Cycles: changes to source code folders structure

* Split render/ into scene/ and session/. The scene/ folder now contains the
  scene and its nodes. The session/ folder contains the render session and
  associated data structures like drivers and render buffers.
* Move top level kernel headers into new folders kernel/camera/, kernel/film/,
  kernel/light/, kernel/sample/, kernel/util/
* Move integrator related kernel headers into kernel/integrator/
* Move OSL shaders from kernel/shaders/ to kernel/osl/shaders/

For patches and branches, git merge and rebase should be able to detect the
renames and move over code to the right file.

4 files changed, 1823 insertions, 0 deletions

diff --git a/intern/cycles/kernel/light/light.h b/intern/cycles/kernel/light/light.h
new file mode 100644
index 00000000000..facbbe23d0f
--- /dev/null
+++ b/intern/cycles/kernel/light/light.h
@@ -0,0 +1,872 @@
+/*
+ * Copyright 2011-2013 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.
+ */
+
+#pragma once
+
+#include "kernel/geom/geom.h"
+#include "kernel/light/light_background.h"
+#include "kernel/sample/sample_mapping.h"
+
+CCL_NAMESPACE_BEGIN
+
+/* Light Sample result */
+
+typedef struct LightSample {
+  float3 P;       /* position on light, or direction for distant light */
+  float3 Ng;      /* normal on light */
+  float3 D;       /* direction from shading point to light */
+  float t;        /* distance to light (FLT_MAX for distant light) */
+  float u, v;     /* parametric coordinate on primitive */
+  float pdf;      /* light sampling probability density function */
+  float eval_fac; /* intensity multiplier */
+  int object;     /* object id for triangle/curve lights */
+  int prim;       /* primitive id for triangle/curve lights */
+  int shader;     /* shader id */
+  int lamp;       /* lamp id */
+  LightType type; /* type of light */
+} LightSample;
+
+/* Regular Light */
+
+template<bool in_volume_segment>
+ccl_device_inline bool light_sample(KernelGlobals kg,
+                                    const int lamp,
+                                    const float randu,
+                                    const float randv,
+                                    const float3 P,
+                                    const uint32_t path_flag,
+                                    ccl_private LightSample *ls)
+{
+  const ccl_global KernelLight *klight = &kernel_tex_fetch(__lights, lamp);
+  if (path_flag & PATH_RAY_SHADOW_CATCHER_PASS) {
+    if (klight->shader_id & SHADER_EXCLUDE_SHADOW_CATCHER) {
+      return false;
+    }
+  }
+
+  LightType type = (LightType)klight->type;
+  ls->type = type;
+  ls->shader = klight->shader_id;
+  ls->object = PRIM_NONE;
+  ls->prim = PRIM_NONE;
+  ls->lamp = lamp;
+  ls->u = randu;
+  ls->v = randv;
+
+  if (in_volume_segment && (type == LIGHT_DISTANT || type == LIGHT_BACKGROUND)) {
+    /* Distant lights in a volume get a dummy sample, position will not actually
+     * be used in that case. Only when sampling from a specific scatter position
+     * do we actually need to evaluate these. */
+    ls->P = zero_float3();
+    ls->Ng = zero_float3();
+    ls->D = zero_float3();
+    ls->pdf = true;
+    ls->t = FLT_MAX;
+    return true;
+  }
+
+  if (type == LIGHT_DISTANT) {
+    /* distant light */
+    float3 lightD = make_float3(klight->co[0], klight->co[1], klight->co[2]);
+    float3 D = lightD;
+    float radius = klight->distant.radius;
+    float invarea = klight->distant.invarea;
+
+    if (radius > 0.0f)
+      D = distant_light_sample(D, radius, randu, randv);
+
+    ls->P = D;
+    ls->Ng = D;
+    ls->D = -D;
+    ls->t = FLT_MAX;
+
+    float costheta = dot(lightD, D);
+    ls->pdf = invarea / (costheta * costheta * costheta);
+    ls->eval_fac = ls->pdf;
+  }
+#ifdef __BACKGROUND_MIS__
+  else if (type == LIGHT_BACKGROUND) {
+    /* infinite area light (e.g. light dome or env light) */
+    float3 D = -background_light_sample(kg, P, randu, randv, &ls->pdf);
+
+    ls->P = D;
+    ls->Ng = D;
+    ls->D = -D;
+    ls->t = FLT_MAX;
+    ls->eval_fac = 1.0f;
+  }
+#endif
+  else {
+    ls->P = make_float3(klight->co[0], klight->co[1], klight->co[2]);
+
+    if (type == LIGHT_POINT || type == LIGHT_SPOT) {
+      float radius = klight->spot.radius;
+
+      if (radius > 0.0f)
+        /* sphere light */
+        ls->P += sphere_light_sample(P, ls->P, radius, randu, randv);
+
+      ls->D = normalize_len(ls->P - P, &ls->t);
+      ls->Ng = -ls->D;
+
+      float invarea = klight->spot.invarea;
+      ls->eval_fac = (0.25f * M_1_PI_F) * invarea;
+      ls->pdf = invarea;
+
+      if (type == LIGHT_SPOT) {
+        /* spot light attenuation */
+        float3 dir = make_float3(klight->spot.dir[0], klight->spot.dir[1], klight->spot.dir[2]);
+        ls->eval_fac *= spot_light_attenuation(
+            dir, klight->spot.spot_angle, klight->spot.spot_smooth, ls->Ng);
+        if (ls->eval_fac == 0.0f) {
+          return false;
+        }
+      }
+      float2 uv = map_to_sphere(ls->Ng);
+      ls->u = uv.x;
+      ls->v = uv.y;
+
+      ls->pdf *= lamp_light_pdf(kg, ls->Ng, -ls->D, ls->t);
+    }
+    else {
+      /* area light */
+      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]);
+      float3 Ng = make_float3(klight->area.dir[0], klight->area.dir[1], klight->area.dir[2]);
+      float invarea = fabsf(klight->area.invarea);
+      bool is_round = (klight->area.invarea < 0.0f);
+
+      if (!in_volume_segment) {
+        if (dot(ls->P - P, Ng) > 0.0f) {
+          return false;
+        }
+      }
+
+      float3 inplane;
+
+      if (is_round || in_volume_segment) {
+        inplane = ellipse_sample(axisu * 0.5f, axisv * 0.5f, randu, randv);
+        ls->P += inplane;
+        ls->pdf = invarea;
+      }
+      else {
+        inplane = ls->P;
+
+        float3 sample_axisu = axisu;
+        float3 sample_axisv = axisv;
+
+        if (klight->area.tan_spread > 0.0f) {
+          if (!light_spread_clamp_area_light(
+                  P, Ng, &ls->P, &sample_axisu, &sample_axisv, klight->area.tan_spread)) {
+            return false;
+          }
+        }
+
+        ls->pdf = rect_light_sample(P, &ls->P, sample_axisu, sample_axisv, randu, randv, true);
+        inplane = ls->P - inplane;
+      }
+
+      ls->u = dot(inplane, axisu) * (1.0f / dot(axisu, axisu)) + 0.5f;
+      ls->v = dot(inplane, axisv) * (1.0f / dot(axisv, axisv)) + 0.5f;
+
+      ls->Ng = Ng;
+      ls->D = normalize_len(ls->P - P, &ls->t);
+
+      ls->eval_fac = 0.25f * invarea;
+
+      if (klight->area.tan_spread > 0.0f) {
+        /* Area Light spread angle attenuation */
+        ls->eval_fac *= light_spread_attenuation(
+            ls->D, ls->Ng, klight->area.tan_spread, klight->area.normalize_spread);
+      }
+
+      if (is_round) {
+        ls->pdf *= lamp_light_pdf(kg, Ng, -ls->D, ls->t);
+      }
+    }
+  }
+
+  ls->pdf *= kernel_data.integrator.pdf_lights;
+
+  return (ls->pdf > 0.0f);
+}
+
+ccl_device bool lights_intersect(KernelGlobals kg,
+                                 ccl_private const Ray *ccl_restrict ray,
+                                 ccl_private Intersection *ccl_restrict isect,
+                                 const int last_prim,
+                                 const int last_object,
+                                 const int last_type,
+                                 const uint32_t path_flag)
+{
+  for (int lamp = 0; lamp < kernel_data.integrator.num_all_lights; lamp++) {
+    const ccl_global KernelLight *klight = &kernel_tex_fetch(__lights, lamp);
+
+    if (path_flag & PATH_RAY_CAMERA) {
+      if (klight->shader_id & SHADER_EXCLUDE_CAMERA) {
+        continue;
+      }
+    }
+    else {
+      if (!(klight->shader_id & SHADER_USE_MIS)) {
+        continue;
+      }
+    }
+
+    if (path_flag & PATH_RAY_SHADOW_CATCHER_PASS) {
+      if (klight->shader_id & SHADER_EXCLUDE_SHADOW_CATCHER) {
+        continue;
+      }
+    }
+
+    LightType type = (LightType)klight->type;
+    float t = 0.0f, u = 0.0f, v = 0.0f;
+
+    if (type == LIGHT_POINT || type == LIGHT_SPOT) {
+      /* Sphere light. */
+      const float3 lightP = make_float3(klight->co[0], klight->co[1], klight->co[2]);
+      const float radius = klight->spot.radius;
+      if (radius == 0.0f) {
+        continue;
+      }
+
+      float3 P;
+      if (!ray_aligned_disk_intersect(ray->P, ray->D, ray->t, lightP, radius, &P, &t)) {
+        continue;
+      }
+    }
+    else if (type == LIGHT_AREA) {
+      /* Area light. */
+      const float invarea = fabsf(klight->area.invarea);
+      const bool is_round = (klight->area.invarea < 0.0f);
+      if (invarea == 0.0f) {
+        continue;
+      }
+
+      const float3 axisu = make_float3(
+          klight->area.axisu[0], klight->area.axisu[1], klight->area.axisu[2]);
+      const float3 axisv = make_float3(
+          klight->area.axisv[0], klight->area.axisv[1], klight->area.axisv[2]);
+      const float3 Ng = make_float3(klight->area.dir[0], klight->area.dir[1], klight->area.dir[2]);
+
+      /* One sided. */
+      if (dot(ray->D, Ng) >= 0.0f) {
+        continue;
+      }
+
+      const float3 light_P = make_float3(klight->co[0], klight->co[1], klight->co[2]);
+
+      float3 P;
+      if (!ray_quad_intersect(
+              ray->P, ray->D, 0.0f, ray->t, light_P, axisu, axisv, Ng, &P, &t, &u, &v, is_round)) {
+        continue;
+      }
+    }
+    else {
+      continue;
+    }
+
+    if (t < isect->t &&
+        !(last_prim == lamp && last_object == OBJECT_NONE && last_type == PRIMITIVE_LAMP)) {
+      isect->t = t;
+      isect->u = u;
+      isect->v = v;
+      isect->type = PRIMITIVE_LAMP;
+      isect->prim = lamp;
+      isect->object = OBJECT_NONE;
+    }
+  }
+
+  return isect->prim != PRIM_NONE;
+}
+
+ccl_device bool light_sample_from_distant_ray(KernelGlobals kg,
+                                              const float3 ray_D,
+                                              const int lamp,
+                                              ccl_private LightSample *ccl_restrict ls)
+{
+  ccl_global const KernelLight *klight = &kernel_tex_fetch(__lights, lamp);
+  const int shader = klight->shader_id;
+  const float radius = klight->distant.radius;
+  const LightType type = (LightType)klight->type;
+
+  if (type != LIGHT_DISTANT) {
+    return false;
+  }
+  if (!(shader & SHADER_USE_MIS)) {
+    return false;
+  }
+  if (radius == 0.0f) {
+    return false;
+  }
+
+  /* a distant light is infinitely far away, but equivalent to a disk
+   * shaped light exactly 1 unit away from the current shading point.
+   *
+   *     radius              t^2/cos(theta)
+   *  <---------->           t = sqrt(1^2 + tan(theta)^2)
+   *       tan(th)           area = radius*radius*pi
+   *       <----->
+   *        \    |           (1 + tan(theta)^2)/cos(theta)
+   *         \   |           (1 + tan(acos(cos(theta)))^2)/cos(theta)
+   *       t  \th| 1         simplifies to
+   *           \-|           1/(cos(theta)^3)
+   *            \|           magic!
