Reflections are a pretty interesting point of discussion in these games, as they all seem to go about it a little differently.
Driveclub without a doubt has some of the best use of screen space I've seen in a racing game with good fallback technique. You can tell where they used the power freed up by running at 30fps.
SSR doesn't always work, especially at extreme angles or when reflected areas are partially occluded. They also don't work if the reflected objects are off-screen. Driveclub uses cubemaps that include the other cars on the track as well so that even when the cars aren't on screen for the SSR, they still appear in the cubemap reflection, though at a lower resolution and LOD.
This video shows the pros and cons to DC's reflections. You can see the reflected areas use screen space until the reflected area leaves the screen, then is replaced with the cubemap. Even the car's self reflected mirror disappears from the reflection as it leaves the screen. I think if Evo had the chance to make a sequel on the pro they would have had it completely nailed down. As it stands I can't think of another racing game that has self-reflection at all. Overall it's easily the best solution I've seen, but again, the game is a closed-track arcade racer reduced to 30fps.
-PCars2 uses a cubemap setup like GTS and Forza 7, but
has no reflections of cars in wet weather in-game. Overall the reflections in the game are pretty dated and don't use any fancy work-arounds or tricks to make things look better than they should. There isn't a whole lot to go into but it should be noted the game has 36 cars on track, all with a very robust damage system, weather system and night racing, not to mention it's running what is arguably the most sim-oriented engine of the big 3 franchises.
-Forza uses cubemaps, but they don't include other cars in them like Driveclub does. In actuality, Forza is already rendering up to 24 duplicate "reflected cars" at any given point along with duplicates of most of the trackside objects in place of screen space reflections normally used. Using this technique instead of SSR the track reflections will not be distorted being blocked by other objects, which is a constant issue with 24 cars on a track. These are always present, as a race in FM7 can have certain random weather variables, meaning the track
can go from completely dry, to a downpour, back to completely dry. This also allows for reflected objects to be manually removed by the devs to keep the framerate solidly above 60fps with rain, night, dynamic headlights and 24 other cars on screen.
The benefit is that the reflected car isn't just a mirrored image, and the "reflected model" has it's own reflection off it's "reflected geometry". This shot of the ferris wheel shows that effect well, the reflections on the real Van and the "reflected van" are coming from different sources and thus appear differently. In Horizon 4 using SSR the reflected van would have a duplicate reflection that matches the original van's reflection.
This method has some big drawbacks though, as seen here, sometimes the objects chosen not to be reflected are quite noticeable like the stands and trees below the animated ferris wheel. Often times even the objects that pass over your car don't make the cut and it can be jarring to not see them in the reflection. Items like the tire walls are never reflected as they are an animated object in FM7 and react to being hit. Finer detail like the fence panels are gone as well. The end result is a bit of a patchy reflection missing a lot of detail, but it was all likely done to preserve the framerate yet still show custom liveries in the reflections. Quite often if you race in the wrong direction you'll see that there are many more missing reflected objects as you were never intended to drive in that direction.
The other drawback isn't as big, but looks odd. Regardless of whether you can see the cars on the track, the "reflected cars" are still being rendered into the wet surfaces. In this shot I'm driving over the overpass on Suzuka, and several AI are passing below me. I get to see their inverted reflected models rendered where the cars are on the track below. This was also visible in
this video linked to above.
Finally, the Forza games use a technique in hood view that isn't SSR or a cubemap. In bonnet view the game takes the previous frame data, and vector maps the resulting image to the hood and any parts visible in that view. This results in a hood reflection that matches the resolution, contains every visible object on the screen including other cars, the driving line, cones, particle effects, literally everything. it also updates at 60fps. The result is extremely clear reflections of everything in that view. This is attainable because the bonnet view is a replacement overlay, not your actual car. The drawback here is that each car has to be mapped so I imagine this takes time to implement. The other drawback is that on certain cars like the Caterham the light pods use the regular cubemap, so on the base Xbone, the headlight pod's reflections update at 30fps while the hood is updating at 60fps. Digital Foundry incorrectly thought this technique was used in other views as well. On several cars, when looking to the extreme right or left, you can see where the hood reflection is replaced with the cubemap.
