Cycles: add texture to sun light

Using area-preserving mapping from cone to disk. Has somewhat distortion
near 90°.
The texture rotates with the transformation of the light object, can
have negative and non-uniform scaling.

Pull Request: https://projects.blender.org/blender/blender/pulls/109842

intern/cycles/kernel/light/distant.h

@@ -10,6 +10,27 @@
 
 CCL_NAMESPACE_BEGIN
 
+ccl_device_inline void distant_light_uv(const ccl_global KernelLight *klight,
+                                        const float3 D,
+                                        ccl_private float *u,
+                                        ccl_private float *v)
+{
+  /* Map direction (x, y, z) to disk [-0.5, 0.5]^2:
+   * r^2 = (1 - z) / (1 - cos(klight->distant.angle))
+   * u_ = 0.5 * x * r / sin_angle(D, -klight->co)
+   * v_ = 0.5 * y * r / sin_angle(D, -klight->co) */
+  const float fac = klight->distant.half_inv_sin_half_angle / len(D - klight->co);
+
+  /* Get u axis and v axis. */
+  const Transform itfm = klight->itfm;
+  const float u_ = dot(D, float4_to_float3(itfm.x)) * fac;
+  const float v_ = dot(D, float4_to_float3(itfm.y)) * fac;
+
+  /* NOTE: Return barycentric coordinates in the same notation as Embree and OptiX. */
+  *u = v_ + 0.5f;
+  *v = -u_ - v_;
+}
+
 ccl_device_inline bool distant_light_sample(const ccl_global KernelLight *klight,
                                             const float2 rand,
                                             ccl_private LightSample *ls)
@@ -24,6 +45,8 @@ ccl_device_inline bool distant_light_sample(const ccl_global KernelLight *klight
 
   ls->eval_fac = klight->distant.eval_fac;
 
+  distant_light_uv(klight, ls->D, &ls->u, &ls->v);
+
   return true;
 }
 
@@ -92,9 +115,6 @@ ccl_device bool distant_light_sample_from_intersection(KernelGlobals kg,
   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;
@@ -104,6 +124,8 @@ ccl_device bool distant_light_sample_from_intersection(KernelGlobals kg,
   ls->pdf = klight->distant.pdf;
   ls->eval_fac = klight->distant.eval_fac;
 
+  distant_light_uv(klight, ray_D, &ls->u, &ls->v);
+
   return true;
 }
 

intern/cycles/kernel/light/light.h

@@ -462,7 +462,6 @@ ccl_device bool light_sample_from_intersection(KernelGlobals kg,
   ls->object = isect->object;
   ls->prim = isect->prim;
   ls->lamp = lamp;
-  /* todo: missing texture coordinates */
   ls->t = isect->t;
   ls->P = ray_P + ray_D * ls->t;
   ls->D = ray_D;

intern/cycles/kernel/types.h

@@ -1379,8 +1379,10 @@ typedef struct KernelAreaLight {
 typedef struct KernelDistantLight {
   float angle;
   float one_minus_cosangle;
+  float half_inv_sin_half_angle;
   float pdf;
   float eval_fac;
+  float pad[3];
 } KernelDistantLight;
 
 typedef struct KernelLight {

intern/cycles/scene/light.cpp

@@ -1253,6 +1253,7 @@ void LightManager::device_update_lights(Device *device, DeviceScene *dscene, Sce
       klights[light_index].distant.eval_fac = (light->normalize && angle > 0) ?
                                                   M_1_PI_F / sqr(sinf(angle)) :
                                                   1.0f;
+      klights[light_index].distant.half_inv_sin_half_angle = 0.5f / sinf(0.5f * angle);
     }
     else if (light->light_type == LIGHT_BACKGROUND) {
       uint visibility = scene->background->get_visibility();