Cycles: First implementation of shadow catcher

It uses an idea of accumulating all possible light reachable across the
light path (without taking shadow blocked into account) and accumulating
total shaded light across the path. Dividing second figure by first one
seems to be giving good estimate of the shadow.

In fact, to my knowledge, it's something really similar to what is
happening in the denoising branch, so we are aligned here which is good.

The workflow is following:

- Create an object which matches real-life object on which shadow is
  to be catched.

- Create approximate similar material on that object.

  This is needed to make indirect light properly affecting CG objects
  in the scene.

- Mark object as Shadow Catcher in the Object properties.

Ideally, after doing that it will be possible to render the image and
simply alpha-over it on top of real footage.

1 files changed, 28 insertions, 2 deletions

diff --git a/intern/cycles/kernel/kernel_path_surface.h b/intern/cycles/kernel/kernel_path_surface.h
index efa23038089..d6bfe9be5f1 100644
--- a/intern/cycles/kernel/kernel_path_surface.h
+++ b/intern/cycles/kernel/kernel_path_surface.h
@@ -16,8 +16,7 @@
 
 CCL_NAMESPACE_BEGIN
 
-#if (defined(__BRANCHED_PATH__) || defined(__SUBSURFACE__)) && !defined(__SPLIT_KERNEL__)
-
+#if (defined(__BRANCHED_PATH__) || defined(__SUBSURFACE__) || defined(__SHADOW_TRICKS__)) && !defined(__SPLIT_KERNEL__)
 /* branched path tracing: connect path directly to position on one or more lights and add it to L */
 ccl_device_noinline void kernel_branched_path_surface_connect_light(KernelGlobals *kg, RNG *rng,
 	ShaderData *sd, ShaderData *emission_sd, PathState *state, float3 throughput,
@@ -66,6 +65,9 @@ ccl_device_noinline void kernel_branched_path_surface_connect_light(KernelGlobal
 							/* accumulate */
 							path_radiance_accum_light(L, throughput*num_samples_inv, &L_light, shadow, num_samples_inv, state->bounce, is_lamp);
 						}
+						else {
+							path_radiance_accum_total_light(L, throughput*num_samples_inv, &L_light);
+						}
 					}
 				}
 			}
@@ -100,6 +102,9 @@ ccl_device_noinline void kernel_branched_path_surface_connect_light(KernelGlobal
 							/* accumulate */
 							path_radiance_accum_light(L, throughput*num_samples_inv, &L_light, shadow, num_samples_inv, state->bounce, is_lamp);
 						}
+						else {
+							path_radiance_accum_total_light(L, throughput*num_samples_inv, &L_light);
+						}
 					}
 				}
 			}
@@ -123,6 +128,9 @@ ccl_device_noinline void kernel_branched_path_surface_connect_light(KernelGlobal
 					/* accumulate */
 					path_radiance_accum_light(L, throughput*num_samples_adjust, &L_light, shadow, num_samples_adjust, state->bounce, is_lamp);
 				}
+				else {
+					path_radiance_accum_total_light(L, throughput*num_samples_adjust, &L_light);
+				}
 			}
 		}
 	}
@@ -197,6 +205,21 @@ ccl_device_inline void kernel_path_surface_connect_light(KernelGlobals *kg, ccl_
 	if(!(kernel_data.integrator.use_direct_light && (sd->flag & SD_BSDF_HAS_EVAL)))
 		return;
 
+#ifdef __SHADOW_TRICKS__
+	if(state->flag & PATH_RAY_SHADOW_CATCHER) {
+		kernel_branched_path_surface_connect_light(kg,
+		                                           rng,
+		                                           sd,
+		                                           emission_sd,
+		                                           state,
+		                                           throughput,
+		                                           1.0f,
+		                                           L,
+		                                           1);
+		return;
+	}
+#endif
+
 	/* sample illumination from lights to find path contribution */
 	float light_t = path_state_rng_1D(kg, rng, state, PRNG_LIGHT);
 	float light_u, light_v;
@@ -221,6 +244,9 @@ ccl_device_inline void kernel_path_surface_connect_light(KernelGlobals *kg, ccl_
 				/* accumulate */
 				path_radiance_accum_light(L, throughput, &L_light, shadow, 1.0f, state->bounce, is_lamp);
 			}
+			else {
+				path_radiance_accum_total_light(L, throughput, &L_light);
+			}
 		}
 	}
 #endif