unfortunately I cannot give a simple answer as in MapBuilder I use different configurations for single skin + detail map terrains, and for multitexturing where blending is stored in the alpha channel. if you are patient enough and want to read a lot, it is within MBteredit_tx.h/.c and MBterrhmp.h/.c. later I'm planning to upload it to GitHub, it would be easier to discuss.

so it is okay now, this mipmap thingie at least fixed some other issues and I can change my shaders to use square atlas maps. currently I'm mostly using an autotexturing shader, which is much better for using with many levels than to save skins to every hmp-s (and I frequently use the same hmp for many levels beside a deformation info file), thus imo it is better if you don't invest too much time into my problem at the moment. and I'm not sure how much I will work with 3dgs in future... the dll basically works fine beside this limitation.