Currently, C++ Lix features decoration as a non-solid gadgets that possibly animates and has no function. If it's not animated, then it should be terrain (otherwise it's misleading, like when the shovel in Matt's goldmine tile set was still decoration). If it is animated, it still misleads the player as everything that moves tends to be important or dangerous. Decoration is always drawn behind terrain.
The only good use case of decoration is L1 legacy support. But L1 multi-part exits should be a single exit objects (possible to hard-code which deco goes with which exit).
What seems more useful to level-designers who wish to decorate their levels would be something that draws on terrain without affecting solidity. In particular, this would be useful with alpha blending. We've had a discussion about alpha-blending [1] before and it is a tricky issue if it is allowed for any kind of terrain pieces as it is unclear what should become solid/non-solid/steel. Instead, I propose a new type of decoration. Given a decoration image (with alpha channel), it would be drawn (with alpha blending) on any terrain/steel below without affecting its solidity/steel-ness, while it would not be drawn on non-solid terrain.
Example usage:
Green jade (from the sandstone tile set) could have its shading using alpha thus allowing it to be placed on any surface, not just sandstone colored ones.
Any kinds of shading/depth maps, like hieroglyphs that could be placed on stuff.
Decay such as moss or rust (caveat: should have a visually distinctive style from terrain which has a different effect when used on steel [2]).
Smoothen seams between terrain pieces.
Decorational stuff should also be allowed to be grouped together with terrain. This could even allow for convenient mix-and-match of textures and shading (shading for blocks, balls, pillars, ...).
Do gadgets already allow for alpha blending in the D port?
[1] Alpha blending:
http://www.lemmingsforums.net/index.php?topic=2420.msg55108#msg55108[2] Terrain on steel:
http://www.lemmingsforums.net/index.php?topic=2557.0