Best way to mask 2D sprites in XNA?

Question

I currently am trying to mask some sprites. Rather than explaining it in words, I've made up some example pictures:

The area to mask (in white)

Now, the red sprite that needs to be cropped.

The final result.

Now, I'm aware that in XNA you can do two things to accomplish this:

1. Use the Stencil Buffer.
2. Use a Pixel Shader.

I have tried to do a pixel shader, which essentially did this:

float4 main(float2 texCoord : TEXCOORD0) : COLOR0
{
    float4 tex = tex2D(BaseTexture, texCoord);
    float4 bitMask = tex2D(MaskTexture, texCoord);

    if (bitMask.a > 0)
    { 
        return float4(tex.r, tex.g, tex.b, tex.a);
    }
    else
    {
        return float4(0, 0, 0, 0);
    }
}

This seems to crop the images (albeit, not correct once the image starts to move), but my problem is that the images are constantly moving (they aren't static), so this cropping needs to be dynamic.

Is there a way I could alter the shader code to take into account it's position?

---

Alternatively, I've read about using the Stencil Buffer, but most of the samples seem to hinge on using a rendertarget, which I really don't want to do. (I'm already using 3 or 4 for the rest of the game, and adding another one on top of it seems overkill)

The only tutorial I've found that doesn't use Rendertargets is one from Shawn Hargreaves' blog over here. The issue with that one, though is that it's for XNA 3.1, and doesn't seem to translate well to XNA 4.0.

It seems to me that the pixel shader is the way to go, but I'm unsure of how to get the positioning correct. I believe I would have to change my onscreen coordinates (something like 500, 500) to be between 0 and 1 for the shader coordinates. My only problem is trying to work out how to correctly use the transformed coordinates.

Thanks in advance for any help!

Answer 1

You can use your pixel shader approach but it is incomplete.

You will also need to add some parameters to it that inform the pixel shader where you want your stencil to be located.

sampler BaseTexture : register(s0);
sampler MaskTexture : register(s1) {
    addressU = Clamp;
    addressV = Clamp;
};

//All of these variables are pixel values
//Feel free to replace with float2 variables
float MaskLocationX;
float MaskLocationY;
float MaskWidth;
float MaskHeight;
float BaseTextureLocationX;  //This is where your texture is to be drawn
float BaseTextureLocationY;  //texCoord is different, it is the current pixel
float BaseTextureWidth;
float BaseTextureHeight;

float4 main(float2 texCoord : TEXCOORD0) : COLOR0
{
    //We need to calculate where in terms of percentage to sample from the MaskTexture
    float maskPixelX = texCoord.x * BaseTextureWidth + BaseTextureLocationX;
    float maskPixelY = texCoord.y * BaseTextureHeight + BaseTextureLocationY;
    float2 maskCoord = float2((maskPixelX - MaskLocationX) / MaskWidth, (maskPixelY - MaskLocationY) / MaskHeight);
    float4 bitMask = tex2D(MaskTexture, maskCoord);

    float4 tex = tex2D(BaseTexture, texCoord);

    //It is a good idea to avoid conditional statements in a pixel shader if you can use math instead.
    return tex * (bitMask.a);
    //Alternate calculation to invert the mask, you could make this a parameter too if you wanted
    //return tex * (1.0 - bitMask.a);
}

In your C# code you pass in variables to the pixel shader prior to the SpriteBatch.Draw call like this:

maskEffect.Parameters["MaskWidth"].SetValue(800);
maskEffect.Parameters["MaskHeight"].SetValue(600);

Answer 2

The first step is to tell the graphics card we need the stencil buffer. To do this when you create GraphicsDeviceManager we set the PreferredDepthStencilFormat to DepthFormat.Depth24Stencil8 so there is actually a stencil to write to.

graphics = new GraphicsDeviceManager(this) {
    PreferredDepthStencilFormat = DepthFormat.Depth24Stencil8
};

The AlphaTestEffect is used to set the coordinate system and filter the pixels with alpha that pass the alpha test. We are not going to set any filters and set the coordinate system to the view port.

var m = Matrix.CreateOrthographicOffCenter(0,
    graphics.GraphicsDevice.PresentationParameters.BackBufferWidth,
    graphics.GraphicsDevice.PresentationParameters.BackBufferHeight,
    0, 0, 1
);
var a = new AlphaTestEffect(graphics.GraphicsDevice) {
    Projection = m
};

Next we need to set up two DepthStencilStates. These states dictate when the SpriteBatch renders to the stencil and when the SpriteBatch renders to the BackBuffer. We are primarily interested in two variables StencilFunction and StencilPass.

• StencilFunction dictates when the SpriteBatch will draw individual pixels and when they will be ignored.
• StencilPass dictates when drawn pixels pixels effect the Stencil.

