Transparency and render methods
Verified in both Cycles and EEVEE:
Material | Result in the pass |
|---|---|
Opaque | Its own colour |
Glass BSDF, default render method | Its own colour, occluding what is behind |
Transmission on Principled | Its own colour, occluding what is behind |
Render Method set to Blended | No ID at all — you see through to whatever is behind |
Fully transparent BSDF | No ID — nothing is there to shade |
Alpha below 1 | Partial, dithered coverage — not a clean matte |
The row to remember is the Blended one, because it is a common setting for archviz glass. A window set to Blended will be missing from your ID pass. Set its Render Method to Dithered and it appears.
For a partly-transparent surface you genuinely need to select, pin it a colour and set it to Dithered for the ID render.