Field of View / Mask

Stage 1

You will need to find your 'graphics' or '_graphics' variable. You will need to add the following parameter:

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

Stage 2

You then need to find your Draw method. You will then need to add the following

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

You then need to create your first DepthStencilState:

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

You then need to create your second DepthStencilState:

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

You could alternatively define all of these within the Game1 class itself, you would need to replace 'var' with the data type.

Stage 3

Now you need to draw your mask using your first DepthStencilState:

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

Now draw the rest with your second DepthStencilState:

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