Set global ray-tracing parameters

1. In the Scene or Hierarchy view, select a GameObject that contains a Volume component to view it in the Inspector.
2. In the Inspector, navigate to Add Override > Ray Tracing and click on Ray Tracing Settings.

For more information about the ray tracing settings properties, refer to Ray Tracing Settings reference.

Add objects to the Ray Tracing Acceleration Structure

HDRP provides a utility function that adds objects to the ray tracing acceleration structure. The function is AddInstanceToRAS and it takes a Renderer) a HDEffectsParameters parameter and two a booleans that tracks changes in the transform and material properties of the included game objects.

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.Experimental.Rendering;
using UnityEngine.Rendering;
using UnityEngine.Rendering.HighDefinition;

public class ManualRTASManager : MonoBehaviour
{
    RayTracingAccelerationStructure rtas = null;
    public List<GameObject> gameObjects = new List<GameObject>();

    void Update()
    {
        HDRenderPipeline hdrp = RenderPipelineManager.currentPipeline is HDRenderPipeline ? (HDRenderPipeline)RenderPipelineManager.currentPipeline : null;
        if (hdrp != null && rtas != null)
        {
            // Get the HDCamera for the current camera
            var hdCamera = HDCamera.GetOrCreate(GetComponent<Camera>());

            // Evaluate the effect params
            HDEffectsParameters hdEffectParams = HDRenderPipeline.EvaluateEffectsParameters(hdCamera, true, false);

            // Clear the contents of rtas from the previous frame
            rtas.ClearInstances();

            // Add all the objects individually
            int numGameObjects = gameObjects.Count;
            for (int i = 0; i < numGameObjects; ++i)
                HDRenderPipeline.AddInstanceToRAS(rtas, gameObjects[i].GetComponent<Renderer>(), hdEffectParams, ref hdCamera.transformsDirty, ref hdCamera.materialsDirty);

            // Build the RTAS
            rtas.Build(transform.position);

            // Assign it to the camera
            hdCamera.rayTracingAccelerationStructure = rtas;
        }
    }

    void Start()
    {
        if (rtas == null)
            rtas = new RayTracingAccelerationStructure();
    }

    void OnDestroy()
    {
        if (rtas != null)
            rtas.Dispose();
    }
}