MonoGame.Samples/StencilsCratersTutorial/Core/StencilCraters.cs

using System;
using System.Collections.Generic;
using System.IO;

using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Audio;
using Microsoft.Xna.Framework.Content;
// using Microsoft.Xna.Framework.GamerServices; // Not available in MonoGame 3.8.*
using Microsoft.Xna.Framework.Graphics;
using Microsoft.Xna.Framework.Input;
using Microsoft.Xna.Framework.Media;

namespace StencilCraters
{
  public class StencilCratersGame : Game
  {
    const int PlanetDataSize = 256;

    bool firstTime = true;
    Vector2 craterPosition;
    Vector2 planetPosition;
    Texture2D drawingTexture;
    KeyboardState currentKeyboardState;
    KeyboardState previousKeyboardState;

    GraphicsDeviceManager graphics;
    SpriteBatch spriteBatch;
        private SpriteFont instructionFont;
        Random random;

    Texture2D planetTexture;
    Texture2D craterTexture;

    RenderTarget2D renderTargetA;
    RenderTarget2D renderTargetB;
    RenderTarget2D activeRenderTarget;
    RenderTarget2D textureRenderTarget;

    AlphaTestEffect alphaTestEffect;
    DepthStencilState stencilAlways;
    DepthStencilState stencilKeepIfZero;



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

      IsMouseVisible = true;

      random = new Random();
    }


    protected override void Initialize()
    {
      base.Initialize();

      // initialize keyboard state
      currentKeyboardState = Keyboard.GetState();
      previousKeyboardState = currentKeyboardState;
    }


    protected override void LoadContent()
    {
      spriteBatch = new SpriteBatch(GraphicsDevice);

      // Load the SpriteFont
      instructionFont = Content.Load<SpriteFont>("font");


      // load assets
      planetTexture = Content.Load<Texture2D>("planet");
      craterTexture = Content.Load<Texture2D>("crater");

      // set up alpha test effect
      Matrix projection = Matrix.CreateOrthographicOffCenter(0, PlanetDataSize, PlanetDataSize, 0, 0, 1);
      Matrix halfPixelOffset = Matrix.CreateTranslation(-0.5f, -0.5f, 0);

      alphaTestEffect = new AlphaTestEffect(GraphicsDevice);
      alphaTestEffect.VertexColorEnabled = true;
      alphaTestEffect.DiffuseColor = Color.White.ToVector3();
      alphaTestEffect.AlphaFunction = CompareFunction.Equal;
      alphaTestEffect.ReferenceAlpha = 0;
      alphaTestEffect.World = Matrix.Identity;
      alphaTestEffect.View = Matrix.Identity;
      alphaTestEffect.Projection = halfPixelOffset * projection;


      // set up stencil state to always replace stencil buffer with 1
      stencilAlways = new DepthStencilState();
      stencilAlways.StencilEnable = true;
      stencilAlways.StencilFunction = CompareFunction.Always;
      stencilAlways.StencilPass = StencilOperation.Replace;
      stencilAlways.ReferenceStencil = 1;
      stencilAlways.DepthBufferEnable = false;


      // set up stencil state to pass if the stencil value is 0
      stencilKeepIfZero = new DepthStencilState();
      stencilKeepIfZero.StencilEnable = true;
      stencilKeepIfZero.StencilFunction = CompareFunction.Equal;
      stencilKeepIfZero.StencilPass = StencilOperation.Keep;
      stencilKeepIfZero.ReferenceStencil = 0;
      stencilKeepIfZero.DepthBufferEnable = false;


      // create render targets
      renderTargetA = new RenderTarget2D(GraphicsDevice, PlanetDataSize, PlanetDataSize,
                                         false, SurfaceFormat.Color, DepthFormat.Depth24Stencil8,
                                         0, RenderTargetUsage.DiscardContents);

      renderTargetB = new RenderTarget2D(GraphicsDevice, PlanetDataSize, PlanetDataSize,
                                         false, SurfaceFormat.Color, DepthFormat.Depth24Stencil8,
                                         0, RenderTargetUsage.DiscardContents);


