How do I set RenderSetting.customReflection using a cubemap RenderTexture?

Hey there!

To achieve what you want, I would suggest creating a giant Reflection Probe around your environment and add a script that uses the ReflectionProbe.BlendCubeMap() function.

This will return a RenderTexture that you can assign by script in the probe. The function is quite efficient (~0.3 ms). Here's a simple proof-of-concept script:

 using UnityEngine;
 using System.Collections;
 
 [RequireComponent (typeof (ReflectionProbe))]
 public class BlendReflection : MonoBehaviour {
 
     [SerializeField] Cubemap cubemap1;
     [SerializeField] Cubemap cubemap2;
     [Range(0.0F, 1F)] [SerializeField] float blendFactor;
     RenderTexture renderTexture;
     ReflectionProbe probe;
 
     public void Start () 
     {
         probe = this.GetComponent<ReflectionProbe> ();
         probe.mode = UnityEngine.Rendering.ReflectionProbeMode.Custom;
         renderTexture = new RenderTexture (cubemap1.height, cubemap1.height, 0, RenderTextureFormat.ARGBHalf);
         renderTexture.useMipMap = true;
         renderTexture.isCubemap = true;
         probe.customBakedTexture = renderTexture;
     }
     
 
     public void Update () 
     {
         if (probe != null && cubemap1 != null && cubemap2 !=null) 
         {
             ReflectionProbe.BlendCubemap (cubemap1, cubemap2, blendFactor, renderTexture);
         }
     }
 }
 

Simply add this script to a probe, set the two cubemap, press play and use the slider to see it blend between the two maps. This will work on all objects affected by the probe.