Cycles / Sky Model:

* Replaced the Preetham model with the newer Hosek / Wilkie model:
"An Analytic Model for Full Spectral Sky-Dome Radiance" http://cgg.mff.cuni.cz/projects/SkylightModelling/ 

* We use the sample code data, which comes with the paper, but removed some unnecessary parts, we only need the xyz version.
* New "Albedo" UI paraemeter, to control the ground albedo (between 0 and 1). 
* Works with SVM only atm (CPU and CUDA). 

Example render:
http://www.pasteall.org/pic/show.php?id=57635

ToDo / Open Questions:
* OSL still uses the old model, will be done later. In the meantime it's useful to compare the two models this way.
* The new model needs a much weaker Strength value (0.01), otherwise it's white. Can this be fixed? 
* Code cleanup. 

2 files changed, 18 insertions, 16 deletions

diff --git a/intern/cycles/kernel/kernel_types.h b/intern/cycles/kernel/kernel_types.h
index 836eacf7cb6..69a3a4fe28c 100644
--- a/intern/cycles/kernel/kernel_types.h
+++ b/intern/cycles/kernel/kernel_types.h
@@ -718,12 +718,11 @@ typedef struct KernelBackground {
 
 typedef struct KernelSunSky {
 	/* sun direction in spherical and cartesian */
-	float theta, phi, pad3, pad4;
+	float theta, phi;
 
 	/* perez function parameters */
-	float zenith_Y, zenith_x, zenith_y, pad2;
-	float perez_Y[5], perez_x[5], perez_y[5];
-	float pad5;
+	float radiance_x, radiance_y, radiance_z;
+	float config_x[9], config_y[9], config_z[9];
 } KernelSunSky;
 
 typedef struct KernelIntegrator {
diff --git a/intern/cycles/kernel/svm/svm_sky.h b/intern/cycles/kernel/svm/svm_sky.h
index 8b4e35816d0..d81f6ece46c 100644
--- a/intern/cycles/kernel/svm/svm_sky.h
+++ b/intern/cycles/kernel/svm/svm_sky.h
@@ -19,8 +19,8 @@
 CCL_NAMESPACE_BEGIN
 
 /*
- * "A Practical Analytic Model for Daylight"
- * A. J. Preetham, Peter Shirley, Brian Smits
+ * "An Analytic Model for Full Spectral Sky-Dome Radiance"
+ * Lukas Hosek, Alexander Wilkie
  */
 
 __device float sky_angle_between(float thetav, float phiv, float theta, float phi)
@@ -29,12 +29,16 @@ __device float sky_angle_between(float thetav, float phiv, float theta, float ph
 	return safe_acosf(cospsi);
 }
 
-__device float sky_perez_function(__constant float *lam, float theta, float gamma)
+/* ArHosekSkyModel_GetRadianceInternal */
+__device float sky_radiance_internal(__constant float *configuration, float theta, float gamma)
 {
-	float ctheta = cosf(theta);
-	float cgamma = cosf(gamma);
+    const float expM = expf(configuration[4] * gamma);
+    const float rayM = cosf(gamma)*cosf(gamma);
+    const float mieM = (1.0f + cosf(gamma)*cosf(gamma)) / powf((1.0f + configuration[8]*configuration[8] - 2.0f*configuration[8]*cosf(gamma)), 1.5f);
+    const float zenith = sqrt(cosf(theta));
 
-	return (1.0f + lam[0]*expf(lam[1]/ctheta)) * (1.0f + lam[2]*expf(lam[3]*gamma)  + lam[4]*cgamma*cgamma);
+    return (1.0f + configuration[0] * expf(configuration[1] / (cosf(theta) + 0.01f))) *
+            (configuration[2] + configuration[3] * expM + configuration[5] * rayM + configuration[6] * mieM + configuration[7] * zenith);
 }
 
 __device float3 sky_radiance(KernelGlobals *kg, float3 dir)
@@ -50,14 +54,13 @@ __device float3 sky_radiance(KernelGlobals *kg, float3 dir)
 	/* clamp theta to horizon */
 	theta = min(theta, M_PI_2_F - 0.001f);
 
-	/* compute xyY color space values */
-	float x = kernel_data.sunsky.zenith_x * sky_perez_function(kernel_data.sunsky.perez_x, theta, gamma);
-	float y = kernel_data.sunsky.zenith_y * sky_perez_function(kernel_data.sunsky.perez_y, theta, gamma);
-	float Y = kernel_data.sunsky.zenith_Y * sky_perez_function(kernel_data.sunsky.perez_Y, theta, gamma);
+	/* compute xyz color space values */
+	float x = sky_radiance_internal(kernel_data.sunsky.config_x, theta, gamma) * kernel_data.sunsky.radiance_x;
+	float y = sky_radiance_internal(kernel_data.sunsky.config_y, theta, gamma) * kernel_data.sunsky.radiance_y;
+	float z = sky_radiance_internal(kernel_data.sunsky.config_z, theta, gamma) * kernel_data.sunsky.radiance_z;
 
 	/* convert to RGB */
-	float3 xyz = xyY_to_xyz(x, y, Y);
-	return xyz_to_rgb(xyz.x, xyz.y, xyz.z);
+	return xyz_to_rgb(x, y, z);
 }
 
 __device void svm_node_tex_sky(KernelGlobals *kg, ShaderData *sd, float *stack, uint dir_offset, uint out_offset)