batched billboard implementation

Geometry/Sprite.cs

@@ -0,0 +1,39 @@
+using Microsoft.Xna.Framework;
+using Microsoft.Xna.Framework.Graphics;
+
+namespace Kav
+{
+    public struct Sprite
+    {
+        public Texture2D Texture { get; }
+        public Vector3 Position { get; }
+        public Vector2 Origin { get; }
+        public float Rotation { get; }
+        public Vector2 Scale { get; }
+
+        public Sprite(
+            Texture2D texture,
+            Vector3 position,
+            Vector2 origin,
+            float rotation,
+            Vector2 scale
+        ) {
+            Texture = texture;
+            Position = position;
+            Origin = origin;
+            Rotation = rotation;
+            Scale = scale;
+        }
+
+        public Sprite(
+            Texture2D texture,
+            Vector3 position
+        ) {
+            Texture = texture;
+            Position = position;
+            Origin = Vector2.Zero;
+            Rotation = 0f;
+            Scale = Vector2.One;
+        }
+    }
+}

Renderer.cs

@@ -33,6 +33,7 @@ namespace Kav
         private LinearDepthEffect LinearDepthEffect { get; }
         private Effect ToneMapEffect { get; }
         private SkyboxEffect SkyboxEffect { get; }
+        private BasicEffect BasicEffect { get; }
 
         private RenderTarget2D gPosition { get; }
         private RenderTarget2D gNormal { get; }
@@ -159,6 +160,7 @@ namespace Kav
             ToneMapEffect = new Effect(graphicsDevice, Resources.ToneMapEffect);
             Deferred_ToonEffect = new Deferred_ToonEffect(GraphicsDevice);
             SkyboxEffect = new SkyboxEffect(GraphicsDevice);
+            BasicEffect = new BasicEffect(GraphicsDevice);
 
             FullscreenTriangle = new VertexBuffer(GraphicsDevice, typeof(VertexPositionTexture), 3, BufferUsage.WriteOnly);
             FullscreenTriangle.SetData(new VertexPositionTexture[3] {
@@ -206,6 +208,7 @@ namespace Kav
         }
 
         public void DeferredToonRender(
+            RenderTarget2D renderTarget,
             PerspectiveCamera camera,
             IEnumerable<(Model, Matrix)> modelTransforms,
             AmbientLight ambientLight,
@@ -230,12 +233,64 @@ namespace Kav
             DirectionalLightToonRender(camera, modelTransforms, directionalLight);
             SkyboxRender(camera, skybox);
 
-            GraphicsDevice.SetRenderTarget(null);
+            GraphicsDevice.SetRenderTarget(renderTarget);
             SpriteBatch.Begin(SpriteSortMode.Deferred, BlendState.Opaque, null, null, null, null);
             SpriteBatch.Draw(ColorRenderTarget, Vector2.Zero, Color.White);
             SpriteBatch.End();
         }
 
+        // billboards sprites into the scene
+        // FIXME: we can frustum cull the sprites probably
+        public void BillboardSpriteRender(
+            RenderTarget2D renderTarget,
+            PerspectiveCamera camera,
+            IEnumerable<(Model, Matrix)> modelTransforms,
+            IEnumerable<Sprite> sprites
+        ) {
+            GraphicsDevice.SetRenderTarget(ColorRenderTarget);
+            GraphicsDevice.Clear(ClearOptions.Target | ClearOptions.DepthBuffer, Color.Black, 1f, 0);
+            GraphicsDevice.DepthStencilState = DepthStencilState.Default;
+
+            DepthRender(camera, modelTransforms);
+            GraphicsDevice.Clear(ClearOptions.Target, new Color(0, 0, 0, 0), 1f, 0);
+
+            Matrix invertY = Matrix.CreateScale(1, -1, 1);
+
+            BasicEffect.World = invertY;
+            BasicEffect.View = Matrix.Identity;
+            BasicEffect.Projection = camera.Projection;
+            BasicEffect.TextureEnabled = true;
+            BasicEffect.VertexColorEnabled = true;
+
+            SpriteBatch.Begin(0, null, null, DepthStencilState.DepthRead, RasterizerState.CullNone, BasicEffect);
+
+            foreach (var sprite in sprites)
+            {
+                // transform view space on CPU so we don't have to break the batch
+                Vector3 viewSpacePosition = Vector3.Transform(sprite.Position, camera.View * Matrix.CreateRotationX(sprite.Rotation) * invertY);
+
+                SpriteBatch.Draw(
+                    sprite.Texture,
+                    new Vector2(viewSpacePosition.X, viewSpacePosition.Y),
+                    null,
+                    Color.White,
+                    0,
+                    sprite.Origin,
+                    sprite.Scale / new Vector2(sprite.Texture.Width, sprite.Texture.Height),
+                    0,
+                    viewSpacePosition.Z
+                );
+            }
+
+            SpriteBatch.End();
+
+            GraphicsDevice.SetRenderTarget(renderTarget);
+            GraphicsDevice.Clear(new Color(0, 0, 0, 0));
+            SpriteBatch.Begin(SpriteSortMode.Deferred, BlendState.AlphaBlend, null, null, null, null);
+            SpriteBatch.Draw(ColorRenderTarget, Vector2.Zero, Color.White);
+            SpriteBatch.End();
+        }
+
         private void DepthRender(
             PerspectiveCamera camera,
             IEnumerable<(Model, Matrix)> modelTransforms