Cycles: initial subsurface multiple scattering support. It's not working as

well as I would like, but it works, just add a subsurface scattering node and
you can use it like any other BSDF.

It is using fully raytraced sampling compatible with progressive rendering
and other more advanced rendering algorithms we might used in the future, and
it uses no extra memory so it's suitable for complex scenes.

Disadvantage is that it can be quite noisy and slow. Two limitations that will
be solved are that it does not work with bump mapping yet, and that the falloff
function used is a simple cubic function, it's not using the real BSSRDF
falloff function yet.

The node has a color input, along with a scattering radius for each RGB color
channel along with an overall scale factor for the radii.

There is also no GPU support yet, will test if I can get that working later.

Node Documentation:
http://wiki.blender.org/index.php/Doc:2.6/Manual/Render/Cycles/Nodes/Shaders#BSSRDF

Implementation notes:
http://wiki.blender.org/index.php/Dev:2.6/Source/Render/Cycles/Subsurface_Scattering

1 files changed, 154 insertions, 0 deletions

diff --git a/intern/cycles/kernel/closure/bssrdf.h b/intern/cycles/kernel/closure/bssrdf.h
new file mode 100644
index 00000000000..1327fbd011e
--- /dev/null
+++ b/intern/cycles/kernel/closure/bssrdf.h
@@ -0,0 +1,154 @@
+/*
+ * Copyright 2013, Blender Foundation.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ */
+
+#ifndef __KERNEL_BSSRDF_H__
+#define __KERNEL_BSSRDF_H__
+
+CCL_NAMESPACE_BEGIN
+
+__device int bssrdf_setup(ShaderClosure *sc)
+{
+	if(sc->data0 < BSSRDF_MIN_RADIUS) {
+		/* revert to diffuse BSDF if radius too small */
+		sc->data0 = 0.0f;
+		sc->data1 = 0.0f;
+		return bsdf_diffuse_setup(sc);
+	}
+	else {
+		/* radius + IOR params */
+		sc->data0 = max(sc->data0, 0.0f);
+		sc->data1 = max(sc->data1, 1.0f);
+		sc->type = CLOSURE_BSSRDF_ID;
+
+		return SD_BSDF|SD_BSDF_HAS_EVAL|SD_BSSRDF;
+	}
+}
+
+/* Simple Cubic BSSRDF falloff */
+
+__device float bssrdf_cubic(float ld, float r)
+{
+	if(ld == 0.0f)
+		return (r == 0.0f)? 1.0f: 0.0f;
+
+	return powf(ld - min(r, ld), 3.0f) * 4.0f/powf(ld, 4.0f);
+}
+
+/* Original BSSRDF fallof function */
+
+typedef struct BSSRDFParams {
+	float eta;		/* index of refraction */
+	float sigma_t_; /* reduced extinction coefficient */
+	float sigma_tr;	/* effective extinction coefficient */
+	float Fdr;		/* diffuse fresnel reflectance */
+	float D;		/* diffusion constant */
+	float A;
+	float alpha_;	/* reduced albedo */
+	float zr;		/* distance of virtual lightsource above surface */
+	float zv;		/* distance of virtual lightsource below surface */
+	float ld;		/* mean free path */
+	float ro;		/* diffuse reflectance */
+} BSSRDFParams;
+
+__device float bssrdf_reduced_albedo_Rd(float alpha_, float A, float ro)
+{
+	float sq;
+
+	sq = sqrt(3.0f*(1.0f - alpha_));
+	return (alpha_/2.0f)*(1.0f + expf((-4.0f/3.0f)*A*sq))*expf(-sq) - ro;
+}
+
+__device float bssrdf_compute_reduced_albedo(float A, float ro)
+{
+	const float tolerance = 1e-8;
+	const int max_iteration_count = 20;
+	float d, fsub, xn_1 = 0.0f, xn = 1.0f, fxn, fxn_1;
+	int i;
+
+	/* use secant method to compute reduced albedo using Rd function inverse
+	 * with a given reflectance */
+	fxn = bssrdf_reduced_albedo_Rd(xn, A, ro);
+	fxn_1 = bssrdf_reduced_albedo_Rd(xn_1, A, ro);
+
+	for (i= 0; i < max_iteration_count; i++) {
+		fsub = (fxn - fxn_1);
+		if (fabsf(fsub) < tolerance)
+			break;
+		d = ((xn - xn_1)/fsub)*fxn;
+		if (fabsf(d) < tolerance)
+			break;
+
+		xn_1 = xn;
+		fxn_1 = fxn;
+		xn = xn - d;
+
+		if (xn > 1.0f) xn = 1.0f;
+		if (xn_1 > 1.0f) xn_1 = 1.0f;
+		
+		fxn = bssrdf_reduced_albedo_Rd(xn, A, ro);
+	}
+
+	/* avoid division by zero later */
+	if (xn <= 0.0f)
+		xn = 0.00001f;
+
+	return xn;
+}
+
+__device void bssrdf_setup_params(BSSRDFParams *ss, float refl, float radius, float ior)
+{
+	ss->eta = ior;
+	ss->Fdr = -1.440f/ior*ior + 0.710f/ior + 0.668f + 0.0636f*ior;
+	ss->A = (1.0f + ss->Fdr)/(1.0f - ss->Fdr);
+	ss->ld = radius;
+	ss->ro = min(refl, 0.999f);
+
+	ss->alpha_ = bssrdf_compute_reduced_albedo(ss->A, ss->ro);
+
+	ss->sigma_tr = 1.0f/ss->ld;
+	ss->sigma_t_ = ss->sigma_tr/sqrtf(3.0f*(1.0f - ss->alpha_));
+
+	ss->D = 1.0f/(3.0f*ss->sigma_t_);
+
+	ss->zr = 1.0f/ss->sigma_t_;
+	ss->zv = ss->zr + 4.0f*ss->A*ss->D;
+}
+
+/* exponential falloff function */
+
+__device float bssrdf_original(const BSSRDFParams *ss, float r)
+{
+	if(ss->ld == 0.0f)
+		return (r == 0.0f)? 1.0f: 0.0f;
+
+	float rr = r*r;
+	float sr, sv, Rdr, Rdv;
+
+	sr = sqrt(rr + ss->zr*ss->zr);
+	sv = sqrt(rr + ss->zv*ss->zv);
+
+	Rdr = ss->zr*(1.0f + ss->sigma_tr*sr)*expf(-ss->sigma_tr*sr)/(sr*sr*sr);
+	Rdv = ss->zv*(1.0f + ss->sigma_tr*sv)*expf(-ss->sigma_tr*sv)/(sv*sv*sv);
+
+	return ss->alpha_*(1.0f/(4.0f*(float)M_PI))*(Rdr + Rdv);
+}
+
+CCL_NAMESPACE_END
+
+#endif /* __KERNEL_BSSRDF_H__ */
+