Currently Threadbare does not have any real concept of height. Apparently-elevated sections of levels like this:

are in fact flat tiles that have collision shapes that prevent walking through them:

(Actually we can see from the collision debug info that the middle 2 tiles are the wrong ones. They should have collision shapes only at the edges.)

Periodically we run into the limitations of doing this. For some examples:

• @manuq tested whether it is possible to grapple up or down a cliff. No – Prototype: Grapple to climb #1904 – and without having a third dimension it is not possible to both grapple up/down cliffs, and also have puzzles which require hooking a needle to get round a cliff corner.
  • This also means we can only grapple across water if the two sides are at ground level. If the two sides have cliff edges between them: no dice.
• If ink-drinkers are on top of a cliff, they have to be very carefully positioned so that the spawn point for their projectiles is off the cliff. Otherwise the projectile gets stuck on top of the cliff. Occasionally you can see this in some of the levels in the game.
  • Early on there were some attempts to fix this - Make projectile collisions with walls be one way #379 / Projectiles: avoid collisions with walls if they spawn in an elevated terrain #364 - which we decided not to pursue because they added too much complexity.
• You cannot walk behind raised areas. In the screenshots above, you should conceptually be able to walk behind the top row of cliff tiles, but you cannot.
• It is not possible to have a bridge that you can both walk under, and walk across:
  image
  because:
  • Visually, the bridge has to be on a higher z-index for the player to walk under it, but once you have done this (and adjusted the cliffs and other level-1 tiles accordingly) the player is now underneath them.
    image
  • Ignoring the visuals, there is nothing in the physics to stop the player walking down under the bridge, then right to walk off the right edge of the bridge
• And surely other problems not yet enumerated here

These problems are not insurmountable. One approach people take is to have different physics layers for walls at different elevations; then they move the player in and out of those layers as they move up and down. Or you can enable and disable collisions on different tilesets based on the player's layer.

Any approach to adding/faking a third dimension in a 2D game adds complexity to the code & more importantly complexity for the level designer. So far we have chosen the path of less complexity, sacrificing the things above.

The question to discuss is, is this the right trade-off? (One example: Minit, which has a couple of special-cases for ladders but otherwise has the same simplification: the world is flat, even if it looks like it's not.)