# Convex hull

3 views (last 30 days)
Raghuram on 10 Mar 2011
Hello, I have two vectors, x, and y:
x = [0.3333 0.5000 0.6667 0.7000]
x =
0.3333 0.5000 0.6667 0.7000
>> y = [0.6667 0.5000 0.3333 0.7000]
y =
0.6667 0.5000 0.3333 0.7000
>> convhull(x,y)
ans =
1
3
4
1
>> I know that (0.5 0.5) should theoretically be in the hull. If I try this, it works: x = single(x)
x =
0.3333 0.5000 0.6667 0.7000
>> y = single(y)
y =
0.6667 0.5000 0.3333 0.7000
>> x = double(x)
x =
0.3333 0.5000 0.6667 0.7000
>> y = double(y)
y =
0.6667 0.5000 0.3333 0.7000
>> convhull(x,y)
ans =
1
2
3
4
1
Now notice point 2 appears. So two questions: 1) Why does switching to single and coming back to double work? 2) Why does (0.5,0.5) not appear initially as part of the hull? Thank you, Raghu

Jan on 10 Mar 2011
The point (0.5, 0.5) is almost an element of the edge. Rounding to SINGLE and increasing the precision back to DOUBLE can move all points a little bit. Remember that 0.6667 is not 2*0.3333 and that floating point numbers in decimal notation do not have necessarily an exactly equal representation in binary notation, which are used for computations.