Simple Circle in XNA

This post is about creating a simple circle in XNA using VertexPositionColor elements and LineStrip primitive.
Lets call our class Circle.

Members:

//List of vertices

List<VertexPositionColor> _Vertices;

//Convert to array to avoid ToArray for every draw

VertexPositionColor[] _VertexArray;

//Color of the vertices

Color _VertexColor;

//Main game class

Game_Game;

BasicEffect _Effect;

//Transforms for the circle

Matrix _Transforms;

Vector3 _Position, _Scale, _Rotation;

//Radius of the circle

float _Radius;

Properties:

public Vector3 Position

{

get{ return_Position; }

set{_Position=value;Transform();}

}

public Vector3 Rotation

{

get{return_Rotation;}

set{_Rotation=value;Transform();}

}

public Vector3 Scale

{

get{return_Scale;}

set{_Scale=value;Transform();}

}

public MatrixTransforms

{

get{return_Transforms;}

set{_Transforms=value;}

}

Constants:

//This indicates that every 5 degree 

//a point on the cirumference will be added

//Ex: 360/5 = 72 points will be used

int DEGREE_INCREMENT = 5;

Constructor:

public Circle(Game game, Color color, float radius)

{

_Game = game;

_Effect = new BasicEffect(_Game.GraphicsDevice, null);

_Effect.VertexColorEnabled = true;

_VertexColor = color;

_Transforms = Matrix.Identity;

_Radius = radius;

_Scale = Vector3.One;

_Vertices = new List<VertexPositionColor>();

Setup();

_VertexArray = _Vertices.ToArray();

}

Setup Function:

void Setup()

{

//For each number of points : 360/ DEGREE_INCREMENT

for (int i = 0; i <= 360; i+= DEGREE_INCREMENT)

{

VertexPositionColor tmp = new VertexPositionColor();

// X => R * Cos(Angle), Y = 0, Z => R * Sin(Angle)

tmp.Position = new Vector3(_Radius * (float)Math.Cos(MathHelper.ToRadians(i)), 0, _Radius * (float)Math.Sin(MathHelper.ToRadians(i)));

tmp.Color = _VertexColor;

_Vertices.Add(tmp);                

} 

}

Transform Function:

//Update transform when pos, rot, scale is changed

void Transform()

{

_Transforms = Matrix.CreateScale(_Scale) * 

Matrix.CreateFromYawPitchRoll(_Rotation.Y, _Rotation.X, _Rotation.Z) * 

Matrix.CreateTranslation(_Position);

}

Draw Function:

public void Draw()

{

//Tell the device we are sending Vertexpositioncolor elements

_Game.GraphicsDevice.VertexDeclaration = new VertexDeclaration(_Game.GraphicsDevice, VertexPositionColor.VertexElements);

_Effect.Begin();

foreach (EffectPass pass in _Effect.CurrentTechnique.Passes)

{

pass.Begin();

//Set the world, view, projection matrices

_Effect.World = _Transforms;

_Effect.View = Camera.View;

_Effect.Projection = Camera.Projection;

//Draw primitive: n points => n-1 lines

_Game.GraphicsDevice.DrawUserPrimitives<VertexPositionColor>(PrimitiveType.LineStrip, _VertexArray, 0, _Vertices.Count - 1);

pass.End();

}

_Effect.End(); 

}

Screen shots with different Degree increments:

Next up - Sphere