Create a way to handle multiple shared primary tilesets between a single secondary tileset

[grunt-lucas]

I am thinking of a case where a secondary metatile, on say the bottom layer, references a fixed tile/rotation/palette in the primary set. And depending on which primary set is in place, that tile might be grey stone, or brown stone, or whatever. So you could have a secondary set called e.g. SecondaryCaveDetails that has e.g. a sign where the background tile of the sign changes depending on if it's paired with PrimaryGreyCave or PrimaryBrownCave, just as an e.g.

This provides a really cool way for users to have a lot of primary tileset variety without having to create tons of duplicate secondary sets. It can be used for more than just caves. E.g. you could do seasons with it too. You could have e.g. PrimaryGeneralSummer and PrimaryGeneralWinter, and then all your secondary tilesets would automatically index in to the correct place, no matter if the game is using the summer or winter primary set.

Josh mentioned that this is done in FireRed for some of the cave tilesets.

At this time I haven't thought about how Porytiles could support this. Since you're expected to put RGBA art on your secondary sheets, it won't work out of the box. Porytiles currently compiles your paired primary and then tries to match up the secondary RGBA art with content from the primary. If primary content is variable, this doesn't really work.