I thought that it was likely that the top of the cube was being illuminated by the lights below, but it looks like I was wrong, and that YAL does in fact check the angle of incidence for light rays hitting the surfaces and attentuates the light properly. So a lumel that's off a surface should still be facing away from the light and be dark, even if there's really nothing between it and the light.
So that can't be the issue.
The only remaining possiblity is bilinear filtering... And looking at the lightmap that is generated, I think I can say that this is our culprit. I'm pretty sure the top of the cube is being put in the top right corner, which means that on each side of it sit the much larger light grey lightmaps. (YAL compresses lightmaps with low contrast more than others)
Unfortunately it looks like YAL must stick it's UV's right up against the map edges. So we can't simply draw extra pixels around it. We'll have to find a way to get it to make the polygons smaller within the lightmaps.
So that can't be the issue.
The only remaining possiblity is bilinear filtering... And looking at the lightmap that is generated, I think I can say that this is our culprit. I'm pretty sure the top of the cube is being put in the top right corner, which means that on each side of it sit the much larger light grey lightmaps. (YAL compresses lightmaps with low contrast more than others)
Unfortunately it looks like YAL must stick it's UV's right up against the map edges. So we can't simply draw extra pixels around it. We'll have to find a way to get it to make the polygons smaller within the lightmaps.