+   *             P
+   */
+
+  float3 lightD = make_float3(klight->co[0], klight->co[1], klight->co[2]);
+  float costheta = dot(-lightD, ray_D);
+  float cosangle = klight->distant.cosangle;
+
+  if (costheta < cosangle)
+    return false;
+
+  ls->type = type;
+  ls->shader = klight->shader_id;
+  ls->object = PRIM_NONE;
+  ls->prim = PRIM_NONE;
+  ls->lamp = lamp;
+  /* todo: missing texture coordinates */
+  ls->u = 0.0f;
+  ls->v = 0.0f;
+  ls->t = FLT_MAX;
+  ls->P = -ray_D;
+  ls->Ng = -ray_D;
+  ls->D = ray_D;
+
+  /* compute pdf */
+  float invarea = klight->distant.invarea;
+  ls->pdf = invarea / (costheta * costheta * costheta);
+  ls->pdf *= kernel_data.integrator.pdf_lights;
+  ls->eval_fac = ls->pdf;
+
+  return true;
+}
+
+ccl_device bool light_sample_from_intersection(KernelGlobals kg,
+                                               ccl_private const Intersection *ccl_restrict isect,
+                                               const float3 ray_P,
+                                               const float3 ray_D,
+                                               ccl_private LightSample *ccl_restrict ls)
+{
+  const int lamp = isect->prim;
+  ccl_global const KernelLight *klight = &kernel_tex_fetch(__lights, lamp);
+  LightType type = (LightType)klight->type;
+  ls->type = type;
+  ls->shader = klight->shader_id;
+  ls->object = PRIM_NONE;
+  ls->prim = PRIM_NONE;
+  ls->lamp = lamp;
+  /* todo: missing texture coordinates */
+  ls->t = isect->t;
+  ls->P = ray_P + ray_D * ls->t;
+  ls->D = ray_D;
+
+  if (type == LIGHT_POINT || type == LIGHT_SPOT) {
+    ls->Ng = -ray_D;
+
+    float invarea = klight->spot.invarea;
+    ls->eval_fac = (0.25f * M_1_PI_F) * invarea;
+    ls->pdf = invarea;
+
+    if (type == LIGHT_SPOT) {
+      /* spot light attenuation */
+      float3 dir = make_float3(klight->spot.dir[0], klight->spot.dir[1], klight->spot.dir[2]);
+      ls->eval_fac *= spot_light_attenuation(
+          dir, klight->spot.spot_angle, klight->spot.spot_smooth, ls->Ng);
+
+      if (ls->eval_fac == 0.0f) {
+        return false;
+      }
+    }
+    float2 uv = map_to_sphere(ls->Ng);
+    ls->u = uv.x;
+    ls->v = uv.y;
+
+    /* compute pdf */
+    if (ls->t != FLT_MAX)
+      ls->pdf *= lamp_light_pdf(kg, ls->Ng, -ls->D, ls->t);
+  }
+  else if (type == LIGHT_AREA) {
+    /* area light */
+    float invarea = fabsf(klight->area.invarea);
+
+    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]);
+    float3 Ng = make_float3(klight->area.dir[0], klight->area.dir[1], klight->area.dir[2]);
+    float3 light_P = make_float3(klight->co[0], klight->co[1], klight->co[2]);
+
+    ls->u = isect->u;
+    ls->v = isect->v;
+    ls->D = ray_D;
+    ls->Ng = Ng;
+
+    const bool is_round = (klight->area.invarea < 0.0f);
+    if (is_round) {
+      ls->pdf = invarea * lamp_light_pdf(kg, Ng, -ray_D, ls->t);
+    }
+    else {
+      float3 sample_axisu = axisu;
+      float3 sample_axisv = axisv;
+
+      if (klight->area.tan_spread > 0.0f) {
+        if (!light_spread_clamp_area_light(
+                ray_P, Ng, &light_P, &sample_axisu, &sample_axisv, klight->area.tan_spread)) {
+          return false;
+        }
+      }
+
+      ls->pdf = rect_light_sample(ray_P, &light_P, sample_axisu, sample_axisv, 0, 0, false);
+    }
+    ls->eval_fac = 0.25f * invarea;
+
+    if (klight->area.tan_spread > 0.0f) {
+      /* Area Light spread angle attenuation */
+      ls->eval_fac *= light_spread_attenuation(
+          ls->D, ls->Ng, klight->area.tan_spread, klight->area.normalize_spread);
+      if (ls->eval_fac == 0.0f) {
+        return false;
+      }
+    }
+  }
+  else {
+    kernel_assert(!"Invalid lamp type in light_sample_from_intersection");
+    return false;
+  }
+
+  ls->pdf *= kernel_data.integrator.pdf_lights;
+
+  return true;
+}
+
+/* Triangle Light */
+
+/* returns true if the triangle is has motion blur or an instancing transform applied */
+ccl_device_inline bool triangle_world_space_vertices(
+    KernelGlobals kg, int object, int prim, float time, float3 V[3])
+{
+  bool has_motion = false;
+  const int object_flag = kernel_tex_fetch(__object_flag, object);
+
+  if (object_flag & SD_OBJECT_HAS_VERTEX_MOTION && time >= 0.0f) {
+    motion_triangle_vertices(kg, object, prim, time, V);
+    has_motion = true;
+  }
+  else {
+    triangle_vertices(kg, prim, V);
+  }
+
+  if (!(object_flag & SD_OBJECT_TRANSFORM_APPLIED)) {
+#ifdef __OBJECT_MOTION__
+    float object_time = (time >= 0.0f) ? time : 0.5f;
+    Transform tfm = object_fetch_transform_motion_test(kg, object, object_time, NULL);
+#else
+    Transform tfm = object_fetch_transform(kg, object, OBJECT_TRANSFORM);
+#endif
+    V[0] = transform_point(&tfm, V[0]);
+    V[1] = transform_point(&tfm, V[1]);
+    V[2] = transform_point(&tfm, V[2]);
+    has_motion = true;
+  }
+  return has_motion;
+}
+
+ccl_device_inline float triangle_light_pdf_area(KernelGlobals kg,
+                                                const float3 Ng,
+                                                const float3 I,
+                                                float t)
+{
+  float pdf = kernel_data.integrator.pdf_triangles;
+  float cos_pi = fabsf(dot(Ng, I));
+
+  if (cos_pi == 0.0f)
+    return 0.0f;
+
+  return t * t * pdf / cos_pi;
+}
+
+ccl_device_forceinline float triangle_light_pdf(KernelGlobals kg,
+                                                ccl_private const ShaderData *sd,
+                                                float t)
+{
+  /* A naive heuristic to decide between costly solid angle sampling
+   * and simple area sampling, comparing the distance to the triangle plane
+   * to the length of the edges of the triangle. */
+
+  float3 V[3];
+  bool has_motion = triangle_world_space_vertices(kg, sd->object, sd->prim, sd->time, V);
+
+  const float3 e0 = V[1] - V[0];
+  const float3 e1 = V[2] - V[0];
+  const float3 e2 = V[2] - V[1];
+  const float longest_edge_squared = max(len_squared(e0), max(len_squared(e1), len_squared(e2)));
+  const float3 N = cross(e0, e1);
+  const float distance_to_plane = fabsf(dot(N, sd->I * t)) / dot(N, N);
+
+  if (longest_edge_squared > distance_to_plane * distance_to_plane) {
+    /* sd contains the point on the light source
+     * calculate Px, the point that we're shading */
+    const float3 Px = sd->P + sd->I * t;
+    const float3 v0_p = V[0] - Px;
+    const float3 v1_p = V[1] - Px;
+    const float3 v2_p = V[2] - Px;
+
+    const float3 u01 = safe_normalize(cross(v0_p, v1_p));
+    const float3 u02 = safe_normalize(cross(v0_p, v2_p));
+    const float3 u12 = safe_normalize(cross(v1_p, v2_p));
+
+    const float alpha = fast_acosf(dot(u02, u01));
+    const float beta = fast_acosf(-dot(u01, u12));
+    const float gamma = fast_acosf(dot(u02, u12));
+    const float solid_angle = alpha + beta + gamma - M_PI_F;
+
+    /* pdf_triangles is calculated over triangle area, but we're not sampling over its area */
+    if (UNLIKELY(solid_angle == 0.0f)) {
+      return 0.0f;
+    }
+    else {
+      float area = 1.0f;
+      if (has_motion) {
+        /* get the center frame vertices, this is what the PDF was calculated from */
+        triangle_world_space_vertices(kg, sd->object, sd->prim, -1.0f, V);
+        area = triangle_area(V[0], V[1], V[2]);
+      }
+      else {
+        area = 0.5f * len(N);
+      }
+      const float pdf = area * kernel_data.integrator.pdf_triangles;
+      return pdf / solid_angle;
+    }
+  }
+  else {
+    float pdf = triangle_light_pdf_area(kg, sd->Ng, sd->I, t);
+    if (has_motion) {
+      const float area = 0.5f * len(N);
+      if (UNLIKELY(area == 0.0f)) {
+        return 0.0f;
+      }
+      /* scale the PDF.
