/* * * * * * * * * * * * * * * * * * * * * * * * * * 
 * Renders 2D primitives (lines) specified in screen/pixel space 
 * using a custom shader to transform vertices into clip space.
 * 
 * By Karn Bianco - www.KarnBianco.co.uk
 * 
 * * * * * * * * * * * * * * * * * * * * * * * * * */

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

namespace YourNameSpaceHere
{
    public class Render2DLines : Microsoft.Xna.Framework.Game
    {
        GraphicsDeviceManager graphics;

        Vector2[] vertices;                     // Vertices defined in screen space
        VertexPositionColor[] verticesForGPU;   // 3D vertices required for rendering
        int[] indices;                          // Indices for defining lines from points
        Effect lineEffect;                      // Simple custom shader for rendering lines

        public Render2DLines()
        {
            graphics = new GraphicsDeviceManager(this);
            Content.RootDirectory = "Content";

            this.graphics.PreferredBackBufferWidth = 450;
            this.graphics.PreferredBackBufferHeight = 300;
        }

        protected override void LoadContent()
        {
            // Load effect file using the content pipeline
            lineEffect = Content.Load<Effect>("LineShader2D");

            // Set up vertices in screen space (just as you would set sprite positions)
            vertices = new Vector2[4];
            vertices[0] = new Vector2(10, 10);
            vertices[1] = new Vector2(400, 10);
            vertices[2] = new Vector2(400, 150);
            vertices[3] = new Vector2(10, 150);

            // Set up 3D vertices using screen space vectors and a 0 component for z-value
            verticesForGPU = new VertexPositionColor[4];

            for (int i = 0; i < vertices.Length; i++)
            {
                verticesForGPU[i].Position = new Vector3(vertices[i], 0);
                verticesForGPU[i].Color = Color.White;
            }

            // Generate two indices for each vertex
            indices = new int[8] { 0, 1, 1, 2, 2, 3, 3, 0 };

            // Set vertex declaration
            GraphicsDevice.VertexDeclaration = new VertexDeclaration(
                GraphicsDevice, VertexPositionColor.VertexElements); ;

            // Set Viewportsize parameter
            lineEffect.Parameters["ViewportSize"].SetValue(new Vector2
                (GraphicsDevice.Viewport.Width, GraphicsDevice.Viewport.Height));

            // Set Transform matrix as Idneity matrix by default (can be used to 
            // translate, rotate and scale vertices if required)
            lineEffect.Parameters["TransformMatrix"].SetValue(Matrix.Identity);

        }

        protected override void Update(GameTime gameTime)
        {
            base.Update(gameTime);
        }

        protected override void Draw(GameTime gameTime)
        {
            GraphicsDevice.Clear(Color.Black);

            lineEffect.Begin(); // Begin using custom effect            
            lineEffect.CurrentTechnique.Passes[0].Begin();  // Only one pass in technique
            
            GraphicsDevice.DrawUserIndexedPrimitives(PrimitiveType.LineList, verticesForGPU, 0,
                verticesForGPU.Length, indices, 0, indices.Length / 2); // Draw lines

            lineEffect.CurrentTechnique.Passes[0].End();
            lineEffect.End();

            base.Draw(gameTime);
        }
    }
}