[2.14] Interactible projected decals

[stgatilov]

The Doom 3 engine has some architectural limitations: everything which can be rendered and lit must be a "render entity", and they are usually tied to game entities. For that reason, projected decals (i.e. the ones generated dynamically using the world geometry) are rendered with some lightweight processing, which excludes any possibility to interact with lights.
The architecture of projected decals has been changed in TDM 2.14, and now they can interact with lights just like any simple func_static, for instance.

What's the point in this change exactly?
Well, it was requested by @Dragofer a few years ago, when he tried to add some depth to guards' footsteps on the snow by applying normal maps. Normal map is only used in lighting equations, thus it had no effect at all on the projected decals previously.
From now on, if decal's material has interactions stages, it will be lit. So normal map and specular map finally make sense for projected decals just like it makes sense for static ones.

The new behavior is enabled by default, but is a potentially breaking change. If we discover that some missions have become visually ugly because previously inactive interaction stages have become active, we'll try to fix missions. If there are too many issues, we'll have to restore legacy behavior by default and hide the new behavior behind material parameter. Right now it is possible to switch between two systems using cvar r_decalInteractive.

 

Here is the testmap originally made by @Dragofer, with normal-mapped footsteps generated as projected decals:

You might have noticed that footsteps are too bright where they overlap.
That's because light interactions are additive in our engine, so each of the overlapping footsteps makes the image brighter. The usual way to avoid this is to make surfaces opaque, but then we'll get all the joy of depth fighting which is very hard to avoid in such a dynamic scenario. So yeah, while technically decal interactions are possible, unfortunately it does not necessarily mean that it works well in every imaginable scenario.

P.S. The tracker issue is tracked as 5867.

[kingsal]

Nice change! This will open up a lot of cool opportunities for us.