For the first DepthStencilState we set StencilFunction to CompareFunction. This causes the StencilTest to succeed and when the StencilTest the SpriteBatch renders that pixel. StencilPass is set to StencilOperation. Replace meaning that when the StencilTest succeed that pixel will written to the StencilBuffer with the value of the ReferenceStencil.

var s1 = new DepthStencilState {
    StencilEnable = true,
    StencilFunction = CompareFunction.Always,
    StencilPass = StencilOperation.Replace,
    ReferenceStencil = 1,
    DepthBufferEnable = false,
};

In summary the StencilTest always passes, the image is drawn to screen normally, and for pixels drawn to the screen a value of 1 is stored in the StencilBuffer.

The second DepthStencilState is slightly more complicated. This time we want to only draw to the screen when the value in the StencilBuffer is. To achieve this we set the StencilFunction to CompareFunction.LessEqual and the ReferenceStencil to 1. This means that when the value in the stencil buffer is 1 the StencilTest will succeed. Setting StencilPass to StencilOperation. Keep causes the StencilBuffer not to update. This allows us to draw multiple times using the same mask.

var s2 = new DepthStencilState {
    StencilEnable = true,
    StencilFunction = CompareFunction.LessEqual,
    StencilPass = StencilOperation.Keep,
    ReferenceStencil = 1,
    DepthBufferEnable = false,
};

In summary the StencilTest only passes when the StencilBuffer is less than 1 (the alpha pixels from the mask) and does not effect the StencilBuffer.

Now that we have our DepthStencilStates set up. We can actually draw using a mask. Simply draw the mask using the first DepthStencilState. This will effect both the BackBuffer and the StencilBuffer. Now that the stencil buffer has a value of 0 where you mask had transparency and 1 where it contained color we can use StencilBuffer to mask later images.

spriteBatch.Begin(SpriteSortMode.Immediate, null, null, s1, null, a);
spriteBatch.Draw(huh, Vector2.Zero, Color.White); //The mask                                   
spriteBatch.End();

The second SpriteBatch uses the second DepthStencilStates. No matter what you draw, only the pixels where the StencilBuffer is set to 1 will pass the stencil test and be drawn to the screen.

spriteBatch.Begin(SpriteSortMode.Immediate, null, null, s2, null, a);            
spriteBatch.Draw(color, Vector2.Zero, Color.White); //The background
spriteBatch.End();

Below is the entirety of the code in the Draw method, don't forget to set PreferredDepthStencilFormat = DepthFormat.Depth24Stencil8 in the game constructor.

GraphicsDevice.Clear(ClearOptions.Target 
    | ClearOptions.Stencil, Color.Transparent, 0, 0);

var m = Matrix.CreateOrthographicOffCenter(0,
    graphics.GraphicsDevice.PresentationParameters.BackBufferWidth,
    graphics.GraphicsDevice.PresentationParameters.BackBufferHeight,
    0, 0, 1
);

var a = new AlphaTestEffect(graphics.GraphicsDevice) {
    Projection = m
};

var s1 = new DepthStencilState {
    StencilEnable = true,
    StencilFunction = CompareFunction.Always,
    StencilPass = StencilOperation.Replace,
    ReferenceStencil = 1,
    DepthBufferEnable = false,
};

var s2 = new DepthStencilState {
    StencilEnable = true,
    StencilFunction = CompareFunction.LessEqual,
    StencilPass = StencilOperation.Keep,
    ReferenceStencil = 1,
    DepthBufferEnable = false,
};

spriteBatch.Begin(SpriteSortMode.Immediate, null, null, s1, null, a);
spriteBatch.Draw(huh, Vector2.Zero, Color.White); //The mask                                   
spriteBatch.End();

spriteBatch.Begin(SpriteSortMode.Immediate, null, null, s2, null, a);            
spriteBatch.Draw(color, Vector2.Zero, Color.White); //The background
spriteBatch.End();

Answer 3

In the answer above, some weird things happened when I did exactly as is explained.

The only thing I needed was the both DepthStencilStates.

var s1 = new DepthStencilState {
    StencilEnable = true,
    StencilFunction = CompareFunction.Always,
    StencilPass = StencilOperation.Replace,
    ReferenceStencil = 1,
    DepthBufferEnable = false,
};

var s2 = new DepthStencilState {
    StencilEnable = true,
    StencilFunction = CompareFunction.LessEqual,
    StencilPass = StencilOperation.Keep,
    ReferenceStencil = 1,
    DepthBufferEnable = false,
};

And the draws without the AlphaTestEffect.

spriteBatch.Begin(SpriteSortMode.Immediate, null, null, s1, null, null);
spriteBatch.Draw(huh, Vector2.Zero, Color.White); //The mask                                   
spriteBatch.End();

spriteBatch.Begin(SpriteSortMode.Immediate, null, null, s2, null, null);            
spriteBatch.Draw(color, Vector2.Zero, Color.White); //The background
spriteBatch.End();

And everything was fine like expected.