Thanx for that link: very interesting. Now I know it's supposed to deal with this problem I'll try a bit harder next time I see it. That image was done quite a while ago, so I guess enblend was missing from Debian at the time, or I had is disabled or something? It has been a while since I saw nasty artifacts so it's probably just working now. Like I say, every time I come back it seems to work noticeably better.