あ、デバッグしたときの無駄なのが残ってる・・・訂正。恥ずかしい。

#define ctype GetY(%1, %2, %3) ((299*%1)+(587*%2)+(114*%1)/1000)

dialog "jpg;*.bmp;*.mag", 16, "画像ファイル"
if stat = 0: end
screen 2: picload refstr: mref vram2, 66
screen 0, winx, winy: mref vram1, 66
repeat winx*winy
peek r, vram2, vpos+2: peek g, vram2, vpos+1: peek b, vram2, vpos
y = GetY(r, g, b)
poke vram1, vpos+2, y: poke vram1, vpos+1, y: poke vram1, vpos, y
vpos += 3
await
loop
redraw
stop