BTW: I have used self-implemented high-precision longint to solve this problem.

1000 0 0 -1000 0 0 1000 1000
VISIBLE

1000 -0.000001 0 -1000 0 0 1000 1000
INVISIBLE

1000 0 -0.000001 -1000 0 0 1000 1000
INVISIBLE

1000 0 0.000001 -1000 0 0 1000 1000
VISIBLE

1000 -0.000001 0.000001 -1000 0 0 1000 1000
VISIBLE

1000 -1000 999.999999 -1000 0 0 1000 1000
INVISIBLE

1000 -1000 999.999999 -999.999999 0 0 1000 1000
VISIBLE

-999.999999 -1000 1000 999.999999 0 0 1000 1000
VISIBLE

1 2 1 0 0 0 0 1
VISIBLE

1.000001 2 1 0 0 0 0 1
VISIBLE

0.999999 2 1 0 0 0 0 1
INVISIBLE

1000 0 1000 -0.000001 0 0 0.000001 1000
VISIBLE

1000 0 1000 -0.000002 0 0 0.000001 1000
VISIBLE

1000 0 1000 -0.000003 0 0 0.000001 1000
INVISIBLE

Thank you

Thank you so much for sharing this.
For bar code issue, you also can refer:
http://www.businessrefinery.com/products/barcode_net/main.html
http://www.businessrefinery.com/products/barcode_reader_net/main.html

2 more test which help me get AC, I fails on checking that both points on 1 side of line.

40 60 59 40 50 30 90 70
VISIBLE

40 60 61 40 50 30 90 70
INVISIBLE