Možda bi ti neko u "Art of Programming" bolje pomogao oko ovoga.
Da probam nešto onako laički, napravi novi monochrome bitmap iste rezolucije i na njemu preko GDI+ iscrtaj popunjenu elipsu. Onda iz originalnog bitmapa izvuci piksele čije su koordinate na monochromu popunjene, tako ćeš imati i izvučene piksele a i transparency map ako to negde treba da pastuješ

(nemojte se smejati, ovo je totalno laički

)
▪ The quicker a phone is answered in sales, the slower it's answered in customer services - Brownridge's Law
▪ Democracy is a device that ensures we shall be governed no better than we deserve - George Bernard Shaw
▪ To err is human--and to blame it on a computer is even more so - Robert Orben