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Crusader Kings in Space

Just a demonstration of how a space mod can be built in CK3, with several natural satellites around Saturn as playable provinces.

It's almost 400MB because I hadn't really thought about optimizing the textures and because the map size is 16384x8192. I wanted to make huge planets with a lot of distance, but I lost motivation...

Built on the Total Conversion Sandbox, I recommend starting from there if you're looking to create a total conversion. The space effect is accomplished by discarding terrain and water to make the ground invisible, then applying the skybox resource by Terrapass to create a spacey skybox.

A few different ideas on how to progress:

Although provinces can't be placed on top of each other, you can still use the heightmap to make certain objects in space much higher than others, making it feel like you're playing in a 3D environment rather than the 2D map it still is behind the scenes.

You can create more interesting planets, adding animated clouds, city lights, or an outer atmosphere using a billboard particle, a 2D image that always faces the camera, and place it in the center of the planet (with a slightly longer radius).

Particles can also be used to add nebulae or asteroid dust, though it could also be accomplished with a raymarched fog if you can access depth information. It's also possible to create a fresnel effect to simulate an atmosphere on the planet itself but that will require some shader work as well.

Any mesh can be made to have province/highlight color as an overlay (try mousing over the asteroids, or look at the color of the exclamation marks next to them!). If you look at trees in the vanilla game, you'll see they get colored using the exact same effect. It relies on the mesh being entirely within the province and is a nice way to still display territories despite there being no terrain to show province borders on.

Since it's very easy to get lost in space, the flatmap can be used to simulate a strategic view, which I've done a little in this mod. If you zoom out far enough, you'll see the flatmap appear which shows the province borders. Maybe you can expand this to be toggleable through a button.

Or instead of just outright discarding terrain, you could use the chasm shader by Terrapass instead and use it to selectively discard terrain, which could be interesting for something like floating islands!

For more information on modding CK3, I highly recommend the modding co-op server on discord, where you'll find a lot of helpful information as well as shared resources (like the shaders)!