Texture paint Shader

[ZuluTangoDelta]

Hi i want to paint a texture on an other texture.So the texture apears on the other texture in the circel.When i change the position of the circel the texture goes away from the old position.I want that the painting texture keeps on its position and overlay the new position too(like painting).I dont know how to solve this problem(i started with shaders few days ago so i am not a pro) Any Tutorials or ideas?

sampler firstSampler; 
texture texture1; // drawmaterial 
float x; 
float y; 
float radius; 
  
sampler Sampler2 = sampler_state  
{  
texture   = <texture1>; 
}; 
  
float4 PSFade(float2 Tex : TEXCOORD0) : COLOR 
{ 
    if ( ((Tex.x-x)*(Tex.x-x)+(Tex.y-y)*(Tex.y-y)) < radius )  // Only in circel 
    { 
  
    float4 color = tex2D(Sampler2, Tex); 
    color.rgb = color.rgb*0.5; 
    return color; 
  
       } 
    else 
    { 
      float4 color = tex2D(firstSampler, Tex); 
       color.rgb = color.rgb*0.5; 
       return color; 
        
       } 
        
} 
  
technique main 
{ 
    pass p1   
    { 
     
        PixelShader = compile ps_2_0 PSFade(); 
    } 
}