3. Paired portals must either be both flipped, or neither flipped, much like teleporter/receiver pairs. If the pair is flipped, the lemming reverses direction when exiting the receiving portal. This is my preferred approach.
+1 for suggestion #3. If it's established for teleporters, it makes sense to keep things the same for portals. Also reduces potential headaches of trying to keep up with which are flipped and which aren't (which suggestion #2 could cause), particularly if the graphics have no left/right-facing indications or characteristics.
It's also consistent with the flipping of other direction-related objects, such as entrance hatches, teleporters, OWW, OWF, etc. Ideally, it follows that there should be some actual change in lemming behaviour when flipping occurs.
Suggestion #1 feels somewhat redundant, although would keep the option to at least flip the graphic without otherwise affecting object behaviour, if this is what is ultimately decided. This feels like it would be similar to flipping a flamethrower graphic, which is sometimes done for aesthetic purposes.
why do we need to flip them?
Good question. I'm thinking that level designers may wish to have the lemming facing a certain direction once they've exited the teleporter/portal. Having it as a characteristic of the object itself reduces the need for messy design.
Therefore, I would throw in the possibility of "prohibiting flipping portals altogether". As why do we need to flip an object that has no visual features that signal the flip?
Strongly opposed to this, for these reasons:
a) All other objects can be flipped, even if only aesthetically. Suggestion #1 has this covered if it's decided that there should be no change in lemming direction upon interaction with the object, so there's no need to outright prevent flipping altogether.
b) Style creators may in fact design their portals to include such visual features/indications, even if default ones don't. If the concern is that designers might include misleading features which suggest direction, this could happen with or without flipping behaviour.
Otherwise I would tend to 3 as well, but here we would need a visual clue that the vortex is indeed flipped just like in the case of teleporters.
Agreed
This could be done with something as simple as arrows, for example.
It doesn't even have to be that clinical, though. A portal designed with a more ethereal effect like a vortex, such as this one below, could be said to be facing "to the right", since the direction of its swirl is right-facing:
Here's what the same portal would look like when facing "to the left":