gistfile1.c

float CalcMipLevel(float2 inUV)
{
    float2 uv = inUV * kAtlasedTextureSize;
    float2 du = ddx(uv0);
    float2 dv = ddy(uv0);
    float  d  = max(dot(du, du), dot(dv, dv));

    const float r = pow(2, (kMipCount - 1) * 2);
    d = clamp(d, 1.0, r);

    return floor(0.5 * log2(d));
}