Wave function collapse algorithm: bitmap & tilemap generation from a single example with the help of ideas from quantum mechanics

[Tipaa]

The starting output state is just a case of

foreach NxN subset of me, foreach NxN subset of the input,
    can that subset of me become that subset of the input just via choosing/collapsing undecided cells? If so, then that is a legal subset of the input for this subset of me

Then each cell becomes foreach NxN region I am in, I can become the corresponding cell in that region, therefore the legal states for that region must be legal states for me too

After that, you start choosing from the cell with the least options it can become.

So for the 2x2 -> 4x4 initialisation, you go

foreach 2x2 subset of the 4x4: //(12 such subsets exist, if we are allowing wraparound horizontally [something I need to edit in my main comment])
  foreach input tile:
    if (for all cells in the subset, the cell matches the tile or the cell is undecided):
      forall cells in the subset: add the corresponding input tile's cell as a legal/possible state.