ref: cde4b93a27af60da423b3ed387de5f7a31ba32b7
dir: /sys/src/libmemdraw/alpha.hoc/
func f(x) { return x-x%1 } func pixel(dr, dg, db, da, sr, sg, sb, sa, m) { M = 255-f((sa*m)/255) print f((sr*m+dr*M)/255), " ", f((sg*m+dg*M)/255), " ", f((sb*m+db*M)/255), " ", f((sa*m+da*M)/255), "\n" return 0 }