+       * area = the area the sample was taken from
+       * area_pre = the are from which pdf_triangles was calculated from */
+      triangle_world_space_vertices(kg, sd->object, sd->prim, -1.0f, V);
+      const float area_pre = triangle_area(V[0], V[1], V[2]);
+      pdf = pdf * area_pre / area;
+    }
+    return pdf;
+  }
+}
+
+template<bool in_volume_segment>
+ccl_device_forceinline void triangle_light_sample(KernelGlobals kg,
+                                                  int prim,
+                                                  int object,
+                                                  float randu,
+                                                  float randv,
+                                                  float time,
+                                                  ccl_private LightSample *ls,
+                                                  const float3 P)
+{
+  /* A naive heuristic to decide between costly solid angle sampling
+   * and simple area sampling, comparing the distance to the triangle plane
+   * to the length of the edges of the triangle. */
+
+  float3 V[3];
+  bool has_motion = triangle_world_space_vertices(kg, object, prim, time, V);
+
+  const float3 e0 = V[1] - V[0];
+  const float3 e1 = V[2] - V[0];
+  const float3 e2 = V[2] - V[1];
+  const float longest_edge_squared = max(len_squared(e0), max(len_squared(e1), len_squared(e2)));
+  const float3 N0 = cross(e0, e1);
+  float Nl = 0.0f;
+  ls->Ng = safe_normalize_len(N0, &Nl);
+  float area = 0.5f * Nl;
+
+  /* flip normal if necessary */
+  const int object_flag = kernel_tex_fetch(__object_flag, object);
+  if (object_flag & SD_OBJECT_NEGATIVE_SCALE_APPLIED) {
+    ls->Ng = -ls->Ng;
+  }
+  ls->eval_fac = 1.0f;
+  ls->shader = kernel_tex_fetch(__tri_shader, prim);
+  ls->object = object;
+  ls->prim = prim;
+  ls->lamp = LAMP_NONE;
+  ls->shader |= SHADER_USE_MIS;
+  ls->type = LIGHT_TRIANGLE;
+
+  float distance_to_plane = fabsf(dot(N0, V[0] - P) / dot(N0, N0));
+
+  if (!in_volume_segment && (longest_edge_squared > distance_to_plane * distance_to_plane)) {
+    /* see James Arvo, "Stratified Sampling of Spherical Triangles"
+     * http://www.graphics.cornell.edu/pubs/1995/Arv95c.pdf */
+
+    /* project the triangle to the unit sphere
+     * and calculate its edges and angles */
+    const float3 v0_p = V[0] - P;
+    const float3 v1_p = V[1] - P;
+    const float3 v2_p = V[2] - P;
+
+    const float3 u01 = safe_normalize(cross(v0_p, v1_p));
+    const float3 u02 = safe_normalize(cross(v0_p, v2_p));
+    const float3 u12 = safe_normalize(cross(v1_p, v2_p));
+
+    const float3 A = safe_normalize(v0_p);
+    const float3 B = safe_normalize(v1_p);
+    const float3 C = safe_normalize(v2_p);
+
+    const float cos_alpha = dot(u02, u01);
+    const float cos_beta = -dot(u01, u12);
+    const float cos_gamma = dot(u02, u12);
+
+    /* calculate dihedral angles */
+    const float alpha = fast_acosf(cos_alpha);
+    const float beta = fast_acosf(cos_beta);
+    const float gamma = fast_acosf(cos_gamma);
+    /* the area of the unit spherical triangle = solid angle */
+    const float solid_angle = alpha + beta + gamma - M_PI_F;
+
+    /* precompute a few things
+     * these could be re-used to take several samples
+     * as they are independent of randu/randv */
+    const float cos_c = dot(A, B);
+    const float sin_alpha = fast_sinf(alpha);
+    const float product = sin_alpha * cos_c;
+
+    /* Select a random sub-area of the spherical triangle
+     * and calculate the third vertex C_ of that new triangle */
+    const float phi = randu * solid_angle - alpha;
+    float s, t;
+    fast_sincosf(phi, &s, &t);
+    const float u = t - cos_alpha;
+    const float v = s + product;
+
+    const float3 U = safe_normalize(C - dot(C, A) * A);
+
+    float q = 1.0f;
+    const float det = ((v * s + u * t) * sin_alpha);
+    if (det != 0.0f) {
+      q = ((v * t - u * s) * cos_alpha - v) / det;
+    }
+    const float temp = max(1.0f - q * q, 0.0f);
+
+    const float3 C_ = safe_normalize(q * A + sqrtf(temp) * U);
+
+    /* Finally, select a random point along the edge of the new triangle
+     * That point on the spherical triangle is the sampled ray direction */
+    const float z = 1.0f - randv * (1.0f - dot(C_, B));
+    ls->D = z * B + safe_sqrtf(1.0f - z * z) * safe_normalize(C_ - dot(C_, B) * B);
+
+    /* calculate intersection with the planar triangle */
+    if (!ray_triangle_intersect(P,
+                                ls->D,
+                                FLT_MAX,
+#if defined(__KERNEL_SSE2__) && defined(__KERNEL_SSE__)
+                                (ssef *)V,
+#else
+                                V[0],
+                                V[1],
+                                V[2],
+#endif
+                                &ls->u,
+                                &ls->v,
+                                &ls->t)) {
+      ls->pdf = 0.0f;
+      return;
+    }
+
+    ls->P = P + ls->D * ls->t;
+
+    /* pdf_triangles is calculated over triangle area, but we're sampling over solid angle */
+    if (UNLIKELY(solid_angle == 0.0f)) {
+      ls->pdf = 0.0f;
+      return;
+    }
+    else {
+      if (has_motion) {
+        /* get the center frame vertices, this is what the PDF was calculated from */
+        triangle_world_space_vertices(kg, object, prim, -1.0f, V);
+        area = triangle_area(V[0], V[1], V[2]);
+      }
+      const float pdf = area * kernel_data.integrator.pdf_triangles;
+      ls->pdf = pdf / solid_angle;
+    }
+  }
+  else {
+    /* compute random point in triangle. From Eric Heitz's "A Low-Distortion Map Between Triangle
+     * and Square" */
+    float u = randu;
+    float v = randv;
+    if (v > u) {
+      u *= 0.5f;
+      v -= u;
+    }
+    else {
+      v *= 0.5f;
+      u -= v;
+    }
+
+    const float t = 1.0f - u - v;
+    ls->P = u * V[0] + v * V[1] + t * V[2];
+    /* compute incoming direction, distance and pdf */
+    ls->D = normalize_len(ls->P - P, &ls->t);
+    ls->pdf = triangle_light_pdf_area(kg, ls->Ng, -ls->D, ls->t);
+    if (has_motion && area != 0.0f) {
+      /* scale the PDF.
