Seeing as I am packing two tilesets into one, I don't know what parts to name the surround currently. But here is how it works...
Lets say you pick a 16x16 square map. Normally you have tiles around the edge which fade out to gray, and I think (correct me if I am wrong) they repeat 3 tiles past the boundary tile. You have ONE, just ONE option for a tile that goes in that place off-map. That means that for the distance of three tiles, you have exactly the same three tiles, and can ONLY have straight lines going away from a rise at the edge of a map.
What I do is offer an edge tile which is placeable at not just the boundary region. The tile is fully, or mostly, unwalkable, and is intended to not be seen past. This surround tile builds a more unique background off the edge of the map, and depending on your needs, could be many decimeters off the map. Because you can use the surround terrain anywhere adjacent to the required tile type, you can create edge boundaries mid-map if you want.
Really this is no different than what they do in the OC forest tileset with the stone wall, except in that I have one or two edges of the tile let loose into tilespace behind them.
What you can then do with the surround tile is make a "wall" or "hole" which the player cannot cross, which also serves as the edge boundary of the map, but may come with distant model parts which can be viewed from the location of the tile. Placeables at a distance fade away completely, basically turning off. On-tile parts do not. So, imagine if you will, a mountain peak matching the mountain terrain color and shape can be added to the surround tile, but be 100 meters in the background, and outside the walkable area of the map. It will stay visible at any distance and remain above the fog collection point on the horizon. Depending on the distance, it can appear rotated toward another point in the sky to appear as though it is further around the bend of the earth.
Basically this makes your surrounding edge far more unique, but doesn't add the clutter of placeables. And because it is a placeable tile on the map, you can make groups of them which perform a certain function, such as the mountain I mentioned. Or an erupting volcano in the background, without having to make an animated skybox. Or place a known castle in the background at the proper distance so it moves in relation to the user's position, not with the sky point.
My initial intent was to simply break the monotony of the gray space outside the area border, but it went beyond that intent.
For the granite lands tileset, I didn't initially know I was going to have basically two tilesets in one. Right now, I have a bottom up tileset, where you start with rolling grass and move up granite bluffs. The other is the top of granite summits down into 25m chasms, in which you can see various terrains and crossers below. With the ground up approach, I assumed there would be a majority surround which was mountain, but that graying edge mountains repeating into the distance would look off, so I intended to have a group which placed a huge granite mountain, where the majority of the surround would simply be unreachable heights of granite. I thought to also make them more fantasic in shape than lower elevation granites.
Unlike the OC forest tileset mountain edge, in the granitelands you can actually reach the top of the rise. So I need another type of high surrounding tile which does not match the base rise, otherwise all you would see is endless flat top granite.
So with the top down subset within this tileset, the 25m chasm may be sufficient. I don't intend to have the chasm area be walkable, so that will save on processor time calculating walkmesh details. But I do intend for the user to place various crossers within the chasm terrain, so that rivers and wooded areas can be seen below, not just naked grass or zero black chasm. In this situation, the distance the user is from the actual area edge should allow me to make use of OC style edge tiles, simply making a common chasm edge tile and calling it good enough.
I had originally did this with my redwood tileset which none of you ever got a chance to download. It was set similar to the OC forest, except that you could have a thick forest as an alternate edge to the mountain. In the space of 30m off the surround tile, I was able to blend a blocking wall of distant trees with actual mesh-built trees, and I was also able to more easily meld that blocking tree wall with a blocking mountain wall, much further out of visible range of the player.
Above image shows appoximate 2.5 tile width edge tile, with 0.5 tile width walkable region. The back curtain wall blends with the mesh trees of the same bark type, and is designed to match most fog colors by using a gray fade texture to represent the distant background. Multiple varities of the eedge replacement tile allows for far more variety than in the OC.
Above image shows 3.5 tile depth path leaving the forest, as well as 2.5 tile depth mountain-forest joining right or left on the tile, while making use of the deeper forest edge depiction.
Forest "surround" in action using short range fog threshold.
This deeper edge also allowed me to more flesh out trails going off the edge of the map. Instead of the rural set coming to an abrupt black hole in the trees with a stupid sign post, what you had was a three tile system which faded into the fog, rather than instantly to gray, and most importantly, was not confined to a repeating straight line of tiles. Because the first tile is placed on the usuable map, more of the area was available for interaction, so I could put a type of door over the first portion of exiting tile, but instead of just a blue rectangle over a black hole in the trees, I could have a toll gate that you could stand at, with appropriate guards, and still see off into the distance multiple tiles away.
In the past set, I had a much lower fog threshold, so I was able to do all this with just the three tiles distance. I am now using a 120m threshold in my areas, so that requires more distant artwork in many cases. However, since I am working with monumental heights, all I have to do is portray more mountainy stuff in the distance and the fog can be used to cancel out what would not be able to be viewed anyway. You'll never find your character on top of one of my surround tiles, so I know exactly the angles the character can see to from points within the surround curtain, so I know what they will be able to see and what I can simply omit.
