Cycles: improve the Oren-Nayar BSDF (roughness value for Diffuse), to avoid

undesired dark rings, and give more accurate lighting when the light is
behind the object. As a bonus, the code is simpler & faster.

Patch by Yasuhiro Fujii, detailed explanation here:
http://mimosa-pudica.net/improved-oren-nayar.html

1 files changed, 6 insertions, 20 deletions

diff --git a/intern/cycles/kernel/svm/bsdf_oren_nayar.h b/intern/cycles/kernel/svm/bsdf_oren_nayar.h
index 0ad7cad06bb..81884de8798 100644
--- a/intern/cycles/kernel/svm/bsdf_oren_nayar.h
+++ b/intern/cycles/kernel/svm/bsdf_oren_nayar.h
@@ -55,25 +55,11 @@ __device float3 bsdf_oren_nayar_get_intensity(const ShaderClosure *sc, float3 n,
 {
 	float nl = max(dot(n, l), 0.0f);
 	float nv = max(dot(n, v), 0.0f);
+	float t = dot(l, v) - nl * nv;
 
-	float3 al = normalize(l - nl * n);
-	float3 av = normalize(v - nv * n);
-	float t = max(dot(al, av), 0.0f);
-
-	float cos_a, cos_b;
-	if(nl < nv) {
-		cos_a = nl;
-		cos_b = nv;
-	}
-	else {
-		cos_a = nv;
-		cos_b = nl;
-	}
-
-	float sin_a = sqrtf(max(1.0f - cos_a * cos_a, 0.0f));
-	float tan_b = sqrtf(max(1.0f - cos_b * cos_b, 0.0f)) / max(cos_b, 1e-8f);
-
-	float is = nl * (sc->data0 + sc->data1 * t * sin_a * tan_b);
+	if (t > 0.0f)
+		t /= max(nl, nv) + FLT_MIN;
+	float is = nl * (sc->data0 + sc->data1 * t);
 	return make_float3(is, is, is);
 }
 
@@ -84,9 +70,9 @@ __device void bsdf_oren_nayar_setup(ShaderData *sd, ShaderClosure *sc, float sig
 
 	sigma = clamp(sigma, 0.0f, 1.0f);
 
-	float div = 1.0f / ((1.0f + 0.5f * sigma) * M_PI_F);
+	float div = 1.0f / (M_PI_F + ((3.0f * M_PI_F - 4.0f) / 6.0f) * sigma);
 
-	sc->data0 =  1.0f * div;
+	sc->data0 = 1.0f * div;
 	sc->data1 = sigma * div;
 }