I'm going to have a go at working through generating a border region to try and show why I think it's better to blend, and how height and slope are your best pals ever. Getting blending to work requires a technique described in the main page on Region Generation.
The region I'm going to look at is where the West Weald rises into Elsweyr, in particular towards the market town of Sheeraln in the Dune foothills.
The effect I want is for the green of the west weald to dry out as you climb south into Elsweyr. I'm using an early set of assets from the Beyond Skyrim project so these will not be available if you're not working on that project but the principles apply anywhere.
Let's start with the West Weald. I really want a more coherent set of textures so this is just a first approximation.
I've set uGridsToLoad to 11 in skyrimeditor.ini - too high for normal use but helpful for screenshots.
This is the general area with nothing done to it
I'm looking south here - so all the region maps later on are effectively upside down. Sorry, but this is the area I want to work on and it's clearer from the north.
I create a region for the West Weald - naming in as logical an order as I can. This is very rough - I just picked a cell I want on the edge and selected an area. It's not easy transferring positions in the render window to the region editor so we'll just kludge it for now.
I've also created a region for Northern Anequina in Elsweyr and a rectangular generation region. I'll improve the layout of these soon, but it doesn't matter for now.
I've added a single grass texture into each region saved and generated the result.
Now you can see the very straight line where these two base textures meet. This is what will happen if you try to use the edge of defined regions to control the blending of landscape.
I can now see that I want the drier Anequina grass to come further much down the slope, and to show on the hills, so I'll move this north by about four cells. I also want the West Weald coming further south by about two cells.
I could show you the result of generating this - but it's just the same, because the first region in the list takes priority if nothing else is set.
Now, by dropping a barrel (or any object - I like barrels) onto the ground I've found that I really want the break between the two textures to happen at around z=-2100 so I'll set a maximum on the West Weald of -2200 and a minimum on the Anequina of -2000.
So I run the region generation again, and this is what we get:
So now the grassy West Weald pushes into Anequina along the valley between the hills, while the higher slopes are drier.
I can use child textures to make for better blending but first, I don't really like the way this cutoff line happens at exactly one particular level. Instead, I want to follow the bottom of the slope of the hill.
I'm going to bump the priority of the West Weald up to 51 and Anequina down to 49. This ensures the West Weald will take priority wherever it can. I think it is anyway but I'm not entirely sure how the engine decides between two regions of the same priority.
Now I'm limiting the maximum slope for the West Weald texture to 40. This may cause me problems elsewhere but I'm concentrating on this as a border. Maybe I'll create another region to use later. I've also set the density to 50. You don't want to do this for a base texture - and I'm doing it to show you why not!
Running the test region now results in this:
See how the dark green is part transparent over the brown in the edge cells? And the edges of the fully green cells show up as really ugly straight lines? Now that's why base textures always go in at 100% density!
Over to the left of the picture you can also see the mess made when I accidentally generated the West Weald region followed by the Anequina region - that gap was generated by the second pass. This is why we use dummy rectangles.
OK - there are still some problems. Several of the green border cells are solid right to a square edge then suddenly switch to brown. This is because we've reached the limit of the Anequina height range. Some of the brown is showing below that range because enough of the cell has been selected to be that colour to cause the engine to decide to make that the base texture for the quarter-cell. As soon as we cross to the next quarter cell the base texture has to switch to West Weald green.
We can stop this happening by allowing the Anequina texture to extend further down the slope. I'm resetting the minimum height to -4000 - just about the bottom of the hilly section.
The transition is still too sharp, but at least it flows around the contour of the land and looks like it's meant to be there.
I'll soften up that edge by using some of the same textures both sides of the transition.
See also: Region Generation