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EEVEE-Next: implement less noisy GGX VNDF sampling for reflections

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from paper "Bounded VNDF Sampling for Smith–GGX Reflections"

Pull Request: https://projects.blender.org/blender/blender/pulls/116773
This commit is contained in:
Weizhen Huang
2024-01-04 20:24:46 +13:00
committed by Gitea
parent ea669cb8dc
commit 8bdb97e5ff
1 changed files with 39 additions and 19 deletions
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58
source/blender/draw/engines/eevee_next/shaders/eevee_bxdf_sampling_lib.glsl
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@@ -15,16 +15,6 @@
#define GGX_USE_VISIBLE_NORMAL 1
float sample_pdf_ggx_reflect(float NH, float NV, float VH, float G1_V, float alpha)
{
float a2 = square(alpha);
#if GGX_USE_VISIBLE_NORMAL
return 0.25 * bxdf_ggx_D(NH, a2) * G1_V / NV;
#else
return NH * bxdf_ggx_D(NH, a2);
#endif
}
float sample_pdf_ggx_refract(
float NH, float NV, float VH, float LH, float G1_V, float alpha, float eta)
{
@@ -83,6 +73,41 @@ vec3 sample_ggx(vec3 rand, float alpha, vec3 Vt)
return sample_ggx(rand, alpha, Vt, G1_unused);
}
/* Similar as `sample_ggx()`, but reduces the number or rejected samples due to reflection in the
* lower hemisphere, and returns `pdf` instead of `G1_V`. Only used for reflection.
*
* Sampling visible GGX normals with bounded spherical caps.
* Eto, Kenta, and Yusuke Tokuyoshi. "Bounded VNDF Sampling for Smith-GGX Reflections."
* SIGGRAPH Asia 2023 Technical Communications. 2023. 1-4.
* https://gpuopen.com/download/publications/Bounded_VNDF_Sampling_for_Smith-GGX_Reflections.pdf */
vec3 sample_ggx_bounded(vec3 rand, float alpha, vec3 Vt, out float pdf)
{
/* Transforming the view direction to the hemisphere configuration. */
vec3 Vh = vec3(alpha * Vt.xy, Vt.z);
float norm = length(Vh);
Vh = Vh / norm;
/* Compute the bounded cap. */
float a2 = square(alpha);
float s2 = square(1.0 + length(Vt.xy));
float k = (1.0 - a2) * s2 / (s2 + a2 * square(Vt.z));
/* Sample a spherical cap in (-Vh.z * k, 1]. */
float cos_theta = mix(-Vh.z * k, 1.0, 1.0 - rand.x);
float sin_theta = sqrt(saturate(1.0 - square(cos_theta)));
vec3 Lh = vec3(sin_theta * rand.yz, cos_theta);
/* Compute unnormalized halfway direction. */
vec3 Hh = Vh + Lh;
/* Transforming the normal back to the ellipsoid configuration. */
vec3 Ht = normalize(vec3(alpha * Hh.xy, max(0.0, Hh.z)));
pdf = 0.5 * bxdf_ggx_D(saturate(Ht.z), a2) / (k * Vt.z + norm);
return Ht;
}
/**
* Returns a reflected ray direction following the GGX distribution.
*
@@ -97,17 +122,12 @@ vec3 sample_ggx(vec3 rand, float alpha, vec3 Vt)
*/
vec3 sample_ggx_reflect(vec3 rand, float alpha, vec3 V, vec3 N, vec3 T, vec3 B, out float pdf)
{
float G1_V;
vec3 tV = world_to_tangent(V, N, T, B);
vec3 tH = sample_ggx(rand, alpha, tV, G1_V);
float NH = saturate(tH.z);
float NV = saturate(tV.z);
float VH = dot(tV, tH);
vec3 H = tangent_to_world(tH, N, T, B);
vec3 Vt = world_to_tangent(V, N, T, B);
vec3 Ht = sample_ggx_bounded(rand, alpha, Vt, pdf);
vec3 H = tangent_to_world(Ht, N, T, B);
if (VH > 0.0) {
if (dot(V, H) > 0.0) {
vec3 L = reflect(-V, H);
pdf = sample_pdf_ggx_reflect(NH, NV, VH, G1_V, alpha);
return L;
}
pdf = 0.0;