How to draw triangles, lines etc in XNA?

user primitives are the ones that DON’T need a vertex buffer

Here’s how. This example isn’t commented, but if you’re familiar with OpenGL and glVertex3f and glColor3f, this should make a lot of sense to you!

Copy and paste into a new XNA project. There is a Main method here already, so you’ll need to delete the one that comes with your XNA project.

using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Graphics;

class G : Game
{
  GraphicsDeviceManager g;
  BasicEffect r;
  Vector3 eye = new Vector3( 2, 2, 2 ); // put the user @ (2,2,2)
  Vector3 look = new Vector3( 0, 0, 0 ) ; // have him stare into the origin.
  Vector3 up = new Vector3( 0, 1, 0 ) ; // define UP as +y.
  VertexPositionColor[] tris = new VertexPositionColor[ 3 ];
  VertexPositionColor[] axisLines = new VertexPositionColor[ 6 ];  // 6 verts for axis lines

  public G()
  {
    g = new GraphicsDeviceManager( this );
  }

  protected override void LoadContent()
  {
    r = new BasicEffect( GraphicsDevice, null );
    r.VertexColorEnabled = true;

    // Set up vertices.  Remember d3d wants clockwise.
    tris[ 0 ] = new VertexPositionColor( new Vector3( -0.5f, -0.25f, 0 ), Color.Black );
    tris[ 1 ] = new VertexPositionColor( new Vector3( 0, 0.5f, 0 ), Color.GreenYellow );
    tris[ 2 ] = new VertexPositionColor( new Vector3( 0.5f, -0.25f, 0 ), Color.Red );

    int axisLength = 2 ;
    axisLines[ 0 ] = new VertexPositionColor( -axisLength * Vector3.UnitX, Color.Red );
    axisLines[ 1 ] = new VertexPositionColor( axisLength * Vector3.UnitX, Color.Red );

    axisLines[ 2 ] = new VertexPositionColor( -axisLength * Vector3.UnitY, Color.Green );
    axisLines[ 3 ] = new VertexPositionColor( axisLength * Vector3.UnitY, Color.Green );

    axisLines[ 4 ] = new VertexPositionColor( -axisLength * Vector3.UnitZ, Color.Blue );
    axisLines[ 5 ] = new VertexPositionColor( axisLength * Vector3.UnitZ, Color.Blue );
  }

  protected override void Draw( GameTime gameTime )
  {
    GraphicsDevice.Clear( Color.Black );
    GraphicsDevice.RenderState.CullMode = CullMode.None ; // don't discard
    // triangles that are ccw.  d3d has opposite winding order (cw) than openGL.

    // Set look.
    r.View = Matrix.CreateLookAt( eye, look, up );

    r.Projection = Matrix.CreatePerspectiveFieldOfView( MathHelper.ToRadians(45.0f), 1.0f, 1.0f, 1000.0f ) ;

    r.GraphicsDevice.VertexDeclaration = new VertexDeclaration( GraphicsDevice, VertexPositionColor.VertexElements ) ;
    r.Begin();
    foreach( EffectPass pass in r.CurrentTechnique.Passes )
    {
      pass.Begin();

      r.GraphicsDevice.DrawUserPrimitives( PrimitiveType.TriangleList, tris, 0, 1 );

      r.GraphicsDevice.DrawUserPrimitives( PrimitiveType.LineList, axisLines, 0, 3 );

      pass.End();
    }
    r.End();
    base.Draw( gameTime );
  }

  static void Main( string[] args )
  {
    G game = new G();
    game.Run();
  }
}

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