Custom WPF ShaderEffect alpha channel has unusual behavior - wpf

Question

I've got a fairly straightforward pixel shader—set alpha channel to zero and return.
sampler2D tex : register(s0);
float4 PS(float2 uv : TEXCOORD) : COLOR
{
float4 color = tex2D(tex, uv);
color.a = 0;
return color;
}
I'd assume this would cause the image it's applied to to be completely invisible. However that's not what appears to be happening. Instead, the resulting image will become invisible over a white background, but over a black background it'll be unchanged. It appears that this shader is somehow causing an "add" function to be called between the foreground and the background.
For example, the following code loads a foreground and background image, applies the above shader effect to the foreground, renders them to a bitmap, and writes the bitmap to file.
public sealed partial class MainWindow
{
public MainWindow()
{
InitializeComponent();
}
void Button_Click(object sender, RoutedEventArgs e)
{
const int width = 1024;
const int height = 768;
var sz = new Size(width, height);
var background = new Image { Source = new BitmapImage(new Uri(#"c:\background.jpg")) };
background.Measure(sz);
background.Arrange(new Rect(sz));
var foreground = new Image { Source = new BitmapImage(new Uri(#"c:\foreground.jpg")), Effect = new Alpha() };
foreground.Measure(sz);
foreground.Arrange(new Rect(sz));
var target = new RenderTargetBitmap(width, height, 96d, 96d, PixelFormats.Default);
target.Render(background);
target.Render(foreground);
var jpg = new JpegBitmapEncoder();
jpg.Frames.Add(BitmapFrame.Create(target));
using (var fileStream = File.OpenWrite(#"c:\output.jpg"))
{
jpg.Save(fileStream);
}
}
}
// Standard ShaderEffect stuff here, nothing exciting.
public sealed class Alpha : ShaderEffect
{
static readonly PixelShader Shader = new PixelShader{UriSource = new Uri("pack://application:,,,/Alpha.ps", UriKind.RelativeOrAbsolute)};
public static readonly DependencyProperty InputProperty = RegisterPixelShaderSamplerProperty("Input", typeof(Alpha), 0);
public Alpha()
{
PixelShader = Shader;
UpdateShaderValue(InputProperty);
}
public Brush Input
{
get { return (Brush)GetValue(InputProperty); }
set { SetValue(InputProperty, value); }
}
}
This produces the following when applied to two of the Win7 sample pictures:
This is the same behavior I see on the screen when I apply the effect to one Image in XAML, with another Image or anything else behind it.
Note the image is the same if foreground and background are reversed, so if it's not "add", it's at least something commutative. I think it's "add".
Computers are usually right, so I assume this is user error, but why is setting alpha to zero not giving me a transparent image? And how do I get a transparent image if so? (I obviously want to do something more complex with the shader eventually (specifically greenscreen), but to get that to work I have to get this shader to work first, so don't just say "Set the Opacity property").

Answer 1

Gah, stackoverflow is better than google. The top "related question" had the answer. Handling alpha channel in WPF pixel shader effect