I just came across the following snippet in an old Sinclair User magazine (issue 20):
Genuine hi-res in space invasion
ODYSSEY Computing has achieved the remarkable feat of producing, with no extra hardware, a genuine high-resolution version of Space Invaders for the 16K ZX-81. Invaders offers superior graphics and allows one or two people to play and provides a choice of missile-firing and bomb-dropping rates. The result is an absorbing game which should satisfy trigger-happy space fighters. Invaders is from Odyssey Computing, Notts. It costs £4.95.
Does anyone recall this game at all (I know I don't) and do we know how they did the hi-res graphics? I can't find it on the web anywhere.