+       * area = the area the sample was taken from
+       * area_pre = the are from which pdf_triangles was calculated from */
+      triangle_world_space_vertices(kg, object, prim, -1.0f, V);
+      const float area_pre = triangle_area(V[0], V[1], V[2]);
+      ls->pdf = ls->pdf * area_pre / area;
+    }
+    ls->u = u;
+    ls->v = v;
+  }
+}
+
+/* Light Distribution */
+
+ccl_device int light_distribution_sample(KernelGlobals kg, ccl_private float *randu)
+{
+  /* This is basically std::upper_bound as used by PBRT, to find a point light or
+   * triangle to emit from, proportional to area. a good improvement would be to
+   * also sample proportional to power, though it's not so well defined with
+   * arbitrary shaders. */
+  int first = 0;
+  int len = kernel_data.integrator.num_distribution + 1;
+  float r = *randu;
+
+  do {
+    int half_len = len >> 1;
+    int middle = first + half_len;
+
+    if (r < kernel_tex_fetch(__light_distribution, middle).totarea) {
+      len = half_len;
+    }
+    else {
+      first = middle + 1;
+      len = len - half_len - 1;
+    }
+  } while (len > 0);
+
+  /* Clamping should not be needed but float rounding errors seem to
+   * make this fail on rare occasions. */
+  int index = clamp(first - 1, 0, kernel_data.integrator.num_distribution - 1);
+
+  /* Rescale to reuse random number. this helps the 2D samples within
+   * each area light be stratified as well. */
+  float distr_min = kernel_tex_fetch(__light_distribution, index).totarea;
+  float distr_max = kernel_tex_fetch(__light_distribution, index + 1).totarea;
+  *randu = (r - distr_min) / (distr_max - distr_min);
+
+  return index;
+}
+
+/* Generic Light */
+
+ccl_device_inline bool light_select_reached_max_bounces(KernelGlobals kg, int index, int bounce)
+{
+  return (bounce > kernel_tex_fetch(__lights, index).max_bounces);
+}
+
+template<bool in_volume_segment>
+ccl_device_noinline bool light_distribution_sample(KernelGlobals kg,
+                                                   float randu,
+                                                   const float randv,
+                                                   const float time,
+                                                   const float3 P,
+                                                   const int bounce,
+                                                   const uint32_t path_flag,
+                                                   ccl_private LightSample *ls)
+{
+  /* Sample light index from distribution. */
+  const int index = light_distribution_sample(kg, &randu);
+  ccl_global const KernelLightDistribution *kdistribution = &kernel_tex_fetch(__light_distribution,
+                                                                              index);
+  const int prim = kdistribution->prim;
+
+  if (prim >= 0) {
+    /* Mesh light. */
+    const int object = kdistribution->mesh_light.object_id;
+
+    /* Exclude synthetic meshes from shadow catcher pass. */
+    if ((path_flag & PATH_RAY_SHADOW_CATCHER_PASS) &&
+        !(kernel_tex_fetch(__object_flag, object) & SD_OBJECT_SHADOW_CATCHER)) {
+      return false;
+    }
+
+    const int shader_flag = kdistribution->mesh_light.shader_flag;
+    triangle_light_sample<in_volume_segment>(kg, prim, object, randu, randv, time, ls, P);
+    ls->shader |= shader_flag;
+    return (ls->pdf > 0.0f);
+  }
+
+  const int lamp = -prim - 1;
+
+  if (UNLIKELY(light_select_reached_max_bounces(kg, lamp, bounce))) {
+    return false;
+  }
+
+  return light_sample<in_volume_segment>(kg, lamp, randu, randv, P, path_flag, ls);
+}
+
+ccl_device_inline bool light_distribution_sample_from_volume_segment(KernelGlobals kg,
+                                                                     float randu,
+                                                                     const float randv,
+                                                                     const float time,
+                                                                     const float3 P,
+                                                                     const int bounce,
+                                                                     const uint32_t path_flag,
+                                                                     ccl_private LightSample *ls)
+{
+  return light_distribution_sample<true>(kg, randu, randv, time, P, bounce, path_flag, ls);
+}
+
+ccl_device_inline bool light_distribution_sample_from_position(KernelGlobals kg,
+                                                               float randu,
+                                                               const float randv,
+                                                               const float time,
+                                                               const float3 P,
+                                                               const int bounce,
+                                                               const uint32_t path_flag,
+                                                               ccl_private LightSample *ls)
+{
+  return light_distribution_sample<false>(kg, randu, randv, time, P, bounce, path_flag, ls);
+}
+
+ccl_device_inline bool light_distribution_sample_new_position(KernelGlobals kg,
+                                                              const float randu,
+                                                              const float randv,
+                                                              const float time,
+                                                              const float3 P,
+                                                              ccl_private LightSample *ls)
+{
+  /* Sample a new position on the same light, for volume sampling. */
+  if (ls->type == LIGHT_TRIANGLE) {
+    triangle_light_sample<false>(kg, ls->prim, ls->object, randu, randv, time, ls, P);
+    return (ls->pdf > 0.0f);
+  }
+  else {
+    return light_sample<false>(kg, ls->lamp, randu, randv, P, 0, ls);
+  }
+}
+
+CCL_NAMESPACE_END
diff --git a/intern/cycles/kernel/light/light_background.h b/intern/cycles/kernel/light/light_background.h
new file mode 100644
index 00000000000..78f8c94f7a3
--- /dev/null
+++ b/intern/cycles/kernel/light/light_background.h
@@ -0,0 +1,453 @@
+/*
+ * 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.
