MonoGame: stencil buffer not working
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21-12-2019 - |
Question
I'm trying to add shadows to my MonoGame-based 2D game. At first I just rendered semitransparent black textures to the frame, but they sometimes overlap and it looks nasty:
I tried to render all shadows into the stencil buffer first, and then use a single semitransparent texture to draw all shadows at once using the stencil buffer. However, it doesn't work as expected:
The two problems are:
- The shadows are rendered into the scene
- The stencil buffer is seemingly unaffected: the semitransparent black texture covers the entire screen
Here's the initialization code:
StencilCreator = new DepthStencilState
{
StencilEnable = true,
StencilFunction = CompareFunction.Always,
StencilPass = StencilOperation.Replace,
ReferenceStencil = 1
};
StencilRenderer = new DepthStencilState
{
StencilEnable = true,
StencilFunction = CompareFunction.Equal,
ReferenceStencil = 1,
StencilPass = StencilOperation.Keep
};
var projection = Matrix.CreateOrthographicOffCenter(0, GraphicsDevice.Viewport.Width, GraphicsDevice.Viewport.Height, 0, 0, 1);
var halfPixel = Matrix.CreateTranslation(-0.5f, -0.5f, 0);
AlphaEffect = new AlphaTestEffect(GraphicsDevice)
{
DiffuseColor = Color.White.ToVector3(),
AlphaFunction = CompareFunction.Greater,
ReferenceAlpha = 0,
World = Matrix.Identity,
View = Matrix.Identity,
Projection = halfPixel * projection
};
MaskingTexture = new Texture2D(GameEngine.GraphicsDevice, 1, 1);
MaskingTexture.SetData(new[] { new Color(0f, 0f, 0f, 0.3f) });
ShadowTexture = ResourceCache.Get<Texture2D>("Skins/Common/wall-shadow");
And the following code is the body of my Draw
method:
// create stencil
GraphicsDevice.Clear(ClearOptions.Stencil, Color.Black, 0f, 0);
var batch = new SpriteBatch(GraphicsDevice);
batch.Begin(SpriteSortMode.Immediate, null, null, StencilCreator, null, AlphaEffect);
foreach (Vector2 loc in ShadowLocations)
{
newBatch.Draw(
ShadowTexture,
loc,
null,
Color.White,
0f,
Vector2.Zero,
2f,
SpriteEffects.None,
0f
);
}
batch.End();
// render shadow texture through stencil
batch.Begin(SpriteSortMode.Immediate, null, null, StencilRenderer, null);
batch.Draw(MaskingTexture, GraphicsDevice.Viewport.Bounds, Color.White);
batch.End();
What could possibly be the problem? The same code worked fine in my XNA project.
La solution
I worked around the issue by using a RenderTarget2D
instead of the stencil buffer. Drawing solid black shadows to a RT2D and then drawing the RT2D itself into the scene with a semitransparent color does the trick and is much simplier to implement.