# Creating 2D geometry from a line strip

During development of an old prototype that never saw the light, I found the need to create some 2D geometry from a set of points that form a line.

After searching for any algorithm that could make what I needed, with no success, I decided to develop my own.

The algorithm is very simple, it just takes a rope and a width and creates a set of vertices tha form a TriangleStrip to be rendered on screen.

[sourcecode language=’csharp’]

///

///

/// List of points that form the line
/// Desired with of the geometry
///

public static List

{

List

Vector2 normal = Vector2.Zero ;

// Generate vertices for each point in the line

for (int i = 0; i < contour.Count; i++)
{
// First and last are special cases, the normal is calculated right away
if (i == contour.Count - 1)
normal = GameMath.Normal(contour[i] - contour[i - 1]);
else if (i == 0)
normal = GameMath.Normal(contour[1] - contour[0]);
else
{
// For the rest of points, determine the normal as the middle angle between the direction of the previous and next segments.
Vector2 delta1 = contour[i] - contour[i - 1];
Vector2 delta2 = contour[i + 1] - contour[i];
if ((delta1.Length()!=0)&&(delta2.Length()!=0))
{
delta1.Normalize();
delta2.Normalize();
normal = GameMath.Normal(delta1 + delta2);
}
}
// Add two vertices to the vertex list
result.Add(contour[i] - normal * width / 2f);
result.Add(contour[i] + normal * width / 2f);
}
return result;
}
///

///

///
///

public static Vector2 Normal(Vector2 line)

{

float x = line.X; float y = line.Y;

Vector2 normal = new Vector2();

if (x != 0)

{

normal.X = -y / x;

normal.Y = 1 / (float)Math.Sqrt(normal.X * normal.X + 1);

normal.Normalize();

}

else if (y != 0)

{

normal.Y = 0;

normal.X = (line.Y<0) ? 1 : -1;
}
else
{
normal.X = 1;
normal.Y = 0;
}
if (x < 0)
{
normal *= -1;
}
return normal;
}
[/sourcecode]
That code is well enough to generate a credible line in almost all situations. As for UVs, what I did was assign the even vertices the value (X,0) and odd vertices (X,1), where X is the distance elapsed since the line start.