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[mission0]

Thought I would start up a discussion about this since the wiki nor any recent articles seem to cover it.

Currently I want to know if a tile being inside or outside affects if a farming plot can be planted with underground crops.

From what I have experienced a farm plot that has been exposed to the outside but then covered up makes it unfarmable, however I want to know if this can be prevented. Some testing has proven that digging into an area, channeling down from above (while still underground) then closing up the hole then letting light hit the floor tile it makes the below tile light as well.

I'm not sure if a wall will block it though, or if some other mechanism will prevent light from shining down or reversing the process(reversal seems impossible without some sort of hacking tool).

[Fourdots]

If a tile is ever touched by light, it is forever contaminated and cannot be used for underground crops.

Pretty sure that light passes through constructions, though not certain about this. Further investigation is indeed necessary.

... hmm. From experiences with an aquifer in a freezing biome, when a new wall forms out of ice, the area below it reverts to below ground (which created some issues with getting the walls to freeze, so that I could try to get through the aquifer). Not certain about this, though, but pretty sure. Would magma and water have the same effect, I wonder? Or was it that the floors were being destroyed and then recreated with a wall above them, preventing light from reaching the tile?

[unit335]

Pretty sure that light passes through constructions, though not certain about this. Further investigation is indeed necessary.

Hmm. I might just try to investigate that if I have the time. Channel some loam, then build a roof over it and see if stuff still grows in it. My guess is, though, that the tile is going to have to be labeled both Dark and Subterranean at once to produce viable cave-crops. Maybe someone knows for sure.

[UnLimiTeD]

Currently have a fortress that could help in that investigation. 
Digged out an underground area, mostly soil, some stone, flodded it a little.
There were two tiles channeled out, the rest just digged.
It appears I either channeled a tile after setting down a farm or I built it with one tile light, but I have a farm plot of around 4x4 that can grow underground crops and above ground crops.
Not all of them, because the list doesn't scroll, but still.

If ground hits light, though, it seems to stay contaminated until new ground is formed.
The completely channeled out fields for underground-above-ground-plant-farming work for just that, above ground farming.

[pushy]

Pretty sure that light passes through constructions, though not certain about this. Further investigation is indeed necessary.

Yes, it does. If you build a wall with a ramp, then build a floor next to the (top of the) wall, and channel the rock/soil directly below the floor tile, it's considered to be "Inside/Light/Aboveground" and cannot be used for growing subterranean crops.

... hmm. From experiences with an aquifer in a freezing biome, when a new wall forms out of ice, the area below it reverts to below ground (which created some issues with getting the walls to freeze, so that I could try to get through the aquifer). Not certain about this, though, but pretty sure. Would magma and water have the same effect, I wonder? Or was it that the floors were being destroyed and then recreated with a wall above them, preventing light from reaching the tile?

Wiki says no, and also calls into question your first statement in that quote. When a tile of water freezes, that tile becomes an "ice wall" and the tile above becomes "ice" (technically, an ice floor). The "ice wall" is considered to be "inside/light/aboveground" so why the hell would any exposed tiles below the ice be any different?

Hmm. I might just try to investigate that if I have the time. Channel some loam, then build a roof over it and see if stuff still grows in it. My guess is, though, that the tile is going to have to be labeled both Dark and Subterranean at once to produce viable cave-crops. Maybe someone knows for sure.

Correct. What crops you can grow on your plot depend on whether it's subterranean or above ground. Inside/outside is not a factor. And before anyone asks, no, I can't remember the specifics for farms that are partly subterranean and partly above ground.

[expwnent]

If a tile is ever touched by light, it is forever contaminated and cannot be used for underground crops.

Pretty sure that light passes through constructions, though not certain about this. Further investigation is indeed necessary.

This is correct.

... hmm. From experiences with an aquifer in a freezing biome, when a new wall forms out of ice, the area below it reverts to below ground (which created some issues with getting the walls to freeze, so that I could try to get through the aquifer). Not certain about this, though, but pretty sure. Would magma and water have the same effect, I wonder? Or was it that the floors were being destroyed and then recreated with a wall above them, preventing light from reaching the tile?

I've tested this, and it is NOT true in version 0.31.03. Ice walls formed outside do NOT restore below-ground status for the tile containing the wall, or for any other tile. However, light does not pass through them, so they can prevent above-ground status.

Suppose z-level 0 is exposed to sunlight. If you used buckets/ponds to make an ice-wall spanning z-level 1, then channeled out part (or all) of z-level 0, then z-level -1 would stay underground, but z-levels 1 and 0 would be above-ground.

[mission0]

Ok but one thing I have seen proposed but not refuted yet is magma being thrown onto the channeled out tile then solidified, would this make the below tile dark once again?

[cdrcjsn]

It's also perfectly viable to plant above-ground crops underground...

[Fourdots]

... hmm. From experiences with an aquifer in a freezing biome, when a new wall forms out of ice, the area below it reverts to below ground (which created some issues with getting the walls to freeze, so that I could try to get through the aquifer). Not certain about this, though, but pretty sure. Would magma and water have the same effect, I wonder? Or was it that the floors were being destroyed and then recreated with a wall above them, preventing light from reaching the tile?

Wiki says no, and also calls into question your first statement in that quote. When a tile of water freezes, that tile becomes an "ice wall" and the tile above becomes "ice" (technically, an ice floor). The "ice wall" is considered to be "inside/light/aboveground" so why the hell would any exposed tiles below the ice be any different?

Ah, fair enough. I must have been confusing light/dark and aboveground/subterranean. Didn't put much effort into investigating/looking at it, and my memory tends to be quite bad. Although, for some reason most of the tiles above the water didn't turn into ice floors, leading me with the situation of having lots of tiles of open space with walls direct below, making it hard for the dwarfs to move around (though flooring over and removing the floor fixed that, leaving an ice floor behind) - which has happened once or twice in my current fortress, with magma and water (and a construction directly above, which wasn't the case in the one with ice). Not saying that the wiki is wrong, just that weird things do sometimes happen.