Basic post-processing shader problem

Question

I'm trying to create a basic post-processing shader, like I did many times before but somehow today it's just not working, I guess it's something basic that I missed.

Anyway my setup:

I have 3 render targets rts[0], rts1, and rts[2], I draw a simple model in each one of them. I then draw the render targets to the screen using the following code:

GraphicsDevice.SetRenderTarget(null);
GraphicsDevice.Clear(Color.CornflowerBlue);
spriteBatch.Begin(SpriteSortMode.Immediate, BlendState.AlphaBlend, null, null, null);

spriteBatch.Draw(rts[0], new Rectangle(0, 0, GraphicsDevice.Viewport.Width / 2, GraphicsDevice.Viewport.Height / 2), Color.White);
spriteBatch.Draw(rts[1], new Rectangle(GraphicsDevice.Viewport.Width / 2, 0, GraphicsDevice.Viewport.Width / 2, GraphicsDevice.Viewport.Height / 2), Color.White);
spriteBatch.Draw(rts[2], new Rectangle(0, GraphicsDevice.Viewport.Height / 2, GraphicsDevice.Viewport.Width / 2, GraphicsDevice.Viewport.Height / 2), Color.White);

spriteBatch.End();

This gives me the following picture: Which is exactly what I expect it to be :)

Now I feed these 3 render targets as textures to my shader like this: (shaderTexture is a black 1x1 texture)

cutAwayEffect.Parameters["lowTexture"].SetValue(rts[0]);
cutAwayEffect.Parameters["highTexture"].SetValue(rts[1]);
cutAwayEffect.Parameters["cutTexture"].SetValue(rts[2]);

spriteBatch.Begin(SpriteSortMode.Immediate, BlendState.Opaque, null, null, null, cutAwayEffect);


spriteBatch.Draw(shaderTexture, new Rectangle(GraphicsDevice.Viewport.Width / 2, GraphicsDevice.Viewport.Height / 2, GraphicsDevice.Viewport.Width / 2, GraphicsDevice.Viewport.Height / 2), Color.White);

spriteBatch.End();

And the shader:

texture lowTexture;
sampler lowSampler = sampler_state
{
    Texture = (lowTexture);
};

texture highTexture;
sampler highSampler = sampler_state
{
    Texture = (highTexture);
};

texture cutTexture;
sampler cutSampler = sampler_state
{
    Texture = (cutTexture);
};

float4 PixelShaderFunction(float2 texCoord: TEXCOORD0) : COLOR0
{    
    float4 baseColour = tex2D(lowSampler, texCoord).rgba;
    float4 highColour = tex2D(highSampler, texCoord).rgba;
    float4 cutColour = tex2D(cutSampler, texCoord).rgba;
    float4 finalColour = baseColour + highColour * cutColour;


    return finalColour;
}

technique CutAway
{
    pass Pass1
    { 
        PixelShader = compile ps_2_0 PixelShaderFunction();
    }
}

Now I would expect to see a combination of all 3 render targets in the bottom right corner, however I don't see any data from baseColour. When I change finalColour to just baseColour I only see black, same goes for just highColour and just cutColour. Even finalColour = highColour*cutColour only gives me black while finalColour = baseColour + highColour*cutColour; does give me something.

The shader does seem to be working, changing final Colour to float4(1,0,1,1) gives me the expected pink square. There seems to be a problem reading the data from the textures, but I've got no clue why.

Answer 1

In the end I've made the following changes and it now works

In the shader I replaced the lowTexture + lowSampler by this

sampler lowSampler : register(s0);

The draw code is now:

 cutAwayEffect.Parameters["highTexture"].SetValue(rts[1]);
            cutAwayEffect.Parameters["cutTexture"].SetValue(rts[2]);
            cutAwayEffect.Parameters["blurWidth"].SetValue(0.07f);

            spriteBatch.Begin(SpriteSortMode.Immediate, BlendState.AlphaBlend, null, null, null, cutAwayEffect);
            spriteBatch.Draw(rts[0], new Rectangle(GraphicsDevice.Viewport.Width / 2, GraphicsDevice.Viewport.Height / 2, GraphicsDevice.Viewport.Width / 2, GraphicsDevice.Viewport.Height / 2), Color.White);
            spriteBatch.End();

Why this solves the problem? I'm not 100% sure. The spritebatch now automatically puts rts[0] in to register(s0) and I don't have to use the dummy texture anymore. Maybe that was giving problems but I'm still not sure. So if someone has a suggestion why this works, or better: why the original code didn't, feel free to post your answer.