[Bf-blender-cvs] [3e7389e] master: Cycles: Speedup of Christensen-Burley SSS falloff function

[Sergey Sharybin]

Commit: 3e7389eaf2d7ffbd180931c117776d3de2da5adb
Author: Sergey Sharybin
Date:   Thu Feb 4 15:25:29 2016 +0100
Branches: master
https://developer.blender.org/rB3e7389eaf2d7ffbd180931c117776d3de2da5adb

Cycles: Speedup of Christensen-Burley SSS falloff function

The idea is simply to pre-compute fitting and parameterization
in the bssrdf_setup() function and re-use the values in both
sample() and eval().

The only trick is where to store the pre-calculated values and
the answer is inside of ShaderClosure->custom{1,2,3}. There's
no memory bump here because we now simply re-use padding fields
for the pre-calculated values. Similar trick we can do for other
BSDFs.

Seems to give nice speedup up to 7% here on my desktop with
Core i7 CPU, SSE4.1 kernel.

===================================================================

M	intern/cycles/kernel/closure/bssrdf.h
M	intern/cycles/kernel/kernel_types.h

===================================================================

diff --git a/intern/cycles/kernel/closure/bssrdf.h b/intern/cycles/kernel/closure/bssrdf.h
index 041832e..522bc36 100644
--- a/intern/cycles/kernel/closure/bssrdf.h
+++ b/intern/cycles/kernel/closure/bssrdf.h
@@ -19,25 +19,6 @@
 
 CCL_NAMESPACE_BEGIN
 
-ccl_device int bssrdf_setup(ShaderClosure *sc, ClosureType type)
-{
-	if(sc->data0 < BSSRDF_MIN_RADIUS) {
-		/* revert to diffuse BSDF if radius too small */
-		sc->data0 = 0.0f;
-		sc->data1 = 0.0f;
-		int flag = bsdf_diffuse_setup(sc);
-		sc->type = CLOSURE_BSDF_BSSRDF_ID;
-		return flag;
-	}
-	else {
-		sc->data1 = saturate(sc->data1); /* texture blur */
-		sc->T.x = saturate(sc->T.x); /* sharpness */
-		sc->type = type;
-
-		return SD_BSDF|SD_BSDF_HAS_EVAL|SD_BSSRDF;
-	}
-}
-
 /* Planar Truncated Gaussian
  *
  * Note how this is different from the typical gaussian, this one integrates
@@ -210,13 +191,24 @@ ccl_device_inline float bssrdf_burley_compatible_mfp(float r)
 	return 0.5f * M_1_PI_F * r;
 }
 
-ccl_device float bssrdf_burley_eval(ShaderClosure *sc, float r)
+ccl_device void bssrdf_burley_setup(ShaderClosure *sc)
 {
 	/* Mean free path length. */
 	const float l = bssrdf_burley_compatible_mfp(sc->data0);
 	/* Surface albedo. */
 	const float A = sc->data2;
 	const float s = bssrdf_burley_fitting(A);
+	const float d = l / s;
+
+	sc->custom1 = l;
+	sc->custom2 = s;
+	sc->custom3 = d;
+}
+
+ccl_device float bssrdf_burley_eval(ShaderClosure *sc, float r)
+{
+	const float l = sc->custom1,
+	            s = sc->custom2;
 	/* Burley refletance profile, equation (3).
 	 *
 	 * Note that surface albedo is already included into sc->weight, no need to
@@ -277,12 +269,7 @@ ccl_device void bssrdf_burley_sample(ShaderClosure *sc,
                                      float *r,
                                      float *h)
 {
-	/* Mean free path length. */
-	const float l = bssrdf_burley_compatible_mfp(sc->data0);
-	/* Surface albedo. */
-	const float A = sc->data2;
-	const float s = bssrdf_burley_fitting(A);
-	const float d = l / s;
+	const float d = sc->custom3;
 	/* This is a bit arbitrary, just need big enough radius so it matches
 	 * the mean free length, but still not too big so sampling is still
 	 * effective. Might need some further tweaks.
@@ -330,6 +317,29 @@ ccl_device void bssrdf_none_sample(ShaderClosure *sc, float xi, float *r, float
 
 /* Generic */
 
+ccl_device int bssrdf_setup(ShaderClosure *sc, ClosureType type)
+{
+	if(sc->data0 < BSSRDF_MIN_RADIUS) {
+		/* revert to diffuse BSDF if radius too small */
+		sc->data0 = 0.0f;
+		sc->data1 = 0.0f;
+		int flag = bsdf_diffuse_setup(sc);
+		sc->type = CLOSURE_BSDF_BSSRDF_ID;
+		return flag;
+	}
+	else {
+		sc->data1 = saturate(sc->data1); /* texture blur */
+		sc->T.x = saturate(sc->T.x); /* sharpness */
+		sc->type = type;
+
+		if(type == CLOSURE_BSSRDF_BURLEY_ID) {
+			bssrdf_burley_setup(sc);
+		}
+
+		return SD_BSDF|SD_BSDF_HAS_EVAL|SD_BSSRDF;
+	}
+}
+
 ccl_device void bssrdf_sample(ShaderClosure *sc, float xi, float *r, float *h)
 {
 	if(sc->type == CLOSURE_BSSRDF_CUBIC_ID)
diff --git a/intern/cycles/kernel/kernel_types.h b/intern/cycles/kernel/kernel_types.h
index 93ff732..42d40e1 100644
--- a/intern/cycles/kernel/kernel_types.h
+++ b/intern/cycles/kernel/kernel_types.h
@@ -642,7 +642,14 @@ typedef ccl_addr_space struct ShaderClosure {
 	float data0;
 	float data1;
 	float data2;
-	int pad1, pad2, pad3;
+
+	/* Following fields could be used to store pre-calculated
+	 * values by various BSDF closures for more effective sampling
+	 * and evaluation.
+	 */
+	float custom1;
+	float custom2;
+	float custom3;
 
 #ifdef __OSL__
 	void *prim, *pad4;