Stealth aka distorsion Post FX - Need Help

Hi,

My goal is to create a stealth effect. In fact it should create a texture on the model that will be transparent & slight distorsion.
It like a "invisible" cloak. I don't want the model to be fully invisible. I found a set of fx used in xna that do exactly what I want with a postfx shader.
creators.xna.com/fr-FR/sample/distortion

I have difficulty to implement it correctly and the documentation around PostFx is light at the moment.

Once it's done, I will share the changes in a resource, as I think it's a nice effect that may be useful for a lot of games.

I took some time to tweak up my code and get some results.

First I apply a custom material to my model to get the distorsion map based on normals:

#define IN_HLSL
#include "../shdrConsts.h"
#include "../hlslStructs.h"

float4  DistortionScale = 0.0003f;
                 
VertexIn_PT main( VertexIn_PN IN, uniform float4x4 modelview : register(VC_WORLD_PROJ) )
{
   VertexIn_PT OUT;  
   OUT.pos = mul(modelview,IN.pos);
   float3 normalWV = IN.normal; 
   float amount = dot(normalWV, float3(0,0,1)) * DistortionScale;
   OUT.uv0 = float2(.5,.5) + float2(amount * normalWV.xy);
   return OUT;
}

Vertex

float4 main(float2 displacement : TEXCOORD0) : COLOR
{  
   return float4(displacement, 0, 1);
}

Next in PostFX Pixel Shader I do:
I started from the selection postFx ressource: http://www.torquepowered.com/community/resources/view/17821
In selection buffer I have the model mapped with the distortion map and all the rest in black.

#include "postFx.hlsl"  
#include "shadergen:/autogenConditioners.h"  
  const float ZeroOffset = 0.5f / 255.0f;

float4 main( PFXVertToPix IN,   
             uniform sampler2D selectionBuffer :register(S0), 
				 uniform sampler2D backBuffer : register(S1),
				 uniform float2 targetSize : register(C0) ) : COLOR0  
{  
float4 DistorsionMap=tex2D( selectionBuffer,IN.uv0);

    // Look up the displacement
    float2 displacement = DistorsionMap.rg;//tex2D(DistortionMap, IN.uv0).rg;
    
    float4 finalColor = 0;
    // We need to constrain the area potentially subjected to the gaussian blur to the
    // distorted parts of the scene texture.  Therefore, we can sample for the color
    // we used to clear the distortion map (black).  We used 0 to avoid any potential
    // rounding errors.
    if ((displacement.x == 0) && (displacement.y == 0))
    {
        finalColor = tex2D(backBuffer, IN.uv0);
    }
    else
    {
        // Convert from [0,1] to [-.5, .5) 
        // .5 is excluded by adjustment for zero
        displacement -= .5 + ZeroOffset;


           // Look up the displaced color, without multisampling
            finalColor = tex2D(backBuffer, IN.uv0.xy + displacement);  
    }

    return finalColor;
}

result:

If you look, I just have a displacement of the background to the left and inverted on Y axis :(

If someone can help me, I will appreciate?

More in Post FX, what is the rendertime/renderbin I should set to get the screen texture with not my models drawn in it?
For the moment I'm PFXAfterDiffuse, so the model is with the mapped displacement texture. I try to put if fully transparent with Alpha = 0 but it is rendered. It might an option, but it's hard to get information in the current documentation.