How the cutout actually works — colour matting, edge cases, and the parts that neural nets get wrong. Every claim here comes from measured behaviour, not benchmarks we invented.
Segmentation models answer a different question than matting does. On flat-colour art the difference shows up as eaten outlines, a 1–2px shrink, and edges that are either fully opaque or fully gone.
A drop shadow is background colour multiplied down, which is why most cutters delete it with the background. Here is the luminance model that keeps it, and where that model stops working.
Anti-aliased edges are a mix of foreground and background colour. Keep them and you get a green halo; delete them and the outline gets thinner. Unmixing is the only way out.