- Edited
Currently:
The transition between two lod levels is done continuously, and if there isn't enough depth available then cracks show up. This is either because the visibility distance is too small, or because there are not enough raster posts available to perform the transition.
Take a look at the screen shot, you can see a lod level transition right in the front where two grid lines from the lowest level merge to the next highest. This goes over >10 posts, enough for a smooth, gradual transition, on a patch of terrain of 4096 by 4096. That was an old version.
Extreme case, assume a raster size of 8 by 8 and three lod levels. No spacial data structure and selection algorithm needed for such a simple thing. Let's say for simplicity the raster size coincides with world distance units, so each post is a meter from the next. Of course in the end these will be distinct things, with conversions in between, trivial on a flat world, challenging on a sphere or even ellipsoid. But back to the example. It is easy to show that 3 lod levels will have a hard time.
The first (highest, the most far away) level would go to a little less than a half of the distance (4 posts), the second (nearer) to a little less than a third. Even if we allow the whole depth of a level to be used for continuous lodding, between the second and the third (closest) there'd only be 1 post or two or even less to perform the actual lodding, which results in cracks in some cases. We could (and actually do) apply a multiplier for the grid, to insert posts linearly between two adjacent posts, but that doesn't solve the problem, and looks strange on a very structured terrain.
Solutions coming up :-)
In the end, world distance units and raster posts are separate things. In the first version one can set a linear factor in x and z to scale the raster to world distances. Like, you have a post every meter, or every ten meters, or so. Configuration must be so that the lodding can actually be performed, or at least a warning is shown when to many levels are being squeezed in too little depth of view. Eventually, camera view distance must be separated from lodding distance, though the lodding is performed based on camera view distance alone. So that sounds like a contradiction.
I will certainly have to write a huge help document for this because there's more.