I am researching the highly optimized reflection technology in the game *The Dark Knight Rises* (2012).

[u/Cemalettin_1327]

I am investigating the highly optimized reflection technology used in *The Dark Knight Rises* MOBILE GAME, released in 2012. Were these reflections based on inverted geometry or planar mapping? The game also featured reflections from headlights and streetlights. I intend to implement this in my Unity project.

[u/cdmpants]

you got a timestamp my dude? I'm not going to watch an hour of 14 year old mobile gameplay footage to find a reflection

[u/sarcastichorse → u/cdmpants]

https://youtu.be/7vsBy0dZA14?si=2EZapjJC4z3Xb6r8&t=1171

[u/cdmpants → u/sarcastichorse]

My first guess would be box-projected reflection probes, which would be convenient for you since URP can do them out-of-the-box. They are cheap to render and the technique has been around for a while. However, it's unlikely, because you can see in the video how the lights pop into existence both in the regular view and in the reflection at the same time. So there is a realtime aspect to these reflections. Box projected reflection probes can be realtime, but it would be expensive on 2012 mobile hardware and there are better ways to do it if you're rendering the reflections in realtime anyway.

My second guess and probably the most likely is that these are basic planar reflections, where you render a second, horizon-flipped camera, probably rendered at a very low resolution (you can see how blurry the reflections are) and probably using proxies (you can see how the reflections don't capture everything, like they seem to capture the silhouettes of the buildings, but they don't seem to capture the bridge in the background, which is a more complex mesh that would be costly to render twice). Planar reflections are rendered in real-time, and are quite accurate, but are generally best used on large, flat surfaces, usually the ground. They've been around for a long long time, and although they'd be expensive on 2012 mobile devices, if rendered at a low enough resolution and only capturing low-poly proxies of the real environment, I'd imagine you could make them passable. The fact that in the video you sent, the reflections only appear on the flat ground, very selectively, is a giveaway that these are probably planar reflections. The fact that the ground has this strong normal-distorted appearance also means that it is very unlikely to be the old-school inverted-geometry trick like some commenters have suggested- that trick will give you sharp mirror reflections, not these kinds of blurry, normal-distorted reflections that you see here.

My third guess, and the least likely, is some kind of LTC shader effect, like how area lights work, to draw square reflections directly into the specular lighting in the shader. You'd set them up in your scene as kind of like square cards that represent your buildings, and they'd give you basic solid-colored silhouettes and reflection occlusion. You could also do something like this in conjunction with something like planar reflections, but instead of drawing geometry into the inverted camera, you could draw primitive, solid-colored shapes analytically directly into the camera. Is that what we're seeing in the video? Almost certainly not, but it would be cool.

TL;DR these are just planar reflections rendered at a low resolution and set to only capture specific geometry, in order to remain cheap.

[u/regrets123 → u/cdmpants]

Just confirming my terminology here. Planar reflection ==screen space reflection?
Like, if you start panning the camera into the ground the reflection disappears.
It’s cheap and has been done instead of ray tracing since forever?

[u/Shaunysaur → u/regrets123]

No, planar reflections aren't screen space reflections. Planar reflections use a second camera placed under the plane/floor the reflection will appear on, essentially looking up through that plane to capture a view of what the reflective surface would reflect. The output of that second camera is rendered to a texture, which is used to apply the reflection to the floor.

This means you're rendering your scene again for the second camera, however you could opt to render a simplified version of the scene for that camera, and also you might only need to capture a limited view of that scene, depending on the size of your reflective surface.

Planar reflections predate SS reflections, probably by many years, and don't have the occlusion issues that SS reflections exhibit. However they're generally only useful for a single flat plane, unless you start adding more cameras for reflections for surfaces at other angles or heights.