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, 448 insertions, 0 deletions

diff --git a/intern/cycles/kernel/kernel_light_background.h b/intern/cycles/kernel/kernel_light_background.h
new file mode 100644
index 00000000000..30e336f0f80
--- /dev/null
+++ b/intern/cycles/kernel/kernel_light_background.h
@@ -0,0 +1,448 @@
+/*
+ * Copyright 2011-2020 Blender Foundation
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "kernel_light_common.h"
+
+CCL_NAMESPACE_BEGIN
+
+/* Background Light */
+
+#ifdef __BACKGROUND_MIS__
+
+ccl_device float3 background_map_sample(KernelGlobals *kg, float randu, float randv, float *pdf)
+{
+  /* for the following, the CDF values are actually a pair of floats, with the
+   * function value as X and the actual CDF as Y.  The last entry's function
+   * value is the CDF total. */
+  int res_x = kernel_data.background.map_res_x;
+  int res_y = kernel_data.background.map_res_y;
+  int cdf_width = res_x + 1;
+
+  /* this is basically std::lower_bound as used by pbrt */
+  int first = 0;
+  int count = res_y;
+
+  while (count > 0) {
+    int step = count >> 1;
+    int middle = first + step;
+
+    if (kernel_tex_fetch(__light_background_marginal_cdf, middle).y < randv) {
+      first = middle + 1;
+      count -= step + 1;
+    }
+    else
+      count = step;
+  }
+
+  int index_v = max(0, first - 1);
+  kernel_assert(index_v >= 0 && index_v < res_y);
+
+  float2 cdf_v = kernel_tex_fetch(__light_background_marginal_cdf, index_v);
+  float2 cdf_next_v = kernel_tex_fetch(__light_background_marginal_cdf, index_v + 1);
+  float2 cdf_last_v = kernel_tex_fetch(__light_background_marginal_cdf, res_y);
+
+  /* importance-sampled V direction */
+  float dv = inverse_lerp(cdf_v.y, cdf_next_v.y, randv);
+  float v = (index_v + dv) / res_y;
+
+  /* this is basically std::lower_bound as used by pbrt */
+  first = 0;
+  count = res_x;
+  while (count > 0) {
+    int step = count >> 1;
+    int middle = first + step;
+
+    if (kernel_tex_fetch(__light_background_conditional_cdf, index_v * cdf_width + middle).y <
+        randu) {
+      first = middle + 1;
+      count -= step + 1;
+    }
+    else
+      count = step;
+  }
+
+  int index_u = max(0, first - 1);
+  kernel_assert(index_u >= 0 && index_u < res_x);
+
+  float2 cdf_u = kernel_tex_fetch(__light_background_conditional_cdf,
+                                  index_v * cdf_width + index_u);
+  float2 cdf_next_u = kernel_tex_fetch(__light_background_conditional_cdf,
+                                       index_v * cdf_width + index_u + 1);
+  float2 cdf_last_u = kernel_tex_fetch(__light_background_conditional_cdf,
+                                       index_v * cdf_width + res_x);
+
+  /* importance-sampled U direction */
+  float du = inverse_lerp(cdf_u.y, cdf_next_u.y, randu);
+  float u = (index_u + du) / res_x;
+
+  /* compute pdf */
+  float sin_theta = sinf(M_PI_F * v);
+  float denom = (M_2PI_F * M_PI_F * sin_theta) * cdf_last_u.x * cdf_last_v.x;
+
+  if (sin_theta == 0.0f || denom == 0.0f)
+    *pdf = 0.0f;
+  else
+    *pdf = (cdf_u.x * cdf_v.x) / denom;
+
+  /* compute direction */
+  return equirectangular_to_direction(u, v);
+}
+
+/* TODO(sergey): Same as above, after the release we should consider using
+ * 'noinline' for all devices.
