A practical advice how to do a realistic coast.
1) First you have to know that a coast line is fractal. This has a huge advantage that you don't have to worry about resolution because a fractal is self similar at all scales. However far you zoom at it, you see a similar fractal shape.
2) The corollary is that because everything is fractal all the way down, you can start at a rather high resolution. For example if your map is 100x100 (any unit), zoom so that your window is 1x1. Choose the typical fractal size around 0.1.
3) At this 1x1 scale draw a fractal all around your continents and islands. Best way to do so is to follow the general (irregular !) outline that you hand Drew at 100x100. Don't worry about realismus at this stage. You will see that when you'll unzoom back to 100x100 many fine structures will disappear anyway but it wil still look fractal.
5) Conclusion : if your coast was created by a fractal at a small scale, then it looks realistic at all scales provided your initial handrawn outline had already an irregular look.
A perfectionnist would start with a fractal at 100x100 (typical size 10) , then superpose another at 10x10 (typical size 1) and finish with the last at 1x1 (typical size 0.1). That would be very realistic but it would take more time. This latter approach could be done with something like GIMP :
- you make a black fractal at 100x100 with a large typical size
- you go to 10x10 and draw a finer red fractal following the black fractal line. When finished, erase the black and keep red.
- you go to 1x1 and draw a black fractal along the red line. When finished erase the red and keep the last black. Your coast is done.
4) Now it's time to take in account geology. Places with mountains and cliffs will have deep fractal structures (fjords). You redo the fractal along a very jagged (hand drawn) coastline. Big river mouths and deserts are much smoother. Replace the initial fractal by smooth shapes (basically shallow elliptic arcs or even hand drawn smooth curves are good). Everywhere else you leave the fractal coast as is.
That's all there is to it.
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