Quad decal on uneven terrain

How can I make a quad follow terrain shape? Is there a gpu solution? Maybe sampling terrain heightmap and transforming decal vertices?

I've read about projectors but I've never worked with them but I believe they may be too heavy performance wise.

 

The legacy projectors work by re-rendering the entire mesh that's being cast upon. For terrain this is obviously bad since the entire terrain is getting re-rendered in this case, regardless of how big the projector is. A single footstep or fake cloud shadows, both re-render the entire terrain mesh.

The "best" solution is to use deferred decals, which use the depth texture and a projection matrix (or just a box) to project a texture onto the world position derived from the depth texture. This works well if you're already doing deferred rendering or on a PC where the depth texture is most likely already being generated.

On mobile ... there's not really a good GPU option.

If you just need a single projected shape, then using a custom shader on the terrain that adds a single offset texture is a good option. If you need several projected, then having a render texture that you render them all into, and then sample that single render texture in the custom terrain shader is a good option.

Or just put a quad roughly above the terrain using a c# script. GetInterpolatedHeight() is pretty fast.

 

Thanks a lot for explaining my options, I finally went with simple quads because my terrain is forward rendered.

 

There's also a "hacky" approach for the forward render path that Easy Decals do. Re-projecting mesh on top (similar to deffered rendering), and baking results into a mesh.

Works pretty well even on mobile. (Baking takes some processing power, but after that its just a mesh)