Normal tiles consist of a set of variant 72x72 tiles used for a given terrain type. The probability for showing the different variants are given in terrain-graphics.cfg. In addition each terrain type has a set of transition tiles:
- 6 basic transitions: n, ne, se, s, sw, nw
- 6 "2-side trans": n-ne, ne-se, se-s, s-sw, sw-nw, nw-n
- 6 "3-side trans": n-ne-se, ne-se-s, se-s-sw, s-sw-nw, sw-nw-n, nw-n-ne
- 6 "4-side trans": n-ne-se-s, ne-se-s-sw, se-s-sw-nw, s-sw-nw-nw, sw-nw-n-ne, nw-n-ne-se
Few terrain types use all these possible transitions, if any.
It is possible to make several transitions and use different ones depending on which terrain type is adjacent. There is also a need for special transitions for terrains adjacent to castles.
Transitions are drawn around tiles according to the definitions in terrain-graphics.cfg. Each terrain is placed in a given layer. This defines which terrain will draw a transition when adjacent to another. Terrains in the "higher" layers will draw their transitions on top of terrain layered further down.
Each transition is stored in a separate image file.
1 tile + the 6 basic transitions:
1 tile + 2 x nw trans and a sw-nw-n trans. (This may seem strange, but the transitions are named after where the image is drawn in the transition tile itself, not where it's "attached" to the terrain tile).
Multihex tiles work differently. They don't have separate transitions. One could say the transitions are attached to the tile itself. They also behave differently. While normal tiles draw their transitions according to layering, a multihex tile will overlap terrain either vertically and/or horizontally. This means, for example, that mountain tops will extend into tiles above, removing the need for a special transition. For other sorts of terrain like grassland (and other flat ones) this must be done in a way that will allow them to mesh into each other. Of course multihex is not necessarily ideal for all terrain types.
Note on overlay tiles: define a background tile which gets drawn first underneath the overlay using the regular layering rules for that particular terrain type.