Cycles: Add new Sky Texture method including direct sunlight

This commit adds a new model to the Sky Texture node, which is based on a
method by Nishita et al. and works by basically simulating volumetric
scattering in the atmosphere.

By making some approximations (such as only considering single scattering),
we get a fairly simple and fast simulation code that takes into account
Rayleigh and Mie scattering as well as Ozone absorption.

This code is used to precompute a 512x128 texture which is then looked up
during render time, and is fast enough to allow real-time tweaking in the
viewport.

Due to the nature of the simulation, it exposes several parameters that
allow for lots of flexibility in choosing the look and matching real-world
conditions (such as Air/Dust/Ozone density and altitude).

Additionally, the same volumetric approach can be used to compute absorption
of the direct sunlight, so the model also supports adding direct sunlight.
This makes it significantly easier to set up Sun+Sky illumination where
the direction, intensity and color of the sun actually matches the sky.

In order to support properly sampling the direct sun component, the commit
also adds logic for sampling a specific area to the kernel light sampling
code. This is combined with portal and background map sampling using MIS.

This sampling logic works for the common case of having one Sky texture
going into the Background shader, but if a custom input to the Vector
node is used or if there are multiple Sky textures, it falls back to using
only background map sampling (while automatically setting the resolution to
4096x2048 if auto resolution is used).

More infos and preview can be found here:
https://docs.google.com/document/d/1gQta0ygFWXTrl5Pmvl_nZRgUw0mWg0FJeRuNKS36m08/view

Underlying model, implementation and documentation by Marco (@nacioss).
Improvements, cleanup and sun sampling by @lukasstockner.

Differential Revision: https://developer.blender.org/D7896

1 files changed, 106 insertions, 17 deletions

diff --git a/intern/cycles/kernel/shaders/node_sky_texture.osl b/intern/cycles/kernel/shaders/node_sky_texture.osl
index 4def237a2e0..08bc8f85120 100644
--- a/intern/cycles/kernel/shaders/node_sky_texture.osl
+++ b/intern/cycles/kernel/shaders/node_sky_texture.osl
@@ -44,13 +44,13 @@ float sky_perez_function(float lam[9], float theta, float gamma)
          (1.0 + lam[2] * exp(lam[3] * gamma) + lam[4] * cgamma * cgamma);
 }
 
-color sky_radiance_old(normal dir,
-                       float sunphi,
-                       float suntheta,
-                       color radiance,
-                       float config_x[9],
-                       float config_y[9],
-                       float config_z[9])
+color sky_radiance_preetham(normal dir,
+                            float sunphi,
+                            float suntheta,
+                            color radiance,
+                            float config_x[9],
+                            float config_y[9],
+                            float config_z[9])
 {
   /* convert vector to spherical coordinates */
   vector spherical = sky_spherical_coordinates(dir);
@@ -88,13 +88,13 @@ float sky_radiance_internal(float config[9], float theta, float gamma)
          (config[2] + config[3] * expM + config[5] * rayM + config[6] * mieM + config[7] * zenith);
 }
 
-color sky_radiance_new(normal dir,
-                       float sunphi,
-                       float suntheta,
-                       color radiance,
-                       float config_x[9],
-                       float config_y[9],
-                       float config_z[9])
+color sky_radiance_hosek(normal dir,
+                         float sunphi,
+                         float suntheta,
+                         color radiance,
+                         float config_x[9],
+                         float config_y[9],
+                         float config_z[9])
 {
   /* convert vector to spherical coordinates */
   vector spherical = sky_spherical_coordinates(dir);
@@ -116,16 +116,103 @@ color sky_radiance_new(normal dir,
   return xyz_to_rgb(x, y, z) * (M_2PI / 683);
 }
 
