I'm working on a game that preferibly should feature a combination of designed and procedually generated terrain where the designer specifies in somewhat detailed terms what type of terrain a given area will have (grasslands, forest etc...) and then a precedual algorithm takes care of the rest. I'm not talking about minecraft style biomoes, but rather the game map for a strategy game.

Each 'area' will not take up that much of the screen, and thus be more akin to a tile whose texture is procedually generated. While procedually generating terrain textures on the GPU are not that difficult, the hard part is making the borders between them look good. Currently, the 'tiles' are large enough to be visible (due to memory constraints mainly, we are talking planetary sized textures for a game taking place in space and on a continental ground view with seamless transitions between them) and creating good borders between them with an algorithm that is fast enough to be useful has proven difficult. Sampling the n-surrounding pixels and using the combiened result did not yield very good borders and was fairly slow on the GPU to boot (ca 12ms for me, that is without any lighning or shading and with very simple terrain texture shaders).

So are there any practical known methods to solve this problem?