+ */
+ccl_device float background_map_pdf(KernelGlobals *kg, float3 direction)
+{
+  float2 uv = direction_to_equirectangular(direction);
+  int res_x = kernel_data.background.map_res_x;
+  int res_y = kernel_data.background.map_res_y;
+  int cdf_width = res_x + 1;
+
+  float sin_theta = sinf(uv.y * M_PI_F);
+
+  if (sin_theta == 0.0f)
+    return 0.0f;
+
+  int index_u = clamp(float_to_int(uv.x * res_x), 0, res_x - 1);
+  int index_v = clamp(float_to_int(uv.y * res_y), 0, res_y - 1);
+
+  /* pdfs in V direction */
+  float2 cdf_last_u = kernel_tex_fetch(__light_background_conditional_cdf,
+                                       index_v * cdf_width + res_x);
+  float2 cdf_last_v = kernel_tex_fetch(__light_background_marginal_cdf, res_y);
+
+  float denom = (M_2PI_F * M_PI_F * sin_theta) * cdf_last_u.x * cdf_last_v.x;
+
+  if (denom == 0.0f)
+    return 0.0f;
+
+  /* pdfs in U direction */
+  float2 cdf_u = kernel_tex_fetch(__light_background_conditional_cdf,
+                                  index_v * cdf_width + index_u);
+  float2 cdf_v = kernel_tex_fetch(__light_background_marginal_cdf, index_v);
+
+  return (cdf_u.x * cdf_v.x) / denom;
+}
+
+ccl_device_inline bool background_portal_data_fetch_and_check_side(
+    KernelGlobals *kg, float3 P, int index, float3 *lightpos, float3 *dir)
+{
+  int portal = kernel_data.background.portal_offset + index;
+  const ccl_global KernelLight *klight = &kernel_tex_fetch(__lights, portal);
+
+  *lightpos = make_float3(klight->co[0], klight->co[1], klight->co[2]);
+  *dir = make_float3(klight->area.dir[0], klight->area.dir[1], klight->area.dir[2]);
+
+  /* Check whether portal is on the right side. */
+  if (dot(*dir, P - *lightpos) > 1e-4f)
+    return true;
+
+  return false;
+}
+
+ccl_device_inline float background_portal_pdf(
+    KernelGlobals *kg, float3 P, float3 direction, int ignore_portal, bool *is_possible)
+{
+  float portal_pdf = 0.0f;
+
+  int num_possible = 0;
+  for (int p = 0; p < kernel_data.background.num_portals; p++) {
+    if (p == ignore_portal)
+      continue;
+
+    float3 lightpos, dir;
+    if (!background_portal_data_fetch_and_check_side(kg, P, p, &lightpos, &dir))
+      continue;
+
+    /* There's a portal that could be sampled from this position. */
+    if (is_possible) {
+      *is_possible = true;
+    }
+    num_possible++;
+
+    int portal = kernel_data.background.portal_offset + p;
+    const ccl_global KernelLight *klight = &kernel_tex_fetch(__lights, portal);
+    float3 axisu = make_float3(
+        klight->area.axisu[0], klight->area.axisu[1], klight->area.axisu[2]);
+    float3 axisv = make_float3(
+        klight->area.axisv[0], klight->area.axisv[1], klight->area.axisv[2]);
+    bool is_round = (klight->area.invarea < 0.0f);
+
+    if (!ray_quad_intersect(P,
+                            direction,
+                            1e-4f,
+                            FLT_MAX,
+                            lightpos,
+                            axisu,
+                            axisv,
+                            dir,
+                            NULL,
+                            NULL,
+                            NULL,
+                            NULL,
+                            is_round))
+      continue;
+
+    if (is_round) {
+      float t;
+      float3 D = normalize_len(lightpos - P, &t);
+      portal_pdf += fabsf(klight->area.invarea) * lamp_light_pdf(kg, dir, -D, t);
+    }
+    else {
+      portal_pdf += rect_light_sample(P, &lightpos, axisu, axisv, 0.0f, 0.0f, false);
+    }
+  }
+
+  if (ignore_portal >= 0) {
+    /* We have skipped a portal that could be sampled as well. */
+    num_possible++;
+  }
+
+  return (num_possible > 0) ? portal_pdf / num_possible : 0.0f;
+}
+
+ccl_device int background_num_possible_portals(KernelGlobals *kg, float3 P)
+{
+  int num_possible_portals = 0;
+  for (int p = 0; p < kernel_data.background.num_portals; p++) {
+    float3 lightpos, dir;
+    if (background_portal_data_fetch_and_check_side(kg, P, p, &lightpos, &dir))
+      num_possible_portals++;
+  }
+  return num_possible_portals;
+}
+
+ccl_device float3 background_portal_sample(KernelGlobals *kg,
+                                           float3 P,
+                                           float randu,
+                                           float randv,
+                                           int num_possible,
+                                           int *sampled_portal,
+                                           float *pdf)
+{
+  /* Pick a portal, then re-normalize randv. */
+  randv *= num_possible;
+  int portal = (int)randv;
+  randv -= portal;
+
+  /* TODO(sergey): Some smarter way of finding portal to sample
+   * is welcome.
+   */
+  for (int p = 0; p < kernel_data.background.num_portals; p++) {
+    /* Search for the sampled portal. */
+    float3 lightpos, dir;
+    if (!background_portal_data_fetch_and_check_side(kg, P, p, &lightpos, &dir))
+      continue;
+
+    if (portal == 0) {
+      /* p is the portal to be sampled. */
+      int portal = kernel_data.background.portal_offset + p;
+      const ccl_global KernelLight *klight = &kernel_tex_fetch(__lights, portal);
+      float3 axisu = make_float3(
+          klight->area.axisu[0], klight->area.axisu[1], klight->area.axisu[2]);
+      float3 axisv = make_float3(
+          klight->area.axisv[0], klight->area.axisv[1], klight->area.axisv[2]);
+      bool is_round = (klight->area.invarea < 0.0f);
+
+      float3 D;
+      if (is_round) {
+        lightpos += ellipse_sample(axisu * 0.5f, axisv * 0.5f, randu, randv);
+        float t;
+        D = normalize_len(lightpos - P, &t);
+        *pdf = fabsf(klight->area.invarea) * lamp_light_pdf(kg, dir, -D, t);
+      }
+      else {
+        *pdf = rect_light_sample(P, &lightpos, axisu, axisv, randu, randv, true);
+        D = normalize(lightpos - P);
+      }
+
+      *pdf /= num_possible;
+      *sampled_portal = p;
+      return D;
+    }
+
+    portal--;
+  }
+
+  return make_float3(0.0f, 0.0f, 0.0f);
+}
+
+ccl_device_inline float3 background_sun_sample(KernelGlobals *kg,
+                                               float randu,
+                                               float randv,
+                                               float *pdf)
+{
+  float3 D;
+  const float3 N = float4_to_float3(kernel_data.background.sun);
+  const float angle = kernel_data.background.sun.w;
+  sample_uniform_cone(N, angle, randu, randv, &D, pdf);
+  return D;
+}
+
+ccl_device_inline float background_sun_pdf(KernelGlobals *kg, float3 D)
+{
+  const float3 N = float4_to_float3(kernel_data.background.sun);
+  const float angle = kernel_data.background.sun.w;
+  return pdf_uniform_cone(N, D, angle);
+}
+
+ccl_device_inline float3
+background_light_sample(KernelGlobals *kg, float3 P, float randu, float randv, float *pdf)
+{
+  float portal_method_pdf = kernel_data.background.portal_weight;
+  float sun_method_pdf = kernel_data.background.sun_weight;
+  float map_method_pdf = kernel_data.background.map_weight;
+
+  int num_portals = 0;
+  if (portal_method_pdf > 0.0f) {
+    /* Check if there are portals in the scene which we can sample. */
+    num_portals = background_num_possible_portals(kg, P);
+    if (num_portals == 0) {
+      portal_method_pdf = 0.0f;
+    }
+  }
+
+  float pdf_fac = (portal_method_pdf + sun_method_pdf + map_method_pdf);
+  if (pdf_fac == 0.0f) {
+    /* Use uniform as a fallback if we can't use any strategy. */
+    *pdf = 1.0f / M_4PI_F;
+    return sample_uniform_sphere(randu, randv);
+  }
+
+  pdf_fac = 1.0f / pdf_fac;
+  portal_method_pdf *= pdf_fac;
+  sun_method_pdf *= pdf_fac;
+  map_method_pdf *= pdf_fac;
+
+  /* We have 100% in total and split it between the three categories.