+/* Nishita improved */
+vector geographical_to_direction(float lat, float lon)
+{
+  return vector(cos(lat) * cos(lon), cos(lat) * sin(lon), sin(lat));
+}
+
+color sky_radiance_nishita(vector dir, float nishita_data[9], string filename)
+{
+  /* definitions */
+  float sun_elevation = nishita_data[6];
+  float sun_rotation = nishita_data[7];
+  float angular_diameter = nishita_data[8];
+  int sun_disc = angular_diameter > 0;
+  float alpha = 1.0;
+  color xyz;
+  /* convert dir to spherical coordinates */
+  vector direction = sky_spherical_coordinates(dir);
+
+  /* render above the horizon */
+  if (dir[2] >= 0.0) {
+    /* definitions */
+    vector sun_dir = geographical_to_direction(sun_elevation, sun_rotation + M_PI_2);
+    float sun_dir_angle = acos(dot(dir, sun_dir));
+    float half_angular = angular_diameter / 2.0;
+    float dir_elevation = M_PI_2 - direction[0];
+
+    /* if ray inside sun disc render it, otherwise render sky */
+    if (sun_dir_angle < half_angular && sun_disc == 1) {
+      /* get 3 pixels data */
+      color pixel_bottom = color(nishita_data[0], nishita_data[1], nishita_data[2]);
+      color pixel_top = color(nishita_data[3], nishita_data[4], nishita_data[5]);
+      float y;
+
+      /* sun interpolation */
+      if (sun_elevation - half_angular > 0.0) {
+        if ((sun_elevation + half_angular) > 0.0) {
+          y = ((dir_elevation - sun_elevation) / angular_diameter) + 0.5;
+          xyz = mix(pixel_bottom, pixel_top, y);
+        }
+      }
+      else {
+        if (sun_elevation + half_angular > 0.0) {
+          y = dir_elevation / (sun_elevation + half_angular);
+          xyz = mix(pixel_bottom, pixel_top, y);
+        }
+      }
+      /* limb darkening, coefficient is 0.6f */
+      float angle_fraction = sun_dir_angle / half_angular;
+      float limb_darkening = (1.0 - 0.6 * (1.0 - sqrt(1.0 - angle_fraction * angle_fraction)));
+      xyz *= limb_darkening;
+    }
+    /* sky */
+    else {
+      /* sky interpolation */
+      float x = (direction[1] + M_PI + sun_rotation) / M_2PI;
+      float y = 1.0 - (dir_elevation / M_PI_2);
+      if (x > 1.0) {
+        x = x - 1.0;
+      }
+      xyz = (color)texture(filename, x, y, "wrap", "clamp", "interp", "linear", "alpha", alpha);
+    }
+  }
+  /* ground */
+  else {
+    if (dir[2] < -0.4) {
+      xyz = color(0, 0, 0);
+    }
+    else {
+      /* black ground fade */
+      float mul = pow(1.0 + dir[2] * 2.5, 3.0);
+      /* interpolation */
+      float x = (direction[1] + M_PI + sun_rotation) / M_2PI;
+      float y = 1.5;
+      if (x > 1.0) {
+        x = x - 1.0;
+      }
+      xyz = (color)texture(
+                filename, x, y, "wrap", "periodic", "interp", "linear", "alpha", alpha) *
+            mul;
+    }
+  }
+  /* convert to RGB and adjust strength */
+  return xyz_to_rgb(xyz[0], xyz[1], xyz[2]) * 120000.0;
+}
+
 shader node_sky_texture(int use_mapping = 0,
                         matrix mapping = matrix(0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0),
                         vector Vector = P,
                         string type = "hosek_wilkie",
                         float theta = 0.0,
                         float phi = 0.0,
+                        string filename = "",
                         color radiance = color(0.0, 0.0, 0.0),
                         float config_x[9] = {0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0},
                         float config_y[9] = {0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0},
                         float config_z[9] = {0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0},
+                        float nishita_data[9] = {0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0},
                         output color Color = color(0.0, 0.0, 0.0))
 {
   vector p = Vector;
@@ -133,8 +220,10 @@ shader node_sky_texture(int use_mapping = 0,
   if (use_mapping)
     p = transform(mapping, p);
 
+  if (type == "nishita_improved")
+    Color = sky_radiance_nishita(p, nishita_data, filename);
   if (type == "hosek_wilkie")
-    Color = sky_radiance_new(p, phi, theta, radiance, config_x, config_y, config_z);
-  else
-    Color = sky_radiance_old(p, phi, theta, radiance, config_x, config_y, config_z);
+    Color = sky_radiance_hosek(p, phi, theta, radiance, config_x, config_y, config_z);
+  if (type == "preetham")
+    Color = sky_radiance_preetham(p, phi, theta, radiance, config_x, config_y, config_z);
 }