User:PancakeIdentity/Opacity

- pressure plates require the block below to have a rim (e.g. hopper) or center point on top (e.g. glass pane) - rails require the block below to have a rim - banners and cakes require the supporting block to be a solid material (materials should probably be documented somewhere) - bells and levers require the supporting face to be "full" - carpet just needs to be above anything but air - lanterns need a center point on top (if place on top) or below (if placed below) -- for example a lantern can hang below a down-pointing hopper but can't be placed on top - redstone dust requires a full face below, but is also hardcoded to allow hoppers

The "rim" and "center point on top" are apparently well-defined in-game predicates; for example, the center point is a 2x2 center square on the hitbox (not visual box) assuming the surface is a 16x16 square grid. Quoting tryashtar on the "rim", "It's a predicate that is created specifically by removing the entire center section from a full cube, leaving only the, well, rim".

In references to other placement quirks (by tryashtar): Sea pickles just check if the below block has a non-empty upward face Not a block but item frames are really interesting, they can be placed anywhere that doesn't intersect with their hitbox (and support block material must be solid (or a repeater/comparator))

By the same person, on leaves' exceptional behavior: Leaves are considered to have faces that are "full" but not "sturdy"