Implement generic decal geometry placement over meshes

[mvaligursky]

The task is to implement a geometry generation for a decal, allowing a rectangular quad to be placed on a mesh (or an array of meshes), with wrapping around. Something similar to this: http://blog.wolfire.com/2009/06/how-to-project-decals/

[kungfooman]

I worked on a live decal system, currently using the camera as "projector":

Front view:

Back, good to see the wrapping:

(the texture UV's don't look so good yet in the bottom/green part, because from the Point of View of the projector there is so little delta to calculate left/right)

It works only on static models, searching all mesh instances in the scene and using their node.worldTransform to transform the mesh faces/positions.

Currently it just generates a globally positioned mesh which is then drawn via app.drawMesh.

Maybe two systems would be useful:

• live decals, like a light source drawing a decal on the entire scene (using tags as filters), with ability to assign custom materials or e.g. fire shader for some nice effects
• static decals, that get generated once and then attached as child to the meshInstance parent entity, e.g. a driving car with decal bullet holes attached to it, so they 'stick'

This would be amazing for better testing/debugging: #2904

[kungfooman]

Finally came around to make it work in the Playcanvas Editor: https://playcanvas.com/project/907728/overview/decals

Thoughts/ideas/improvements are welcome, personally there are so many things I would like to add/change, but thats never-ending. Its at least a starting point.

The editor doesn't seem to be able to set the material attributes slopeDepthBias / depthBias, so I hardcoded that e.g.

Since the script currently regenerates all decals every frame (so you can move the camera in editor and see live results), normals/tangents calculation slows the scene a bit down:

[mvaligursky]

That runs nicely!

[yaustar]

Other examples:

https://forum.playcanvas.com/t/project-decal-texture-where-the-character-at/28472
https://forum.playcanvas.com/t/spray-decal-script/25778