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Fix #114515: Cycles: Numerical precision issues in triangle light sampling

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The refactor in 97d9bbbc97 changed the way q is computed in the spherical triangle sampling code. While the new approach is more efficient and saves a few operations, it introduces numerical precision issues for skinny/small (spherical) triangles.

Therefore, this change moves the computation of q back to the method from the paper, while keeping the more efficient solid angle computation.

Pull Request: https://projects.blender.org/blender/blender/pulls/119224
This commit is contained in:
Lukas Stockner 2024-05-23 02:27:48 +02:00 • committed by Lukas Stockner
parent 79847acd0c
commit 392b84f879
1 changed files with 12 additions and 12 deletions
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24
intern/cycles/kernel/light/triangle.h
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@ -177,27 +177,27 @@ ccl_device_forceinline bool triangle_light_sample(KernelGlobals kg,
const float cos_a = dot(B, C);
const float cos_b = dot(A, C);
const float cos_c = dot(A, B);
const float sin_b_sin_c_2 = (1.0f - sqr(cos_b)) * (1.0f - sqr(cos_c));
const float mixed_product = fabsf(dot(A, cross(B, C)));
/* The area of the spherical triangle is equal to the subtended solid angle. */
const float solid_angle = 2.0f * fast_atan2f(mixed_product, (1.0f + cos_a + cos_b + cos_c));
/* Compute the angle at A. */
const float cos_alpha = dot(safe_normalize(cross(A, B)), safe_normalize(cross(A, C)));
const float sin_alpha = sin_from_cos(cos_alpha);
const float alpha = safe_acosf(cos_alpha);
/* Select a random sub-area of the spherical triangle and calculate the third vertex C_ of that
* new triangle. */
const float A_hat = rand.x * solid_angle;
float sin_A_hat, cos_A_hat;
fast_sincosf(A_hat, &sin_A_hat, &cos_A_hat);
/* These values lack a `sin_b * sin_c` factor, will divide when computing `temp`. */
const float cos_alpha = cos_a - cos_b * cos_c;
const float sin_alpha = mixed_product;
const float t = cos_A_hat * cos_alpha + sin_A_hat * sin_alpha;
const float temp = (cos_c - 1.0f) * t * cos_alpha / sin_b_sin_c_2;
const float q = (cos_A_hat - cos_c + temp) / (1.0f - cos_A_hat * cos_c + temp);
float sin_phi, cos_phi;
fast_sincosf(A_hat - alpha, &sin_phi, &cos_phi);
const float u = cos_phi - cos_alpha;
const float v = sin_phi + sin_alpha * cos_c;
const float num = (v * cos_phi - u * sin_phi) * cos_alpha - v;
const float den = (v * sin_phi + u * cos_phi) * sin_alpha;
const float q = (den == 0.0f) ? 1.0f : num / den;
const float3 U = safe_normalize(C - cos_b * A);
const float3 C_ = safe_normalize(q * A + sin_from_cos(q) * U);