Compute Shader DirtyWindow

// General
#pragma kernel Dirt
#pragma kernel Brush
RWTexture2D<float4> Output;

// Dirt
float MaxValue;
float AddValue;

[numthreads(32,32,1)]
void Dirt (uint3 id : SV_DispatchThreadID)
{
    float4 src = Output[id.xy];
    src.x = min(MaxValue, src.x + AddValue);
    Output[id.xy] = src;
}

// Brush
float4 Position;
float Radius;

[numthreads(32,32,1)]
void Brush (uint3 id : SV_DispatchThreadID)
{
    float4 src = Output[id.xy];
    float dist = length(id.xy - Position.xy) - Radius;
    Output[id.xy] = float4(saturate(dist * 9999.999) * src.x, src.y, src.z, src.w);
}