Days Gone is one of the more polished PlayStation exclusives to come to PC. Although the game doesn’t feature full-fledged hardware-accelerated ray-tracing like Metro Exodus or Control, it does come with what’s called Screen Space Ray Traced Global Illumination. This is basically a shader-based form of ray-tracing that is a huge improvement over Screen Space Ambient Occlusion. However, while ray-tracing generally considers objects off the screen, this particular method doesn’t. It only takes into account data that’s in the Z-buffer (or screen space). Regardless, it’s still much more accurate than SSAO and its derivatives and can be compared to McFly’s SSRTGI ReShade shader. In fact, it wouldn’t be unfair to compare this implementation to Resident Evil Village’s RTGI.
RTGI is a global illumination (lighting) algorithm that calculates the diffuse lighting in the scene (indirect lighting/rays that light objects after already being reflected once) as well as direct illumination due to the sun, sky, or a light source.
Traditional lighting shaders don’t account for the illumination due to light that penetrates a scene after being reflected once. This is known as indirect lighting or diffuse lighting and tends to be quite intensive due to its recursive nature. Till now, it’s been implemented using voxelization and ray-tracing. There are other methods as well but they aren’t as effective. RTGI is the most accurate as it essentially mimics the natural global illumination phenomena. With that out of the way, let’s have a look at the comparisons:
As you can see, the impact of Screen Space Ray Traced Global Illumination is quite prominent in scenes with vegetation and other tiny objects. Here, traditional SSAO misses the polygons due to the limited coverage provided by it, and such remains largely ineffective. Furthermore, as evident in the below image, standard SSAO also misses objects that have limited depth information such as the bars on the top of the water tank. SSRTGI is able to properly estimate the shadows cast by them.
In interiors, the impact is less noticeable, but nonetheless, SSRTGI results in smoother, much softer shadows that appear more natural and realistic.
In the above image, SSAO once again misses out on the shadows cast by the underside of the truck. This is because there isn’t sufficient data available to the algorithm regarding that part of the truck and the part casting the shadow is essentially hidden away behind the exterior. SSRTGI is very effective here because of the way it’s calculated.
The performance figures are a bit surprising. Yes, SSRTGI does cost a fair bit of GPU processing power, but SSAO Medium which looks significantly worse off than SSAO High performs nearly the same. SSAO Low which is essentially no AO is however much faster. As such, if you’re still somehow unable to hit 60 FPS, I’d recommend turning down Lighting to High, but avoid switching to Medium or Low as they really have an adverse impact on the visual fidelity.