如何显示当前Mipmap级别？

truct v2f {
    float4 pos : SV_POSITION;
    float2 uv : TEXCOORD0;
    float2 mipuv : TEXCOORD1;
};
float2 mainTextureSize;
v2f vert (float4 vertex : POSITION, float2 uv : TEXCOORD0)
{
    v2f o;
    o.pos = mul (matrix_mvp, vertex);
    o.uv = uv;
    o.mipuv = uv * mainTextureSize / mipTextureSize;
    return o;
}
half4 frag (v2f i) : COLOR0
{
    half4 col = tex2D (mainTexture, i.uv);
    half4 mip = tex2D (mipColorsTexture, i.mipuv);
    half4 res;
    res.rgb = lerp (col.rgb, mip.rgb, mip.a);
    res.a = col.a;
    return res;    
}