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

1 files changed, 21 insertions, 0 deletions

diff --git a/intern/cycles/kernel/kernel_bake.h b/intern/cycles/kernel/kernel_bake.h
index 1473e9ba8bf..30a41b9d3e3 100644
--- a/intern/cycles/kernel/kernel_bake.h
+++ b/intern/cycles/kernel/kernel_bake.h
@@ -96,4 +96,25 @@ ccl_device void kernel_background_evaluate(KernelGlobals kg,
   output[offset * 3 + 2] += color.z;
 }
 
+ccl_device void kernel_curve_shadow_transparency_evaluate(
+    KernelGlobals kg,
+    ccl_global const KernelShaderEvalInput *input,
+    ccl_global float *output,
+    const int offset)
+{
+  /* Setup shader data. */
+  const KernelShaderEvalInput in = input[offset];
+
+  ShaderData sd;
+  shader_setup_from_curve(kg, &sd, in.object, in.prim, __float_as_int(in.v), in.u);
+
+  /* Evaluate transparency. */
+  shader_eval_surface<KERNEL_FEATURE_NODE_MASK_SURFACE_SHADOW &
+                      ~(KERNEL_FEATURE_NODE_RAYTRACE | KERNEL_FEATURE_NODE_LIGHT_PATH)>(
+      kg, INTEGRATOR_STATE_NULL, &sd, NULL, PATH_RAY_SHADOW);
+
+  /* Write output. */
+  output[offset] = clamp(average(shader_bsdf_transparency(kg, &sd)), 0.0f, 1.0f);
+}
+
 CCL_NAMESPACE_END