
Japanese: 
答えは透明性です　他の3つは1つの面の
照明の値を決定し それが色に反映されます
透明性は まず不透明なマテリアルに色を描き出し
その色を透明性フィルタが処理するピクセルに
混ぜ合わせて新しい色を貼りつけます
でもベタな色に対して陰影のあるピクセルを混ぜても
うまくいくとは思えません
セルシェーディングにおいては画像処理の方が
1つの値に最終的な影の色を与えるだけなので
やりやすいことがよくあります

English: 
Transparency is the main point of failure. The other methods changed the
illumination on a single surface. The final value is then mapped in some
fashion to a color. With transparency, you're first mapping the opaque material
to a color then you're blending this color with whatever transparent filter is
covering a pixel, then mapping that result to some new color. It's mixing
results from shaded pixels with solid colors which is unlikely to work well.
We'll see in a later lesson that image processing is often a better way to
perform cel shading, as this maps from some final shaded color to a solid value
just once.
