This commit adds support for *decal projectors* to Bevy, allowing for
textures to be projected on top of geometry. Decal projectors are
clusterable objects, just as punctual lights and light probes are. This
means that decals are only evaluated for objects within the conservative
bounds of the projector, and they don't require a second pass.
These clustered decals require support for bindless textures and as such
currently don't work on WebGL 2, WebGPU, macOS, or iOS. For an
alternative that doesn't require bindless, see PR #16600. I believe that
both contact projective decals in #16600 and clustered decals are
desirable to have in Bevy. Contact projective decals offer broader
hardware and driver support, while clustered decals don't require the
creation of bounding geometry.
A new example, `decal_projectors`, has been added, which demonstrates
multiple decals on a rotating object. The decal projectors can be scaled
and rotated with the mouse.
There are several limitations of this initial patch that can be
addressed in follow-ups:
1. There's no way to specify the Z-index of decals. That is, the order
in which multiple decals are blended on top of one another is arbitrary.
A follow-up could introduce some sort of Z-index field so that artists
can specify that some decals should be blended on top of others.
2. Decals don't take the normal of the surface they're projected onto
into account. Most decal implementations in other engines have a feature
whereby the angle between the decal projector and the normal of the
surface must be within some threshold for the decal to appear. Often,
artists can specify a fade-off range for a smooth transition between
oblique surfaces and aligned surfaces.
3. There's no distance-based fadeoff toward the end of the projector
range. Many decal implementations have this.
This addresses #2401.
## Showcase
fc831c390d