Cycles: bake transparent shadows for hair

These transparent shadows can be expansive to evaluate. Especially on the
GPU they can lead to poor occupancy when only some pixels require many kernel
launches to trace and evaluate many layers of transparency.

Baked transparency allows tracing a single ray in many cases by accumulating
the throughput directly in the intersection program without recording hits
or evaluating shaders. Transparency is baked at curve vertices and
interpolated, for most shaders this will look practically the same as actual
shader evaluation.

Fixes T91428, performance regression with spring demo file due to transparent
hair, and makes it render significantly faster than Blender 2.93.

Differential Revision: https://developer.blender.org/D12880

2 files changed, 7 insertions, 0 deletions

diff --git a/intern/cycles/integrator/shader_eval.cpp b/intern/cycles/integrator/shader_eval.cpp
index cfc30056f7d..3de7bb6fd16 100644
--- a/intern/cycles/integrator/shader_eval.cpp
+++ b/intern/cycles/integrator/shader_eval.cpp
@@ -122,6 +122,9 @@ bool ShaderEval::eval_cpu(Device *device,
         case SHADER_EVAL_BACKGROUND:
           kernels.shader_eval_background(kg, input_data, output_data, work_index);
           break;
+        case SHADER_EVAL_CURVE_SHADOW_TRANSPARENCY:
+          kernels.shader_eval_curve_shadow_transparency(kg, input_data, output_data, work_index);
+          break;
       }
     });
   });
@@ -144,6 +147,9 @@ bool ShaderEval::eval_gpu(Device *device,
     case SHADER_EVAL_BACKGROUND:
       kernel = DEVICE_KERNEL_SHADER_EVAL_BACKGROUND;
       break;
+    case SHADER_EVAL_CURVE_SHADOW_TRANSPARENCY:
+      kernel = DEVICE_KERNEL_SHADER_EVAL_CURVE_SHADOW_TRANSPARENCY;
+      break;
   };
 
   /* Create device queue. */
diff --git a/intern/cycles/integrator/shader_eval.h b/intern/cycles/integrator/shader_eval.h
index 013fad17d4f..43b6b1bdd47 100644
--- a/intern/cycles/integrator/shader_eval.h
+++ b/intern/cycles/integrator/shader_eval.h
@@ -30,6 +30,7 @@ class Progress;
 enum ShaderEvalType {
   SHADER_EVAL_DISPLACE,
   SHADER_EVAL_BACKGROUND,
+  SHADER_EVAL_CURVE_SHADOW_TRANSPARENCY,
 };
 
 /* ShaderEval class performs shader evaluation for background light and displacement. */