+ */
+
+#pragma once
+
+#include "kernel/light/light_common.h"
+
+CCL_NAMESPACE_BEGIN
+
+/* Background Light */
+
+#ifdef __BACKGROUND_MIS__
+
+ccl_device float3 background_map_sample(KernelGlobals kg,
+                                        float randu,
+                                        float randv,
+                                        ccl_private 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, ccl_private float3 *lightpos, ccl_private 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, ccl_private 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,
+                                           ccl_private int *sampled_portal,
+                                           ccl_private 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 zero_float3();
+}
+
+ccl_device_inline float3 background_sun_sample(KernelGlobals kg,
+                                               float randu,
+                                               float randv,
+                                               ccl_private 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, ccl_private 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
diff --git a/intern/cycles/kernel/light/light_common.h b/intern/cycles/kernel/light/light_common.h
new file mode 100644
index 00000000000..207e89090cc
--- /dev/null
+++ b/intern/cycles/kernel/light/light_common.h
@@ -0,0 +1,227 @@
+/*
+ * 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.
+ */
+
+#pragma once
+
+#include "kernel/sample/sample_mapping.h"
+
+CCL_NAMESPACE_BEGIN
+
+/* Area light sampling */
+
+/* Uses the following paper:
+ *
+ * Carlos Urena et al.
+ * An Area-Preserving Parametrization for Spherical Rectangles.
+ *
+ * https://www.solidangle.com/research/egsr2013_spherical_rectangle.pdf
+ *
+ * Note: light_p is modified when sample_coord is true.
+ */
+ccl_device_inline float rect_light_sample(float3 P,
+                                          ccl_private float3 *light_p,
+                                          float3 axisu,
+                                          float3 axisv,
+                                          float randu,
+                                          float randv,
+                                          bool sample_coord)
+{
+  /* In our name system we're using P for the center,
+   * which is o in the paper.
+   */
+
+  float3 corner = *light_p - axisu * 0.5f - axisv * 0.5f;
+  float axisu_len, axisv_len;
+  /* Compute local reference system R. */
+  float3 x = normalize_len(axisu, &axisu_len);
+  float3 y = normalize_len(axisv, &axisv_len);
+  float3 z = cross(x, y);
+  /* Compute rectangle coords in local reference system. */
+  float3 dir = corner - P;
+  float z0 = dot(dir, z);
+  /* Flip 'z' to make it point against Q. */
+  if (z0 > 0.0f) {
+    z *= -1.0f;
+    z0 *= -1.0f;
+  }
+  float x0 = dot(dir, x);
+  float y0 = dot(dir, y);
+  float x1 = x0 + axisu_len;
+  float y1 = y0 + axisv_len;
+  /* Compute internal angles (gamma_i). */
+  float4 diff = make_float4(x0, y1, x1, y0) - make_float4(x1, y0, x0, y1);
+  float4 nz = make_float4(y0, x1, y1, x0) * diff;
+  nz = nz / sqrt(z0 * z0 * diff * diff + nz * nz);
+  float g0 = safe_acosf(-nz.x * nz.y);
+  float g1 = safe_acosf(-nz.y * nz.z);
+  float g2 = safe_acosf(-nz.z * nz.w);
+  float g3 = safe_acosf(-nz.w * nz.x);
+  /* Compute predefined constants. */
+  float b0 = nz.x;
+  float b1 = nz.z;
+  float b0sq = b0 * b0;
+  float k = M_2PI_F - g2 - g3;
+  /* Compute solid angle from internal angles. */
+  float S = g0 + g1 - k;
+
+  if (sample_coord) {
+    /* Compute cu. */
+    float au = randu * S + k;
+    float fu = (cosf(au) * b0 - b1) / sinf(au);
+    float cu = 1.0f / sqrtf(fu * fu + b0sq) * (fu > 0.0f ? 1.0f : -1.0f);
+    cu = clamp(cu, -1.0f, 1.0f);
+    /* Compute xu. */
+    float xu = -(cu * z0) / max(sqrtf(1.0f - cu * cu), 1e-7f);
+    xu = clamp(xu, x0, x1);
+    /* Compute yv. */
+    float z0sq = z0 * z0;
+    float y0sq = y0 * y0;
+    float y1sq = y1 * y1;
+    float d = sqrtf(xu * xu + z0sq);
+    float h0 = y0 / sqrtf(d * d + y0sq);
+    float h1 = y1 / sqrtf(d * d + y1sq);
+    float hv = h0 + randv * (h1 - h0), hv2 = hv * hv;
+    float yv = (hv2 < 1.0f - 1e-6f) ? (hv * d) / sqrtf(1.0f - hv2) : y1;
+
+    /* Transform (xu, yv, z0) to world coords. */
+    *light_p = P + xu * x + yv * y + z0 * z;
+  }
+
+  /* return pdf */
+  if (S != 0.0f)
+    return 1.0f / S;
+  else
+    return 0.0f;
+}
+
+ccl_device_inline float3 ellipse_sample(float3 ru, float3 rv, float randu, float randv)
+{
+  to_unit_disk(&randu, &randv);
+  return ru * randu + rv * randv;
+}
+
+ccl_device float3 disk_light_sample(float3 v, float randu, float randv)
+{
+  float3 ru, rv;
+
+  make_orthonormals(v, &ru, &rv);
+
+  return ellipse_sample(ru, rv, randu, randv);
+}
+
+ccl_device float3 distant_light_sample(float3 D, float radius, float randu, float randv)
+{
+  return normalize(D + disk_light_sample(D, randu, randv) * radius);
+}
+
+ccl_device float3
+sphere_light_sample(float3 P, float3 center, float radius, float randu, float randv)
+{
+  return disk_light_sample(normalize(P - center), randu, randv) * radius;
+}
+
+ccl_device float spot_light_attenuation(float3 dir, float spot_angle, float spot_smooth, float3 N)
+{
+  float attenuation = dot(dir, N);
+
+  if (attenuation <= spot_angle) {
+    attenuation = 0.0f;
+  }
+  else {
+    float t = attenuation - spot_angle;
+
+    if (t < spot_smooth && spot_smooth != 0.0f)
+      attenuation *= smoothstepf(t / spot_smooth);
+  }
+
+  return attenuation;
+}
+
+ccl_device float light_spread_attenuation(const float3 D,
+                                          const float3 lightNg,
+                                          const float tan_spread,
+                                          const float normalize_spread)
+{
+  /* Model a soft-box grid, computing the ratio of light not hidden by the
