My immediate thought was to use the “Static Mesh Decal Function” which is a material meant to be used on a translucent box that basically does depth reprojection to create UVs for decals. But it appears that raytraced translucency is having issues with the function because it can’t properly sample the scene depth. Plugging it into a scalar debug node output a flat value of 1,000,000… but its just black. With that node broken, I don’t see any way of using depth reprojection to create a decal.
If it did work, then you could use it in place of dbuffer decals - or what I’d probably do is use the dbuffer in the main pass and make the decal function only visible in RT (since dbuffer has more features).
Annoyingly RT translucency is all or nothing. AFAIK so you can’t set the decal function box to be raster while glass is RT to get around the bug either.
Not sure if there’s a good reason why you can’t mix raster and RT translucency in the same PPV but I really think it should be allowed. I’d like to be able to pick and choose… But that aside, the bigger issue is that the scene texture/depth nodes appear broken in RT translucency.
Until one of those two things are fixed, the only way I can see it working is if you had a conforming mesh with an opaque masked material that has the decal texture on it and lined it up closely to the real decal, kind of like this image from the mesh decal documentation:
