RealityKit how to filter entities in post process

Question

I am implementing a "glow" effect in RealityKit. I followed this open source library which is in SceneKit: https://github.com/laanlabs/SCNTechniqueGlow. The basic idea is to create a "mask" texture that includes all the nodes that we want to "glow", then do a gaussian blur (in both horizontal and vertical direction) of the mask, then combine the blurred mask and the original texture to create the effect.

I was able to replicate the same result in RealityKit using post-process. However, I am not sure how to apply the "glow" to only selected entities (For example, only those entities with a GlowComponent). In SceneKit, this can be easily done by includeCategoryMask, as documented here: https://developer.apple.com/documentation/scenekit/scntechnique:

A bitfield used for including nodes in the drawing pass if the draw key specifies DRAW_SCENE or DRAW_NODE. Defaults to all bits set. See Category Masks.

There doesn't seem to be similar API in RealityKit.

I have uploaded a sample project here: https://drive.google.com/file/d/1-CakPFTvc4ckbIWrirGxx8EQI94Q_051/view?usp=sharing

When you run it, you can see 2 views (screenshot): the top view is a SCNView that uses the SCNTechnique; the bottom view is a ARView that uses postprocess to achieve the same effect:

• For SceneKit (top), the left box has the category bit mask specified in includeCategoryMask, so only left box has the contour.

• For RealityKit (bottom), I'd like to only apply the contour to the left box which has a GlowComponent, and leave the right box out (Right now it's applied to all entities)

enter image description here

• OMGPOP: Note that I don't want to add a "outline" entity that's slightly larger than the original entity to mimic the effect, because i want the outline to always be on top of all entities, and that it should be the same thickness, regardless of how far they are from the camera. So post-process is likely the only solution in RealityKit that's comparable to SCNTechnique.
• Jandi: Unlike SceneKit with its fine-grained controls, in RealityKit (26.1), post-processing methods still apply filters, be it CI or MPS, to the entire rendered scene. To apply a blur to specific rendered model entity (in order to get a desired glow effect) you need an alpha and/or depth targets for masking. Creating an accurate mask requires from you a well-designed algorithm in the Metal kernel. I think category masks are useless in this case.
• Jandi: Of course, there's always a second approach, although it's even more processor-intensive and it's outside the scope of standard post-processing API. To achieve a blur effect on a single model entity, you'd need to render that specific entity into a separate texture and then combine it with the main scene's render output (a.k.a. premultiplied rgb*a image compositing).