CODEWITHSUNDEEP
xnashader
Orteil
Orteil

Reputation: 441

How do I make pixel shaders with multiple passes in XNA?

I'm very new to shaders and am very confused about the whole thing, even after following several tutorials (in fact this is my second question about shaders today).

I'm trying to make a shader with two passes :

technique Technique1  
{  
    pass Pass1
    {  
        PixelShader = compile ps_2_0 HorizontalBlur();  
    }  

    pass Pass2
    {  
        PixelShader = compile ps_2_0 VerticalBlur();  
    }
}

However, this only applies VerticalBlur(). If I remove Pass2, it falls back to the HorizontalBlur() in Pass1. Am I missing something? Maybe it's simply not passing the result of the first pass to the second pass, in which case how would I do that?

Also, in most of the tutorials I've read, I'm told to put effect.CurrentTechnique.Passes[0].Apply(); after I start my spritebatch with the effect. However, this doesn't seem to change anything; I can set it to Passes[1] or even remove it entirely, and I still get only Pass2. (I do get an error when I try to set it to Passes[2], however.) What's the use of that line then? Has the need for it been removed in recent versions?

Thanks a bunch!

Upvotes: 1

Views: 2426

Answers (2)

default
default

Reputation: 2757

To render multiple passes:

For the first pass, render your scene onto a texture.

For the second, third, fourth, etc passes:

  • Draw a quad that uses the texture from the previous pass. If there are more passes, to follow, render this pass to another texture, otherwise if this is the last pass, render it to the back buffer.

In your example, say you are rendering a car.

  • First you render the car to a texture.

  • Then you draw a big rectangle, the size of the screen in pixels, place at a z depth of 0.5, with identity world, view, and projection matrices, and with your car scene as the texture, and apply the horizontal blur pass. This is rendered to a new texture that now has a horizontally blurred car.

  • Finally, render the same rectangle but with the "horizontally burred car" texture, and apply the vertical blur pass. Render this to the back buffer. You have now drawn a blurred car scene.

Upvotes: 2

chue x
chue x

Reputation: 18803

The reason for the following

effect.CurrentTechnique.Passes[0].Apply();

is that many effects only have a single pass.

To run multiple passes, I think you have to do this instead:

foreach (EffectPass pass in effect.CurrentTechnique.Passes)
{
    pass.Apply();

    //Your draw code here:

}

Upvotes: 2

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