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I am trying to make a pixel shader that achieves an effect similar to this video

My basic idea is render the scene to a temp render target then Render the previously rendered image with a slight fade on to another temp render target Draw the current scene on top of that Draw the results on to a render target that persists between draws Draw the results on to the screen

But I am having problems with the fading portion. If I have my pixel shader return a color with its A component set to 0, shouldn't that basically amount to drawing nothing? (Assuming that sprite batch blend mode is set to AlphaBlend)

To test this I have my pixel shader return a transparent red color. Instead of nothing being drawn, it draws a partially transparent red box.

I hope that my question makes sense, but if it doesnt please ask me to clarify

Here is the drawing code

    public override void Draw(GameTime gameTime)
        GraphicsDevice.SamplerStates[1] = SamplerState.PointWrap;

        drawImageOnClearedRenderTarget(presentationTarget, tempRenderTarget, fadeEffect);
        drawImageOnRenderTarget(sceneRenderTarget, tempRenderTarget);
        drawImageOnClearedRenderTarget(tempRenderTarget, presentationTarget);




    private void drawImage(Texture2D image, Effect effect = null)
        spriteBatch.Begin(0, BlendState.AlphaBlend, SamplerState.PointWrap, null, null, effect);
        spriteBatch.Draw(image, new Rectangle(0, 0, width, height), Color.White);

    private void drawImageOnRenderTarget(Texture2D image, RenderTarget2D target, Effect effect = null)
        drawImage(image, effect);

    private void drawImageOnClearedRenderTarget(Texture2D image, RenderTarget2D target, Effect effect = null)
        drawImage(image, effect);

Here is the fade pixel shader

sampler TextureSampler : register(s0);

float4 PixelShaderFunction(float2 texCoord : TEXCOORD0) : COLOR0
    float4 c = 0;
    c = tex2D(TextureSampler, texCoord);
    //c.a = clamp(c.a - 0.05, 0, 1);
    c.r = 1;
    c.g = 0;
    c.b = 0;
    c.a = 0;
    return c;

technique Fade
    pass Pass1
        PixelShader = compile ps_2_0 PixelShaderFunction();
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up vote 5 down vote accepted

BlendState.AlphaBlend uses premultiplied blending so check out this for why you can a value of 0 for alpha and still have the pixel not be transparent (especially the last paragraph).

So I believe what you need is in your shader is (I'm bad with shaders so take this with a grain of salt).

c.a = clamp(c.a - 0.05, 0, 1);
c.r = c.r * c.a;
c.g = c.g * c.a;
c.b = c.b * c.a;
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