In this game shot you can see a reflection in the suspension components, as well as the tire surface. When stopped, the tire reflection is almost lost in the rubber shader. Hood view in Forza with a wheel setup is the best way to play though I would like to see this reflection technique used on the guages/no wheel view so I can deactivate the HUD permanently.
-Forza Horizon 4 made the move to more widespread use of screen space for everything and reflections as a whole are better than in FM7, but SSR has it's own problems as noted above. Because in the game you're constantly driving through trees, bushes, etc, your car is constantly being covered by objects. These would block and distort the screen space reflections so the game uses a cubemap instead for the cars. Horizon 4 also keeps the vector map technique used for the hood view. The cubemap includes a lot more detail than the one in FM7 down to cones and small roadside details, but still doesn't include other cars. I'll assume 12 cars with custom liveries + traffic cars in an open world all being included in the reflections may have been a lot to ask, although the drone does appear in reflective surfaces.
The first image shows the best thing about SSR. The reflection includes everything and maps itself to every reflective surface. In normal gameplay this is pretty much what you see most of the time but SSR falls short in several instances.
1. SSR has issues rendering complex objects in space properly. This Can-Am Maverick is open wheel, and open top with a roll cage. SSR can't handle the spaces in between the parts, so it does what it can, usually filling the space with a gradient between the two surfaces. In gameplay you'll never notice it as the "filler" changes constantly and when moving there is a large wake and a splash effect created by the car.
2. When SSR is blocked by anything, it is incorporated into the reflection, or it distorts the reflection. As seen here, the tree branches are being included in the reflection of the mountain. Again this isn't a major issue as you seldom find yourself in the trees for very long and if you do, you're probably travelling at a high enough velocity that the issue will only be on screen for a fraction of a second.
3.This is the most common problem with SSR, and likely the reason Driveclub uses both SSR and cubemaps on the cars, SSR can't include what isn't on screen. In this image the mountain {seen in the first image} is only reflecting the very base section as the rest has left the screen. In-game it's seldom seen as all the in-game cameras are pointed in a way that the horizon is visible, but off large jumps, or in drone mode, the culled reflections become apparent. Horizon 3 didn't use SSR, and it's reflections are significantly less detailed than Horizon 4, but they don't have any of the above issues.
While not at the same level as the reflections in Driveclub, the game runs the same engine as FM7, dynamic TOD, a full weather system and much higher resolution assets, and it's just a much newer game in general designed to work beyond the Xbox One S.
-Gran Turismo Sport is a game I have little time with, and as I've only spent a short time with the game I'm mostly working off DF and other online information sources. If DF is wrong, I'm sure someone who actually owns the game can fill everyone in with any info. DF was incorrect about the car reflections in FM7 so it's not unfathomable they may have made other mistakes.
GTS appears to use a single cubemap for every car. If the player car crosses under something, that object will appear in the reflection of every car on the track at the same time. Reflections also appear to have a variable framerate but I assume are supposed to be running at 60fps. This video is a year old now though so that may have been patched.
There are wet surface challenges, but the game doesn't appear to reflect the car, similar to Project Cars 2
The great thing, assuming that it can be properly implemented efficiently in time, is that ray traced reflections and lighting can solve almost all of these issues entirely without relying on multiple techniques or cut corners. Polyphony has been working on their own tech for a while, and I have no doubt they'll have something to show for the next Gran Turismo. Microsoft has been on a dev acquiring spree, and with that many studios now collaborating, they may have a solution before long as well to get to that standard without a huge performance hit. Slightly Mad is currently making a custom
PC marketed as a console, which is a really good idea for a studio that makes their games with donation money. I'm guessing they won't survive this idea, but maybe they'll surprise us all.