What: With 3D models and interblock culling enabled, certain configurations of iron bars/any sort of glass pane can cause the tops/bottoms of above/below matching blocks (different states) to have an exposed face culled (which is incorrect).

Why it matters: By having issues such as this, it makes attempting to optimize models less appealing, a little bit more frustrating, and at the very least might make pack makers remove interblock culling from iron bars/glass panes to prevent this issue, causing more of a performance drain than needed, especially in maps/saves that use tons of those blocks.

Note: bars/panes are the few "transparent" blocks that can cause interblock culling on itself. In reeds_not_culling.png you can see a 3D sugarcane model (with top/bottom interblock culling enabled) not culling the top/bottom of sugarcane when touching other sugarcane. Carl Nathans has noted (and I can confirm) that repeaters/comparators cull the top/bottom face if any cullface is specified (even if that case isn't true) and the sides (which do have culling arguments on the default models!) can't be culled at all.

Simple fix: Make it so certain models of iron bars/panes (n, ne, nse) do not cull faces of their other states.

Advanced fix: Remove all hardcoded cull restrictions/quirkiness and allow all blocks to have culled faces if they are specified by the model and are touching either 1)an opaque block or 2)the same state of themself. This is however, still a primitive fix, because there are cases where different states of blocks obscure faces. Examples possibly being the sides of rotated repeaters/comparators, corner+line panes/bars, or even something simple as a repeater being on and another being off.

A full separate cull control system would be fairly useful. For instance, on bars/panes you could make the sides cull as they do now (no limitations on state, since it doesn't cause issues there), but then the tops/bottoms have the state restriction. You also could make them only culled by themself (not by opaque blocks) which would allow a border around all of themself, including over opaque blocks (like they do now, but culled in the middle).

It would also help in the case, as I have done with farmland, where an artist switches a block to render as a full cube. Currently if you do this, the rendering is treated the same (in this case, farmland won't cause other blocks to be culled since it is still a transparent block) even though this could be taken advantage of for better performance. Similarly, in default the enchanting table (and repeaters/comparators, half slabs, and stairs) has a full bottom (or top/sides for the orientables) but isn't an opaque block... so it does obscure faces so it could cull those faces, but does not. These are (and should be) changeable, but optimizing these things should be a possibility.