Faking Shadows from a Cloud Cover in Unity URP


I was trying to add some shadows from clouds to my 2.5D RPG to give a more lively feeling to the world map so I’m sharing a quick writeup on the 2 ways I’ve tried.

The first option I’ve tried is to simply create a plane on which I applied scrolling noise and alpha cutout, using the noise as alpha. This does block some light from reaching the ground and is quite easy to do.

cloud shadows using a plane
Blocking Light using a Plane with Alpha Cutout

This solution has a problem though: Unity creates a shadowmask for real time moving objects, so objects below are either lit or non-lit; this gives a constant level of shadowing where light is obstructed by our cloud cover.

This was not bad but I really wanted something less… uniform. The way to solve this was to inject a custom renderer feature in our pipeline and add screenspace shadows.

Thankfully we talked about using custom renderer feature in our article on Tilt Shift. So all we have to do is to create a suitable material.

What I did for my first try was to get the color from the main texture, convert the color from RGB to HSV, adjust the Value channel according to a generated noisemap and an exposed multiplier, than convert it back to RGB and output to master node. We also add some scrolling to our noise, and can also expose a property to set the player position so that the scrolling noise looks like it’s scrolling in world space and not screen space.

Here’s the result: we can see that the shadows are not clear cut anymore and there is more or less light passing through depending on the location.

renderer feature cloud shadows
Fake Cloud Shadows using a Renderer Feature