Crysis is one of the most demanding games ever released, and it can still chug at times even on modern extreme hardware. But that hasn't stopped one modder, Pascal Gilcher, from pushing things to the next level, attempting to inject path tracing calculations into the mix. It's currently not quite as polished as the Minecraft path tracing mod, but the results are nevertheless impressive.
Path tracing and ray tracing use similar concepts with differences in the specific techniques. Our guide to ray tracing goes into more detail about what it is, how it works, and the various types of ray tracing that can be used in games.
Pascal's ReShade filter (in alpha form—you have to be a Patreon backer to get access) attempts to apply path traced global illumination to games. Yes, any compatible game can in theory add global illumination, not just Crysis. But an older game like Crysis seems like the perfect playground for such shenanigans, and EuroGamer / DigitalFoundry put it to the test—in co-op mode, just to take things to the next level.
It's still a bit unclear as to how this particular filter accomplishes its magic. ReShade typically applies post-process filters to improve the way a game looks—adding SMAA to remove jaggies, SSAO for more realistic shadows, depth of field effects, and more. ReShade injects a DLL into the DirectX or OpenGL libraries to provide access to frame color and depth data, but ray tracing normally needs a lot more than that. Specifically, ray tracing (or path tracing) relies on having access to the geometry in a scene, and runs ray/triangle intersection calculations to help determine shadows, reflections, global illumination, and more.
This 'path tracing ReShade filter' focuses on global illumination for now, but this is not actually path tracing (or ray tracing) in the traditional sense. It doesn't have access to all the geometry and other data that would be necessary to do that sort of ray tracing. I also gather that it doesn't require or even make use of features that accelerate ray tracing, like the RT cores in Nvidia's RTX graphics cards. Doing so would require the use of DirectX Raytracing (DXR) or Vulkan-RT, which would cause problems with an old DirectX 9/10/11 game like Crysis. Instead, computations are done using traditional shaders to simulate path tracing.
Again, this is all a bit fuzzy and there's no detailed explanation right now. I've reached out to Pascal to try to get some additional explanation about what's going on and will update if I receive a response. The information available suggests the filter is using screen space data to approximate some of what ray tracing does, with less of a performance hit and in a way that can be applied to existing games. If that's correct, it's perhaps better to characterize this 'path tracing' as a form of screen space ray tracing—it sounds similar to Unigen 2's SSRTGI at least, which is used in the Superposition benchmark.
Whatever the case, this ReShade filter makes the 11-years-old Crysis look even better. That's in part because Crysis was released before things like global illumination were even attempted. It was the first game to do ambient occlusion, trying to make things look less 'flat,' and GI kicks the shadows and lighting quality up another level. Just don't expect performance to improve, because 'path tracing' of any sort only makes Crysis even more demanding.