Realtime reflections in Unity 5

Here's a quick tutorial on how to create realtime reflections in Unity 5. This approach is very hacky, but you can use the new reflection probes in Unity 5 to easily create realtime reflections for something like a reflective floor.

You can probably use this for more things, like wall mirrors and multiple reflections in the same scene, but that would require quite a bit more scripting magic. This is just a quick and dirty way which can be used for something like the floor in an architectural visualization or similar. For more elaborate effects you should consider something like screen space reflections.

Scroll to the bottom if you just want to see the effect in action or download the project folder.

When you first fire up Unity 5 and look at the inspector for a reflection probe you notice that you can set the type to "Realtime" as well as set it to update every frame. You think that's neat and immediately try it out only to be hugely disappointed when it doesn't look like your reflection probe is updating in realtime at all. All the reflections are wonky and they don't line up with the geometry. It's not at all what you were expecting.

But the reflection probe is actually updating every frame, it's just not what you, or at least I expected when I first tried it out. A reflection probe is sort of a point in space with 6 virtual cameras that render your scene from all directions and then assemble those 6 images into a cubemap. Your materials can query this cubemap for reflection data. At least that's how I think it works. By setting the reflection probe to realtime and the refresh mode to every frame you're telling Unity that you want a new cubemap generated every frame. The problem is that you're generating all these cubemaps at the probe's position. That's why things look wonky and don't line up. You want to generate these cubemaps from the position of the player's camera.

To accomplish this we need to write a small script which we attach to our reflection probe:

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using UnityEngine;
using System.Collections;
 
public class RealtimeReflection : MonoBehaviour {
    
    ReflectionProbe probe;
    
    void Awake() {
        probe = GetComponent<ReflectionProbe>();
    }
    
    void Update () {
        probe.transform.position = new Vector3(
            Camera.main.transform.position.x, 
            Camera.main.transform.position.y * -1, 
            Camera.main.transform.position.z
        );
 
        probe.RenderProbe();
    }
}

The above script makes sure the reflection probe is always positioned where the camera is, but at the "opposite" height. So if the camera is 1.8 meters above the floor then the reflection probe is 1.8 meters below it. That way it renders everything the player's camera sees except from the opposite side of the floor. It's sort of like the duplicate room mirror hack in very old games. Here I make the assumption that your floor is at y = 0. If your floor is at a different y-value you have to change the code to account for that, or move your floor.

I originally wanted to move just the origin for the probe so that the projection bounds would stay in place, but I wasn't able to do that easily through scripting. If you know how I would be interested to hear it. Basically the way I'm doing it here means you'll be moving the bounds with you so you must ensure that no matter where you move in your scene the bounds for the box projection encompass the area you want to reflect.

After attaching the script to your reflection probe you'll notice you get no reflections at all. This is because the reflection probe is now below the floor. How can it render what's going on above the floor when it can't see what's up there? To let the probe see through the floor we need to assign the floor to a layer and then uncheck that layer in the reflection probe's "Culling Mask" setting in the inspector.

Now things should start to look good. If you feel like the reflections are lagging behind when you move around you can change the "Time slicing" setting in the inspector to "No time slicing". You can check out the official documentation about reflection probes to see what time slicing is. I have put a link at the bottom. But basically it's a performance setting and "No time slicing" means: "Fuck it! I don't care about performance, I just want it to look good!".

Here are my reflection probe settings:

This is a massively expensive effect. You can bring even on a beastly machine to a halt with the simplest test scene if you set the reflection probe to render at 2048 resolution. After all you're rendering six 2048x2048 resolution textures every frame if I understand this correctly. Not something you want in a game. But I'm using 1024x1024 without problem in an architectural visualization scene I'm working on. And If you have blurry or rough reflections on your floor you can go much lower as well. 1024 or higher is just to get that perfect mirror look.

Obviously you could speed up this technique a ton if you were able to just render from 1 camera instead of 6. We don't need the left, right, back, forward and down cameras from the reflection probe because our probe is below the ground. We only care about the camera facing up. But that would probably require custom shaders and a bunch of scripting, ie. much more work than just using what we already have in a hacky way.

You can see that there are a number of limitations to this effect, but it's a neat trick which can come in handy. And perhaps it can be used in other ways as well. And you can of course use texture maps or glossy/rough reflections with this effect. I just thought the mirror effect was the most impressive for the example scene seeing as mirrors have forever been the bane of realtime graphics.

Check out an example webplayer here:
/projects/realtime-reflection-example-scene

Or download the entire project folder here if you want to have a look:
/sites/default/files/unity/project/real-time-reflections-1.0.1.zip

And lastly there's a massive entry in the official Unity documentation about reflection probes. It goes in detail about the different reflection probe types, refresh modes, time slicing and more:
https://docs.unity3d.com/Manual/class-ReflectionProbe.html

About Øyvind Strømsvik (TwiiK)

I've been passionate about games all my life and started dabbling in game development about 15 years ago with BlitzBasic,... read more but I quickly lost interest and began doing 3d modeling instead. 3d modeling remained a hobby and I picked up game development again around the release of Unity 2.0. My driving force behind wanting to get back into game development was my lost interest in commercial games as they started appealing to a group of gamers I was no longer in. Indie games were the only games that still looked interesting, but at the same time some of them looked like they would be just as fun to make as to actually play. And many of them were made by just one guy.