+   * slats of the grid at a given angle. (see D10594). */
+  const float cos_a = -dot(D, lightNg);
+  const float sin_a = safe_sqrtf(1.0f - sqr(cos_a));
+  const float tan_a = sin_a / cos_a;
+  return max((1.0f - (tan_spread * tan_a)) * normalize_spread, 0.0f);
+}
+
+/* Compute subset of area light that actually has an influence on the shading point, to
+ * reduce noise with low spread. */
+ccl_device bool light_spread_clamp_area_light(const float3 P,
+                                              const float3 lightNg,
+                                              ccl_private float3 *lightP,
+                                              ccl_private float3 *axisu,
+                                              ccl_private float3 *axisv,
+                                              const float tan_spread)
+{
+  /* Closest point in area light plane and distance to that plane. */
+  const float3 closest_P = P - dot(lightNg, P - *lightP) * lightNg;
+  const float t = len(closest_P - P);
+
+  /* Radius of circle on area light that actually affects the shading point. */
+  const float radius = t / tan_spread;
+
+  /* TODO: would be faster to store as normalized vector + length, also in rect_light_sample. */
+  float len_u, len_v;
+  const float3 u = normalize_len(*axisu, &len_u);
+  const float3 v = normalize_len(*axisv, &len_v);
+
+  /* Local uv coordinates of closest point. */
+  const float closest_u = dot(u, closest_P - *lightP);
+  const float closest_v = dot(v, closest_P - *lightP);
+
+  /* Compute rectangle encompassing the circle that affects the shading point,
+   * clamped to the bounds of the area light. */
+  const float min_u = max(closest_u - radius, -len_u * 0.5f);
+  const float max_u = min(closest_u + radius, len_u * 0.5f);
+  const float min_v = max(closest_v - radius, -len_v * 0.5f);
+  const float max_v = min(closest_v + radius, len_v * 0.5f);
+
+  /* Skip if rectangle is empty. */
+  if (min_u >= max_u || min_v >= max_v) {
+    return false;
+  }
+
+  /* Compute new area light center position and axes from rectangle in local
+   * uv coordinates. */
+  const float new_center_u = 0.5f * (min_u + max_u);
+  const float new_center_v = 0.5f * (min_v + max_v);
+  const float new_len_u = max_u - min_u;
+  const float new_len_v = max_v - min_v;
+
+  *lightP = *lightP + new_center_u * u + new_center_v * v;
+  *axisu = u * new_len_u;
+  *axisv = v * new_len_v;
+
+  return true;
+}
+
+ccl_device float lamp_light_pdf(KernelGlobals kg, const float3 Ng, const float3 I, float t)
+{
+  float cos_pi = dot(Ng, I);
+
+  if (cos_pi <= 0.0f)
+    return 0.0f;
+
+  return t * t / cos_pi;
+}
+
+CCL_NAMESPACE_END
diff --git a/intern/cycles/kernel/light/light_sample.h b/intern/cycles/kernel/light/light_sample.h
new file mode 100644
index 00000000000..4ae5d9e1944
--- /dev/null
+++ b/intern/cycles/kernel/light/light_sample.h
@@ -0,0 +1,271 @@
+/*
+ * Copyright 2011-2013 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.
+ */
+
+#pragma once
+
+#include "kernel/integrator/integrator_path_state.h"
+#include "kernel/integrator/integrator_shader_eval.h"
+
+#include "kernel/light/light.h"
+
+#include "kernel/sample/sample_mapping.h"
+
+CCL_NAMESPACE_BEGIN
+
+/* Evaluate shader on light. */
+ccl_device_noinline_cpu float3
+light_sample_shader_eval(KernelGlobals kg,
+                         IntegratorState state,
+                         ccl_private ShaderData *ccl_restrict emission_sd,
+                         ccl_private LightSample *ccl_restrict ls,
+                         float time)
+{
+  /* setup shading at emitter */
+  float3 eval = zero_float3();
+
+  if (shader_constant_emission_eval(kg, ls->shader, &eval)) {
+    if ((ls->prim != PRIM_NONE) && dot(ls->Ng, ls->D) > 0.0f) {
+      ls->Ng = -ls->Ng;
+    }
+  }
+  else {
+    /* Setup shader data and call shader_eval_surface once, better
+     * for GPU coherence and compile times. */
+    PROFILING_INIT_FOR_SHADER(kg, PROFILING_SHADE_LIGHT_SETUP);
+#ifdef __BACKGROUND_MIS__
+    if (ls->type == LIGHT_BACKGROUND) {
+      shader_setup_from_background(kg, emission_sd, ls->P, ls->D, time);
+    }
+    else
+#endif
+    {
+      shader_setup_from_sample(kg,
+                               emission_sd,
+                               ls->P,
+                               ls->Ng,
+                               -ls->D,
+                               ls->shader,
+                               ls->object,
+                               ls->prim,
+                               ls->u,
+                               ls->v,
+                               ls->t,
+                               time,
+                               false,
+                               ls->lamp);
+
+      ls->Ng = emission_sd->Ng;
+    }
+
+    PROFILING_SHADER(emission_sd->object, emission_sd->shader);
+    PROFILING_EVENT(PROFILING_SHADE_LIGHT_EVAL);
+
+    /* No proper path flag, we're evaluating this for all closures. that's
+     * weak but we'd have to do multiple evaluations otherwise. */
+    shader_eval_surface<KERNEL_FEATURE_NODE_MASK_SURFACE_LIGHT>(
+        kg, state, emission_sd, NULL, PATH_RAY_EMISSION);
+
+    /* Evaluate closures. */
+#ifdef __BACKGROUND_MIS__
+    if (ls->type == LIGHT_BACKGROUND) {
+      eval = shader_background_eval(emission_sd);
+    }
+    else
+#endif
+    {
+      eval = shader_emissive_eval(emission_sd);
+    }
+  }
+
+  eval *= ls->eval_fac;
+
+  if (ls->lamp != LAMP_NONE) {
+    ccl_global const KernelLight *klight = &kernel_tex_fetch(__lights, ls->lamp);
+    eval *= make_float3(klight->strength[0], klight->strength[1], klight->strength[2]);
+  }
+
+  return eval;
+}
+
+/* Test if light sample is from a light or emission from geometry. */
+ccl_device_inline bool light_sample_is_light(ccl_private const LightSample *ccl_restrict ls)