+   * Therefore, we pick portals if randu is between 0 and portal_method_pdf,
+   * sun if randu is between portal_method_pdf and (portal_method_pdf + sun_method_pdf)
+   * and map if randu is between (portal_method_pdf + sun_method_pdf) and 1. */
+  float sun_method_cdf = portal_method_pdf + sun_method_pdf;
+
+  int method = 0;
+  float3 D;
+  if (randu < portal_method_pdf) {
+    method = 0;
+    /* Rescale randu. */
+    if (portal_method_pdf != 1.0f) {
+      randu /= portal_method_pdf;
+    }
+
+    /* Sample a portal. */
+    int portal;
+    D = background_portal_sample(kg, P, randu, randv, num_portals, &portal, pdf);
+    if (num_portals > 1) {
+      /* Ignore the chosen portal, its pdf is already included. */
+      *pdf += background_portal_pdf(kg, P, D, portal, NULL);
+    }
+
+    /* Skip MIS if this is the only method. */
+    if (portal_method_pdf == 1.0f) {
+      return D;
+    }
+    *pdf *= portal_method_pdf;
+  }
+  else if (randu < sun_method_cdf) {
+    method = 1;
+    /* Rescale randu. */
+    if (sun_method_pdf != 1.0f) {
+      randu = (randu - portal_method_pdf) / sun_method_pdf;
+    }
+
+    D = background_sun_sample(kg, randu, randv, pdf);
+
+    /* Skip MIS if this is the only method. */
+    if (sun_method_pdf == 1.0f) {
+      return D;
+    }
+    *pdf *= sun_method_pdf;
+  }
+  else {
+    method = 2;
+    /* Rescale randu. */
+    if (map_method_pdf != 1.0f) {
+      randu = (randu - sun_method_cdf) / map_method_pdf;
+    }
+
+    D = background_map_sample(kg, randu, randv, pdf);
+
+    /* Skip MIS if this is the only method. */
+    if (map_method_pdf == 1.0f) {
+      return D;
+    }
+    *pdf *= map_method_pdf;
+  }
+
+  /* MIS weighting. */
+  if (method != 0 && portal_method_pdf != 0.0f) {
+    *pdf += portal_method_pdf * background_portal_pdf(kg, P, D, -1, NULL);
+  }
+  if (method != 1 && sun_method_pdf != 0.0f) {
+    *pdf += sun_method_pdf * background_sun_pdf(kg, D);
+  }
+  if (method != 2 && map_method_pdf != 0.0f) {
+    *pdf += map_method_pdf * background_map_pdf(kg, D);
+  }
+  return D;
+}
+
+ccl_device float background_light_pdf(KernelGlobals *kg, float3 P, float3 direction)
+{
+  float portal_method_pdf = kernel_data.background.portal_weight;
+  float sun_method_pdf = kernel_data.background.sun_weight;
+  float map_method_pdf = kernel_data.background.map_weight;
+
+  float portal_pdf = 0.0f;
+  /* Portals are a special case here since we need to compute their pdf in order
+   * to find out if we can sample them. */
+  if (portal_method_pdf > 0.0f) {
+    /* Evaluate PDF of sampling this direction by portal sampling. */
+    bool is_possible = false;
+    portal_pdf = background_portal_pdf(kg, P, direction, -1, &is_possible);
+    if (!is_possible) {
+      /* Portal sampling is not possible here because all portals point to the wrong side.
+       * If other methods can be used instead, do so, otherwise uniform sampling is used as a
+       * fallback. */
+      portal_method_pdf = 0.0f;
+    }
+  }
+
+  float pdf_fac = (portal_method_pdf + sun_method_pdf + map_method_pdf);
+  if (pdf_fac == 0.0f) {
+    /* Use uniform as a fallback if we can't use any strategy. */
+    return kernel_data.integrator.pdf_lights / M_4PI_F;
+  }
+
+  pdf_fac = 1.0f / pdf_fac;
+  portal_method_pdf *= pdf_fac;
+  sun_method_pdf *= pdf_fac;
+  map_method_pdf *= pdf_fac;
+
+  float pdf = portal_pdf * portal_method_pdf;
+  if (sun_method_pdf != 0.0f) {
+    pdf += background_sun_pdf(kg, direction) * sun_method_pdf;
+  }
+  if (map_method_pdf != 0.0f) {
+    pdf += background_map_pdf(kg, direction) * map_method_pdf;
+  }
+
+  return pdf * kernel_data.integrator.pdf_lights;
+}
+
+#endif
+
+CCL_NAMESPACE_END
\ No newline at end of file