[Award Winner] Technique for adding coastal detail as you zoom in.
I have been playing with a few techniques in GIMP (applicable to any raster program) to allow me to zoom in and "refractalize" a coastline without significantly changing the shape...
This technique is designed to work with a coastal outline (white land on black water) and will not directly translate to working with heightfields....
1) Make a rectangular selection of the desired area, and copy it to a new image.
2) Enlarge the new image to the desired size
3) Blur it enough to get rid of the jaggies, but still maintaining some detail. I sued ~20 px in the examples. The bigger the blur, the more randomization will occur, and the less "true" the new coastline will be.
3) Create a new layer on top and set its mode to burn
4) Render clouds using the max detail and check turbulent. (I don't know what the photoshop equivalent of this is).
5) Invert (optional). Depends how it looks :)
6) repeat 4 and optionally 5 with different seeds until you get one you like. some just seem to work better than others. I kept a copy of the starting image open for reference to try and ensure coarse features were maintained.
7) Flatten the image (or just merge down the clouds layer).
8) Threshold with black at 1 or very low, again, maximizing detail while keeping the shape close to the original.
Here is a set of examples, with each showing the next one's selection. I started with a nice spherical mapped noise layer: