Mipmap level calculation using dFdx/dFdy

OpenGL
2

Hi,
take a look at the OpenGL 4.2 spec chapter 3.9.11 equation 3.21. The mip map level is calculated based on the lengths of the derivative vectors:


float
mip_map_level(in vec2 texture_coordinate)
{
    // The OpenGL Graphics System: A Specification 4.2
    //  - chapter 3.9.11, equation 3.21


    vec2  dx_vtc        = dFdx(texture_coordinate);
    vec2  dy_vtc        = dFdy(texture_coordinate);
    float delta_max_sqr = max(dot(dx_vtc, dx_vtc), dot(dy_vtc, dy_vtc));


    //return max(0.0, 0.5 * log2(delta_max_sqr) - 1.0); // == log2(sqrt(delta_max_sqr));
    return 0.5 * log2(delta_max_sqr); // == log2(sqrt(delta_max_sqr));
}