From a14fe128fccd9f5d55afd6345e3dad19a7b4ea9f Mon Sep 17 00:00:00 2001
From: Michael Jones <michael_p_jones@apple.com>
Date: Tue, 23 Sep 2025 12:19:44 +0200
Subject: [PATCH] Fix: Cycles: MetalRT motion curves setup bug

MTLAccelerationStructureMotionCurveGeometryDescriptor.controlPointCount should specify the per-step control point count. Although the previous initialisation wasn't manifesting as incorrect behaviour it was technically wrong.

Pull Request: https://projects.blender.org/blender/blender/pulls/146568
---
 intern/cycles/device/metal/bvh.mm | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/intern/cycles/device/metal/bvh.mm b/intern/cycles/device/metal/bvh.mm
index 3506d9ab579..d9c96740898 100644
--- a/intern/cycles/device/metal/bvh.mm
+++ b/intern/cycles/device/metal/bvh.mm
@@ -500,7 +500,8 @@ bool BVHMetal::build_BLAS_hair(Progress &progress,
       geomDescCrv.radiusBuffers = [NSArray arrayWithObjects:radius_ptrs.data()
                                                       count:radius_ptrs.size()];
 
-      geomDescCrv.controlPointCount = cpData.size();
+      /* controlPointCount should specify the *per-step* control point count. */
+      geomDescCrv.controlPointCount = cpData.size() / num_motion_steps;
       geomDescCrv.controlPointStride = sizeof(float3);
       geomDescCrv.controlPointFormat = MTLAttributeFormatFloat3;
       geomDescCrv.radiusStride = sizeof(float);