Sinking the extra islands

In my last post I mentioned an issue with excessive islands appearing around coastlines. Happily, it's proven to be a very easy problem to fix (at least I think so...), but I thought it worth explaining.

Here's an example of the problem:

In itself there's nothing wrong with this, but it's a problem when all coastlines look like this - and as you can see in the last post, they do. But why? If you look through earlier posts on this blog you'll see mostly much less busy coasts. Somewhere along the line, I've made some kind of change to tackle some other problem, and it's had the unexpected side effect of replicating the Hebrides everywhere. Which is fine on occasion, but we don't want it everywhere. It's another illustration of the difficulty of making changes to what is now such a complicated piece of code, as it has so many moving parts that a small tweak in one place can have unlooked-for ramifications elsewhere.

How are coastlines made, anyway? Well, the terrain at the regional level is basically done by seeding the corners of the tiles with values taken from the corresponding cells at the global level, and then applying the diamond-square algorithm to fill in all the rest. Where terrain higher than sea level meets terrain that's lower, we get coasts. There are then endless tweaks to make them look better, but the diamond-square is at the heart of the process.

I think that what's happened has something to do with the changes I made a while ago to the sea bed, especially the addition of continental shelves. This means shallower seas near the coasts on the global map. Shallower seas mean more islands on the regional map, because the diamond-algorithm basically makes the terrain bumpy, and if the terrain is near the surface of the water to start with, lots of those bumps will stick out of it. (This is especially so given that the algorithm is set to tend to bump land up, in order to emphasise river valleys.) The solution, then, is simple. I add to this diamond-square algorithm a check to see whether a point at the corners or on the side of the tile is below sea level. If it is, we make it a lot lower. As a result, the algorithm tends to produce lower land around the sea, and far fewer islands are produced.

But doesn't this result in undesirably deep seas around the coasts? No, because the undersea terrain that this algorithm produces isn't actually used for the final map. Instead, there's another diamond-square-based function that creates undersea terrain. UW calls the first function first, and works out what's land and what's sea, and the heightmap for the land. It then calls another function to create seabed, and then pastes that onto the sea regions of the original heightmap. This allows me to make changes to the land terrain generation without messing up the seabed, and vice versa. So in this case, we can artificially lower the seabed in the way described above during the initial terrain generation, and it won't affect the final seabed - but it will affect the coastlines.

Here's what the above map looks like after doing this:

This looks much more plausible. Some islands still remain, and of course we don't want to get rid of them entirely. I've set it so that the island-reducing tweak varies in strength. It's weaker in areas that have higher roughness, and much weaker in arctic-type areas where one might expect fjords. So crinkly little islands are more likely in such areas. As always, variety helps to make the maps more believable, at least in theory.

(Also, notice how in the first picture there is one river that doesn't reach the coast. I think this is because the land has been extended out into tiles that correspond to ocean cells on the global map, and the river simply doesn't reach that far. There are functions that are supposed to deal with such cases by extending the rivers out to the coast if need be, but they seem to have failed here. In any case, you can see in the second picture that the coast has retreated inland and the river now reaches the sea properly, so that's a bonus from this tweak.)