Added information about ray tracing light culling

com.unity.render-pipelines.high-definition/Documentation~/Ray-Tracing-Getting-Started.md

@@ -186,20 +186,6 @@ To check whether it is possible to use ray tracing in a Scene, HDRP includes a m
 1. Click **Edit > Rendering > Check Scene Content for HDRP Ray Tracing**.
 2. In the Console window (menu: **Window > General > Console**), check if there are any warnings.
 
-<a name="RayTracingMeshes"></a>
-
-# Ray tracing and Meshes
-
-HDRP changes how it handles Meshes in your scene when you integrate a ray traced effect into your project.
-
-When you enable ray tracing, HDRP automatically creates a ray tracing acceleration structure. This structure allows Unity to calculate ray tracing for Meshes in your scene efficiently in real time.
-
-As a result, ray tracing can change how some Meshes appear in your scene in the following ways:
-
-- If your Mesh has a Material assigned that does not have the HDRenderPipeline tag, HDRP does not add it to the acceleration structure and does not apply any ray traced effects to the mesh as a result.
-- If your Mesh has a Decal Material assigned, HDRP does not add it to the acceleration structure and the Mesh does not appear in your scene.
-- If a Mesh has a combination of Materials that are single and double-sided, HDRP flags all Materials you have assigned to this mesh as double-sided.
-
 <a name="RayTracingEffectsOverview"></a>
 
 ## Ray tracing effects overview
@@ -214,8 +200,24 @@ HDRP uses ray tracing to replace some of its screen space effects, shadowing tec
 - [Recursive Ray Tracing](Ray-Tracing-Recursive-Rendering.md) replaces the rendering pipeline for Meshes. Meshes that use this feature cast refraction and reflection rays recursively.
 - [Ray-Traced Subsurface Scattering](Ray-Traced-Subsurface-Scattering.md) replaces [subsurface scattering](Subsurface-Scattering.md) with a more accurate, ray-traced, subsurface scattering technique that can use off screen data.
 
-## Ray tracing mode
+<a name="RayTracingMeshes"></a>
+
+## Ray tracing and Meshes
+
+HDRP changes how it handles Meshes in your scene when you integrate a ray traced effect into your project.
+
+When you enable ray tracing, HDRP automatically creates a ray tracing acceleration structure. This structure allows Unity to calculate ray tracing for Meshes in your scene efficiently in real time.
+
+As a result, ray tracing can change how some Meshes appear in your scene in the following ways:
 
+- If your Mesh has a Material assigned that does not have the HDRenderPipeline tag, HDRP does not add it to the acceleration structure and does not apply any ray traced effects to the mesh as a result.
+- If your Mesh has a Decal Material assigned, HDRP does not add it to the acceleration structure and the Mesh does not appear in your scene.
+- If a Mesh has a combination of Materials that are single and double-sided, HDRP flags all Materials you have assigned to this mesh as double-sided.
+
+## Ray tracing light culling
+Ray tracing requires HDRP to cull lights differently to how it culls lights for rasterization. With rasterization, only lights that affect the current frustum matter. Since ray tracing uses off-screen data for effects such as reflection, HDRP needs to take into account lights that affect off screen geometry. For this reason, HDRP defines a range around the camera where it gathers light. To control this range, use the [Light Cluster](Ray-Tracing-Light-Cluster.md) Volume override. It is important to set a range that accurately represents the environment scale. A higher range makes HDRP include lights further away, but it also increases the resource intensity of light culling for ray tracing.
+
+## Ray tracing mode
 HDRP includes two ray tracing modes that define how it evaluates certain ray-traced effects. The modes are:
 
 * **Performance**: This mode targets real-time applications. If you select this mode, ray-traced effects include presets that you can change to balance performance with quality.