textureLod

[ytgui]

https://zhuanlan.zhihu.com/p/26393153 https://blog.csdn.net/u010468553/article/details/80572641 https://blog.csdn.net/chy555chy/article/details/80177015

[ytgui]

• In the PS the LOD is determined from the derivatives of the texCoords automatically(tex2D). You can also specify the derivatives explicitly as two extra arguments tex2D(textureMap, texCoord, ddx(texCoord), ddy(texCoord)) is equivalent to your tex2D, though of course you could use something else as the derivative.
• Alternately, you can use tex2Dlod to explicitly select the LOD speicifed by the ‘w’ component of texCoord; eg, something like: tex2Dlod(textureMap, float4(texCoord.xy, 0, lod))

texture2D

tex2D(sampler2D tex, float4 uv)
{
    float lod = CalcLod(ddx(uv), ddy(uv));
    uv.w = lod;
    return tex2Dlod(tex, uv);
}

float mipmapLevel(float2 uv, float2 textureSize)
{
    float dx = ddx(uv * textureSize.x);
    float dy = ddy(uv * textureSize.y);
    float d = max(dot(dx, dx), dot(dy, dy));  
    return 0.5 * log2(d);  // 0.5是技巧，本来是d的平方。
}