+{
+  /* return if it's a lamp for shadow pass */
+  return (ls->prim == PRIM_NONE && ls->type != LIGHT_BACKGROUND);
+}
+
+/* Early path termination of shadow rays. */
+ccl_device_inline bool light_sample_terminate(KernelGlobals kg,
+                                              ccl_private const LightSample *ccl_restrict ls,
+                                              ccl_private BsdfEval *ccl_restrict eval,
+                                              const float rand_terminate)
+{
+  if (bsdf_eval_is_zero(eval)) {
+    return true;
+  }
+
+  if (kernel_data.integrator.light_inv_rr_threshold > 0.0f) {
+    float probability = max3(fabs(bsdf_eval_sum(eval))) *
+                        kernel_data.integrator.light_inv_rr_threshold;
+    if (probability < 1.0f) {
+      if (rand_terminate >= probability) {
+        return true;
+      }
+      bsdf_eval_mul(eval, 1.0f / probability);
+    }
+  }
+
+  return false;
+}
+
+/* This function should be used to compute a modified ray start position for
+ * rays leaving from a surface. The algorithm slightly distorts flat surface
+ * of a triangle. Surface is lifted by amount h along normal n in the incident
+ * point. */
+
+ccl_device_inline float3 shadow_ray_smooth_surface_offset(
+    KernelGlobals kg, ccl_private const ShaderData *ccl_restrict sd, float3 Ng)
+{
+  float3 V[3], N[3];
+  triangle_vertices_and_normals(kg, sd->prim, V, N);
+
+  const float u = sd->u, v = sd->v;
+  const float w = 1 - u - v;
+  float3 P = V[0] * u + V[1] * v + V[2] * w; /* Local space */
+  float3 n = N[0] * u + N[1] * v + N[2] * w; /* We get away without normalization */
+
+  object_normal_transform(kg, sd, &n); /* Normal x scale, world space */
+
+  /* Parabolic approximation */
+  float a = dot(N[2] - N[0], V[0] - V[2]);
+  float b = dot(N[2] - N[1], V[1] - V[2]);
+  float c = dot(N[1] - N[0], V[1] - V[0]);
+  float h = a * u * (u - 1) + (a + b + c) * u * v + b * v * (v - 1);
+
+  /* Check flipped normals */
+  if (dot(n, Ng) > 0) {
+    /* Local linear envelope */
+    float h0 = max(max(dot(V[1] - V[0], N[0]), dot(V[2] - V[0], N[0])), 0.0f);
+    float h1 = max(max(dot(V[0] - V[1], N[1]), dot(V[2] - V[1], N[1])), 0.0f);
+    float h2 = max(max(dot(V[0] - V[2], N[2]), dot(V[1] - V[2], N[2])), 0.0f);
+    h0 = max(dot(V[0] - P, N[0]) + h0, 0.0f);
+    h1 = max(dot(V[1] - P, N[1]) + h1, 0.0f);
+    h2 = max(dot(V[2] - P, N[2]) + h2, 0.0f);
+    h = max(min(min(h0, h1), h2), h * 0.5f);
+  }
+  else {
+    float h0 = max(max(dot(V[0] - V[1], N[0]), dot(V[0] - V[2], N[0])), 0.0f);
+    float h1 = max(max(dot(V[1] - V[0], N[1]), dot(V[1] - V[2], N[1])), 0.0f);
+    float h2 = max(max(dot(V[2] - V[0], N[2]), dot(V[2] - V[1], N[2])), 0.0f);
+    h0 = max(dot(P - V[0], N[0]) + h0, 0.0f);
+    h1 = max(dot(P - V[1], N[1]) + h1, 0.0f);
+    h2 = max(dot(P - V[2], N[2]) + h2, 0.0f);
+    h = min(-min(min(h0, h1), h2), h * 0.5f);
+  }
+
+  return n * h;
+}
+
+/* Ray offset to avoid shadow terminator artifact. */
+
+ccl_device_inline float3 shadow_ray_offset(KernelGlobals kg,
+                                           ccl_private const ShaderData *ccl_restrict sd,
+                                           float3 L)
+{
+  float NL = dot(sd->N, L);
+  bool transmit = (NL < 0.0f);
+  float3 Ng = (transmit ? -sd->Ng : sd->Ng);
+  float3 P = ray_offset(sd->P, Ng);
+
+  if ((sd->type & PRIMITIVE_ALL_TRIANGLE) && (sd->shader & SHADER_SMOOTH_NORMAL)) {
+    const float offset_cutoff =
+        kernel_tex_fetch(__objects, sd->object).shadow_terminator_geometry_offset;
+    /* Do ray offset (heavy stuff) only for close to be terminated triangles:
+     * offset_cutoff = 0.1f means that 10-20% of rays will be affected. Also
+     * make a smooth transition near the threshold. */
+    if (offset_cutoff > 0.0f) {
+      float NgL = dot(Ng, L);
+      float offset_amount = 0.0f;
+      if (NL < offset_cutoff) {
+        offset_amount = clamp(2.0f - (NgL + NL) / offset_cutoff, 0.0f, 1.0f);
+      }
+      else {
+        offset_amount = clamp(1.0f - NgL / offset_cutoff, 0.0f, 1.0f);
+      }
+      if (offset_amount > 0.0f) {
+        P += shadow_ray_smooth_surface_offset(kg, sd, Ng) * offset_amount;
+      }
+    }
+  }
+
+  return P;
+}
+
+ccl_device_inline void shadow_ray_setup(ccl_private const ShaderData *ccl_restrict sd,
+                                        ccl_private const LightSample *ccl_restrict ls,
+                                        const float3 P,
+                                        ccl_private Ray *ray)
+{
+  if (ls->shader & SHADER_CAST_SHADOW) {
+    /* setup ray */
+    ray->P = P;
+
+    if (ls->t == FLT_MAX) {
+      /* distant light */
+      ray->D = ls->D;
+      ray->t = ls->t;
+    }
+    else {
+      /* other lights, avoid self-intersection */
+      ray->D = ray_offset(ls->P, ls->Ng) - P;
+      ray->D = normalize_len(ray->D, &ray->t);
+    }
+  }
+  else {
+    /* signal to not cast shadow ray */
+    ray->P = zero_float3();
+    ray->D = zero_float3();
+    ray->t = 0.0f;
+  }
+
+  ray->dP = differential_make_compact(sd->dP);
+  ray->dD = differential_zero_compact();
+  ray->time = sd->time;
+}
+
+/* Create shadow ray towards light sample. */
+ccl_device_inline void light_sample_to_surface_shadow_ray(
+    KernelGlobals kg,
+    ccl_private const ShaderData *ccl_restrict sd,
+    ccl_private const LightSample *ccl_restrict ls,
+    ccl_private Ray *ray)
+{
+  const float3 P = shadow_ray_offset(kg, sd, ls->D);
+  shadow_ray_setup(sd, ls, P, ray);
+}
+
+/* Create shadow ray towards light sample. */
+ccl_device_inline void light_sample_to_volume_shadow_ray(
+    KernelGlobals kg,
+    ccl_private const ShaderData *ccl_restrict sd,
+    ccl_private const LightSample *ccl_restrict ls,
+    const float3 P,
+    ccl_private Ray *ray)
+{
+  shadow_ray_setup(sd, ls, P, ray);
+}
+
+CCL_NAMESPACE_END