?
        • If yes, would also need to be supported at SSS exit
      • Scale albedo based on albedo of other layers (specular, coatings) to conserve energy
        • Maybe optional?
    • Subsurface
      • Replace "Subsurface" factor with scale input, zero disables SSS
      • Try refracting into volume instead of a diffuse entry bounce
        • The problem with the current approach appears to be that at high radius, many random walks go directly from entry to exit
        • Surface color is controlled using single-scatter albedo, but if there is no scatter event it can't have any impact
        • Due to the diffuse entry bounce, near edges many parts start towards the adjacent face and immediately leave, leading to white color
        • Actual refraction at the entrance would prevent that, unless the edge is towards a backfacing face (where the white is realistic)
      • Still do diffuse exit to maintain smooth transition to diffuse at scale->0 and have efficient light connection
    • Dielectric specular
      • Use proper microsurface Fresnel term
    • Transmission
      • Shares IOR and roughness controls with dielectric specular
      • Consider supporting embedded volume automatically
        • Absorption and/or scattering
      • TODO: Should this be influenced by the base color? Really unrealistic, tint of glass objects should be volumetric.
      • Instead of computing macrosurface Fresnel during shader evaluation and using it to mix closures, use single closure based on microsurface Fresnel
        • Requires shared roughness
      • Also have thin surface mode where entry and exit refraction are handled in one shader
        • Could maybe automatically become transparent to shadow rays and secondary bounces if roughness is low enough?
  • Coatings
  • Translucent?
    • Only in thin sheet model
    • Try if we even need this, maybe rough transmission and/or subsurface is enough
  • Additional elements
    • Emission
      • Use opportunity to change defaults: Color defaults to white, Strength defaults to zero
    • Transparency

Inputs of the new model

  • Base color (RGB)
    • Controls diffuse/subsurface color and metallic reflectance
  • Roughness (0-1)
    • Affects transmission and both specular lobes
  • Metallic (0-1)
    • Blends the base layer between metallic specular and diffuse+subsurface+dielectric+transmission
  • Transmission (0-1)
    • Blends the non-metallic part between transmission and diffuse+subsurface+dielectric
  • IOR (0.1-10 or so)
    • Affects dielectric specular, refraction and subsurface
  • Maybe: Diffuse roughness (0-1)
    • Zero for Lambertian, nonzero for Oren-Nayar
  • Subsurface radius (RGB vector), scale (0-inf)
    • Zero scale disables SSS, nonzero gets multiplied with radius
  • Anisotropy strength/rotation
    • Affects specular lobes (arguably only metallic would be more realistic, but this way we can share the closure)
  • Metallic edge color (RGB) and falloff (0-1)
    • Controls metallic color gradient from perpendicular to grazing angles
    • Default: White and 0.2, matches standard Schlick metallic
    • Maybe: Fallback 0 enters physical conductive Fresnel mode (using artist-friendly parametrization)?
  • Sheen strength (0-1), tint (RGB), roughness (0-1)
  • Clearcoat stength (0-1), tint (RGB), roughness (0-1)
    • Unclear: Disney BSDF scales the strength by 0.25. Do we want to keep that? If yes, maybe make range 0-4.
  • Normal
  • Clearcoat Normal
  • Tangent
  • TBD: Extra tint controls
    • Extra non-physical multipliers on top of specific lobes
    • Can be used to adjust brightness and/or color for artistic control
    • Default to white for physically-based and energy-conserving model
  • Thin surface mode (boolean)
    • Most components remain the same
    • Notable difference: Transmission does not actually refract
      • Sample microfacet and compute Fresnel as usual, but when we pick refraction, compute reflection and flip to other surface side instead
  • Alpha
  • Emission
  • Emission Strength

Compatibility with old model

  • The basic outline of the model is the mostly the same, notable differences are:
    • Sheen is significantly different, despite the same name
    • Subsurface IOR is merged into the regular IOR
    • Subsurface strength control is no longer 0-1
    • Subsurface color is gone
      • In order to control the color of the scattered light (e.g. reddish for skin), use the radius controls
    • Dielectric specular strength input is gone
      • Used to be converted to an IOR, now the IOR input is used directly
      • For non-physical control where strength does not match IOR, use the non-physical tint input
  • Visual difference is TBD

Energy conservation considerations

  • Make use of albedo scaling
  • Precompute correct albedo for given cosI and roughness using multiscattering model
  • Avoiding missing energy:
    • Adjust closure weight to account for energy loss in BSDF (divide by albedo)
  • Avoiding extra energy:
    • Adjust closure weight of "lower" layers according to albedo of upper layers
    • For example, clearcoat, sheen and specular reflection all end up dimming diffuse/subsurface

Future ideas

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