Every reel carried a band of sky along its bottom edge. The room wall stopped a hundred units under the floor — enough to hide the coplanar seam it was sized for, and enough for 16:9, but a 9:16 reel sees far below the fighters' feet and looked straight past it into the clear layer. Three thousand units of buried wall costs the same quads and ends the gap: sampled along the reel, the bottom row now reads plaster and grass where it used to read (0,0,20) sky. Not the terrain skirt, which was the obvious suspect and the wrong one — tinting it magenta proved it never reaches the frame at this lens at all.