      // set up the ping-pong texture pointers
      activeRenderTarget = renderTargetA;
      textureRenderTarget = renderTargetB;
      drawingTexture = planetTexture;
    }


    protected override void UnloadContent()
    {
    }



    protected override void Update(GameTime gameTime)
    {
      // check for exit
      if (currentKeyboardState.IsKeyDown(Keys.Escape)
      || GamePad.GetState(PlayerIndex.One).Buttons.Back == ButtonState.Pressed || Keyboard.GetState().IsKeyDown(Keys.Escape))
        this.Exit();



      // update keyboard state
      previousKeyboardState = currentKeyboardState;
      currentKeyboardState = Keyboard.GetState();


      // calculate planet position, centered in screen
      planetPosition = new Vector2(GraphicsDevice.PresentationParameters.BackBufferWidth * 0.5f - PlanetDataSize * 0.4f,
                                   GraphicsDevice.PresentationParameters.BackBufferHeight * 0.5f - PlanetDataSize * 0.4f);


      // add a random crater if space bar is pressed
      if ((currentKeyboardState.IsKeyDown(Keys.Space) && previousKeyboardState.IsKeyUp(Keys.Space)))
      {
        craterPosition = new Vector2(random.Next(PlanetDataSize), random.Next(PlanetDataSize));
      }


      base.Update(gameTime);
    }


    public void AddCrater(Vector2 position)
    {
      // set up rendering to the active render target
      GraphicsDevice.SetRenderTarget(activeRenderTarget);

      // clear the render target to opaque black,
      // and initialize the stencil buffer with all zeroes
      GraphicsDevice.Clear(ClearOptions.Target | ClearOptions.Stencil,
                           new Color(0, 0, 0, 1), 0, 0);


      // draw the new craters into the stencil buffer
      // stencilAlways makes sure we'll always write a 1
      // to the stencil buffer wherever we draw the alphaTestEffect
      // is set up to only write a pixel if the alpha value
      // of the source texture is zero
      spriteBatch.Begin(SpriteSortMode.Immediate, BlendState.Opaque,
                        null, stencilAlways, null, alphaTestEffect);

      Vector2 origin = new Vector2(craterTexture.Width * 0.5f,
                                   craterTexture.Height * 0.5f);

      float rotation = (float)random.NextDouble() * MathHelper.TwoPi;
      Rectangle r = new Rectangle((int)position.X, (int)position.Y, 50, 50);

      spriteBatch.Draw(craterTexture, r, null, Color.White, rotation, 
                       origin, SpriteEffects.None, 0);

      spriteBatch.End();


      // now draw the latest planet texture, excluding the stencil
      // buffer, resulting in the new craters being excluded from
      // the drawing the first time through we don't have a latest
      // planet texture, so draw from the original texture
      spriteBatch.Begin(SpriteSortMode.Immediate, BlendState.Opaque,
                        null, stencilKeepIfZero, null, null);

      if (firstTime)
      {
        spriteBatch.Draw(planetTexture, Vector2.Zero, Color.White);
        firstTime = false;
      }
      else
        spriteBatch.Draw(textureRenderTarget, Vector2.Zero, Color.White);

      spriteBatch.End();


      // restore main render target - this lets us get at the render target we just drew and use it as a texture
      GraphicsDevice.SetRenderTarget(null);


      //// save image for testing
      //using (FileStream f = new FileStream("planet.png", FileMode.Create))
      //{
      //  activeRenderTarget.SaveAsPng(f, 256, 256);
      //}



      // swap render targets, so the next time our source texture is the render target we just drew,
      // and the one we'll be drawing on is the one we just used as our source texture this time
      RenderTarget2D t = activeRenderTarget;
      activeRenderTarget = textureRenderTarget;
      textureRenderTarget = t;

      drawingTexture = textureRenderTarget;
    }



    protected override void Draw(GameTime gameTime)
    {
      // we have to draw render targets first 
      if (craterPosition != Vector2.Zero)
      {
        AddCrater(craterPosition);
        craterPosition = Vector2.Zero;
      }


      // draw the cratered planet texture
      GraphicsDevice.Clear(Color.MonoGameOrange);

      spriteBatch.Begin();
      spriteBatch.Draw(drawingTexture, planetPosition, Color.White);

      spriteBatch.DrawString(
                instructionFont,
                $"Press Space to add a crater",
                new Vector2(20, 105),
                Color.Yellow
            );
      spriteBatch.End();

      base.Draw(gameTime);
    }
  }
}