How to derived from the Effect class

Textblock does not implement the stroke property, and is a sealed class. The most common work-around for this is to create your own textblock class from FrameworkElement. However, I've recently stumbled across the DropShadowEffect, and wondered if it was possible to use a custom effect to achieve the same outlined text result without the work of implementing the entire outlined text block. (I want a crisper outline that DropShadow will give me.)

To that end, I tried creating a class inheriting from Effect, but immediately ran into problems:

namespace MyNamespace;

public class OutlineEffect : Effect
{
    internal override Channel GetChannelCore(int index)
    {
        //How am I supposed to override an internal class in a Microsoft namespace?
    }

    //...
}

It does say in the documentation:

Derive from the Effect class to implement a custom bitmap effect. In most cases, you will derive from ShaderEffect

so I would assume this is possible. So, How do you derive from Effect?

Solution

You have to inherit from ShaderEffect instead of Effect. Here's an example for outlining text use an edge-detection filter effect:

Combining this Shader tutorial and the Prewitt Edge Detection Filter I managed to get a decent outline effect around text. There are other ways to get a similar effect, like creating your own outline text block, but using an effect has the advantage of rendering using the GPU, and applying generically to any UIElement. However, I had to play a lot with the EdgeResponse property to get a nice outline.

The end result in XAML:

<Grid>
    <Grid.Resources>
        <local:EdgeDetectionEffect x:Key="OutlineEffect"
            x:Shared="false"
            EdgeResponse="4.0"
            ActualHeight="{Binding RelativeSource={RelativeSource AncestorType=TextBlock}, Path=ActualHeight}"
            ActualWidth="{Binding RelativeSource={RelativeSource AncestorType=TextBlock}, Path=ActualWidth}"/>
    </Grid.Resources>
    <TextBlock Text="The Crazy Brown Fox Jumped Over the Lazy Dog."
        FontWeight="Bold"
        Foreground="Yellow"
        Effect="{StaticResource OutlineEffect}"/>
</Grid>

To create the effect, I first created the EdgeDetectionColorEffect.fx (hdld) file - this is the code the GPU uses to filter the image. I compiled it in Visual Studio Command Prompt with the command:

fxc /T ps_2_0 /E main /Focc.ps EdgeDetectionColorEffect.fx

sampler2D Input : register(s0);
float ActualWidth : register(c0);
float ActualHeight : register(c1);
float4 OutlineColor : register(c2);
float EdgeDetectionResponse : register(c3);

float4 GetNeighborPixel(float2 pixelPoint, float xOffset, float yOffset)
{
    float2 NeighborPoint = {pixelPoint.x + xOffset, pixelPoint.y + yOffset};
    return tex2D(Input, NeighborPoint);
}

// pixel locations:
// 00 01 02
// 10 11 12
// 20 21 22
float main(float2 pixelPoint : TEXCOORD) : COLOR
{

     float wo = 1 / ActualWidth; //WidthOffset
     float ho = 1 / ActualHeight; //HeightOffset

    float4 c00 = GetNeighborPixel(pixelPoint, -wo, -ho); // color of the pixel up and to the left of me.
    float4 c01 = GetNeighborPixel(pixelPoint,  00, -ho);        
    float4 c02 = GetNeighborPixel(pixelPoint,  wo, -ho);
    float4 c10 = GetNeighborPixel(pixelPoint, -wo,   0);
    float4 c11 = tex2D(Input, pixelPoint); // this is the current pixel
    float4 c12 = GetNeighborPixel(pixelPoint,  wo,   0);
    float4 c20 = GetNeighborPixel(pixelPoint, -wo,  ho);
    float4 c21 = GetNeighborPixel(pixelPoint,   0,  ho);
    float4 c22 = GetNeighborPixel(pixelPoint,  wo,  ho);

    float t00 = c00.r + c00.g + c00.b; //total of color channels
    float t01 = c01.r + c01.g + c01.b;
    float t02 = c02.r + c02.g + c02.b;
    float t10 = c10.r + c10.g + c10.b;
    float t11 = c11.r + c11.g + c11.b;
    float t12 = c12.r + c12.g + c12.b;
    float t20 = c20.r + c20.g + c20.b;
    float t21 = c21.r + c21.g + c21.b;
    float t22 = c22.r + c22.g + c22.b;

    //Prewitt - convolve the 9 pixels with:
    //       01 01 01        01 00 -1
    // Gy =  00 00 00   Gx = 01 00 -1
    //       -1 -1 -1        01 00 -1

    float gy = 0.0;  float gx = 0.0;
    gy += t00;       gx += t00;
    gy += t01;       gx += t10;
    gy += t02;       gx += t20;
    gy -= t20;       gx -= t02;
    gy -= t21;       gx -= t12;
    gy -= t22;       gx -= t22;

    if((gy*gy + gx*gx) > EdgeDetectionResponse)
    {
        return OutlineColor;
    }

    return c11;
}

Here's the wpf effect class:

public class EdgeDetectionEffect : ShaderEffect
{
    private static PixelShader _shader = new PixelShader { UriSource = new Uri("path to your compiled shader probably called cc.ps", UriKind.Absolute) };

public EdgeDetectionEffect()
{
    PixelShader = _shader;
    UpdateShaderValue(InputProperty);
    UpdateShaderValue(ActualHeightProperty);
    UpdateShaderValue(ActualWidthProperty);
    UpdateShaderValue(OutlineColorProperty);
    UpdateShaderValue(EdgeResponseProperty);
}

public Brush Input
{
     get => (Brush)GetValue(InputProperty);
     set => SetValue(InputProperty, value);
}
public static readonly DependencyProperty InputProperty = 
    ShaderEffect.RegisterPixelShaderSamplerProperty(nameof(Input), 
    typeof(EdgeDetectionEffect), 0);

public double ActualWidth
{
     get => (double)GetValue(ActualWidthProperty);
     set => SetValue(ActualWidthProperty, value);
}
public static readonly DependencyProperty ActualWidthProperty =
    DependencyProperty.Register(nameof(ActualWidth), typeof(double), typeof(EdgeDetectionEffect),
    new UIPropertyMetadata(1.0, PixelShaderConstantCallback(0)));

//notice the PixelShaderConstantCallback(#) - this tells it which GPU register to use (compare the number here to the first few lines of the EdgeDetectionColorEffect.fx file above.

public double ActualHeight
{
     get => (double)GetValue(ActualHeightProperty);
     set => SetValue(ActualHeightProperty, value);
}
public static readonly DependencyProperty ActualHeightProperty =
    DependencyProperty.Register(nameof(ActualHeight), typeof(double), typeof(EdgeDetectionEffect),
    new UIPropertyMetadata(1.0, PixelShaderConstantCallback(1)));

public Color OutlineColor
{
     get => (Color)GetValue(OutlineColorProperty);
     set => SetValue(OutlineColorProperty, value);
}
public static readonly DependencyProperty OutlineColorProperty=
    DependencyProperty.Register(nameof(OutlineColor), typeof(Color), typeof(EdgeDetectionEffect),
    new UIPropertyMetadata(Colors.Black, PixelShaderConstantCallback(2)));

public double EdgeResponse
{
     get => (double)GetValue(EdgeResponseProperty);
     set => SetValue(EdgeResponseProperty, value);
}
public static readonly DependencyProperty EdgeResponseProperty =
    DependencyProperty.Register(nameof(EdgeResponse), typeof(double), typeof(EdgeDetectionEffect),
    new UIPropertyMetadata(4.0, PixelShaderConstantCallback(3)));