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EEVEE-Next: implement more efficient GGX VNDF sampling

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from paper "Sampling the GGX Distribution of Visible Normals"
This commit is contained in:
Weizhen Huang
2024-01-04 14:17:06 +13:00
parent 950759a526
commit ea669cb8dc
1 changed files with 20 additions and 22 deletions
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42
source/blender/draw/engines/eevee_next/shaders/eevee_bxdf_sampling_lib.glsl
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@@ -39,35 +39,33 @@ float sample_pdf_ggx_refract(
*
* \param rand: random point on the unit cylinder (result of sample_cylinder).
* \param alpha: roughness parameter.
* \param tV: tangent space view vector.
* \param Vt: tangent space view vector.
* \param G1_V: output G1_V factor to be reused.
*/
vec3 sample_ggx(vec3 rand, float alpha, vec3 Vt, out float G1_V)
{
#if GGX_USE_VISIBLE_NORMAL
/* From:
* "A Simpler and Exact Sampling Routine for the GGXDistribution of Visible Normals"
* by Eric Heitz.
* http://jcgt.org/published/0007/04/01/slides.pdf
/* Sampling Visible GGX Normals with Spherical Caps.
* Jonathan Dupuy and Anis Benyoub, HPG Vol. 42, No. 8, 2023.
* https://diglib.eg.org/bitstream/handle/10.1111/cgf14867/v42i8_03_14867.pdf
* View vector is expected to be in tangent space. */
/* Stretch view. */
vec3 Th, Bh, Vh = normalize(vec3(alpha * Vt.xy, Vt.z));
make_orthonormal_basis(Vh, Th, Bh);
/* Sample point with polar coordinates (r, phi). */
float r = sqrt(rand.x);
float x = r * rand.y;
float y = r * rand.z;
float s = 0.5 * (1.0 + Vh.z);
G1_V = Vh.z / s;
y = (1.0 - s) * sqrt(1.0 - x * x) + s * y;
float z = sqrt(saturate(1.0 - x * x - y * y));
/* Compute normal. */
vec3 Hh = x * Th + y * Bh + z * Vh;
/* Unstretch. */
vec3 Ht = normalize(vec3(alpha * Hh.xy, saturate(Hh.z)));
/* Microfacet Normal. */
return Ht;
/* Transforming the view direction to the hemisphere configuration. */
vec3 Vh = normalize(vec3(alpha * Vt.xy, Vt.z));
/* Visibility term. */
G1_V = 2.0 * Vh.z / (1.0 + Vh.z);
/* Sample a spherical cap in (-Vh.z, 1]. */
float cos_theta = mix(-Vh.z, 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. */
return normalize(vec3(alpha * Hh.xy, max(0.0, Hh.z)));
#else
/* Theta is the cone angle. */
float z = sqrt((1.0 - rand.x) / (1.0 + square(alpha) * rand.x - rand.x)); /* cos theta */