SURF comparison code giving issues - computer-vision

Question

I am doing SURF comparison to identify objects in images by calculating euclidean distances between the desriptors. but the following code isnt working. IPoint is a SURF feature point, Any help apreciated.
List<IPoint> ipts = new List<IPoint>();
Dictionary<string, List<IPoint>> objs = new Dictionary<string, List<IPoint>>();
double distance(IPoint a, IPoint b)
{
double dis = 0;
for (int i = 0; i < 64; i++)
{
dis += Math.Sqrt(Math.Pow((a.descriptor[i] - b.descriptor[i]), 2));
}
return (dis);
}
bool matchpoint(IPoint a, List<IPoint> l, out string e)
{
e = "";
double smallest = double.MaxValue;
string s = string.Empty;
for (int i = 0; i < l.Count; i++)
{
var d = distance(a, l[i]);
if (d < smallest)
{
smallest = d;
s = i.ToString();
}
}
if (smallest < 0.5)
{
e = s;
return true;
}
else
{
return false;// null;
}
return false;
}
string match(out double per)
{
string h;
Dictionary<string, double> torn = new Dictionary<string, double>();
foreach (string s in objs.Keys.ToList())
{
int count = 0;
for (int i = 0; i < objs[s].Count; i++)
{
if (matchpoint(objs[s][i], ipts,out h))
{
count++;
}
}
torn[s] = count / objs[s].Count;
count = 0;
}
string smalln = "";
double smallest = double.MaxValue;
foreach (string s in torn.Keys.ToList())
{
if (torn[s] < smallest)
{
smallest = torn[s];
smalln = s;
}
}
per = smallest;
return smalln;
}
private void button1_Click(object sender, EventArgs e)
{
double d;
match(out d);
MessageBox.Show(match(out d) + " " + d.ToString());
}

Answer 1

Should be:
double distance(IPoint a, IPoint b)
{
double dis = 0;
for (int i = 0; i < 64; i++)
{
dis += Math.Pow((a.descriptor[i] - b.descriptor[i]), 2);
}
return Math.Sqrt(dis);
}
You are squaring and then taking the root of each difference, which is basically doing an absolute value. Try to remember the simple example of Pythagoras: C=SQRT(A*A+B*B) and not C=SQRT(A*A)+SQRT(B*B)

Answer 2

Thanks for your help, also taking the ratio of the two distances works perfectly. I'm posting working code here because you cant find the answer to this question anywhere else.
void getMatches(List<IPoint> ipts1, List<IPoint> ipts2,out List<IPoint> mats)
{
List<IPoint> matches = new List<IPoint>();
float dist, d1, d2;
IPoint match;
matches.Clear();
for(int i = 0; i < ipts1.Count; i++)
{
d1 = d2 = float.MaxValue;
for(int j = 0; j < ipts2.Count; j++)
{
dist = (float)distance(ipts1[i], ipts2[j]);//ipts1[i] - ipts2[j];
if(dist<d1) // if this feature matches better than current best
{
d2 = d1;
d1 = dist;
match = ipts2[j];
}
else if(dist<d2) // this feature matches better than second best
{
d2 = dist;
}
}
// If match has a d1:d2 ratio < 0.65 ipoints are a match
if(d1/d2 < Convert.ToSingle(textBox2.Text))
{
matches.Add(ipts1[i]);
}
}
mats = matches;
}