README: describe the output outside the refiner band precisely

README.md

@@ -60,8 +60,9 @@ Its receptive field is ±9 pixels. Per native pixel it receives:
 | 10 | `a_gf`: alpha of a colour guided filter driven by CorridorKey's low-resolution alpha |
 
 It outputs residuals on top of `a_gf` (alpha) and `F` (foreground). The result is used only inside an
-uncertainty band around CorridorKey's edges, so interiors and clean background stay exactly as CorridorKey keyed
-them. The band is weighted by nearby foreground, which keeps the refiner out of dark or noisy screen areas.
+uncertainty band around CorridorKey's edges. Outside the band the output is the colour-guided upsampling of
+CorridorKey's matte, which in solid foreground and clean background is CorridorKey's own 0 or 1. The band is weighted
+by nearby foreground, which keeps the refiner out of dark or noisy screen areas.
 
 In the real-time engine the refiner only runs on 32×32 tiles that intersect the band, with a fixed tile budget
 (most uncertain tiles first). In fp16 that costs about 5–6 ms per frame with 256 tiles and 10–11 ms with 512 tiles