Is there a way to directly sample a "latlong" HDR texture in GLSL for reflections?

Is there a way/method/function that takes a World Space Direction vector, and gets the correct pixel on a "latlong" format HDR texture?

It's easy with a cubemap (textureCube) but I need to do this in one pass, using the texture2D latlong HDR, in World Space.

I found this online, but for me it produces a messed up projection:

const vec2 invAtan = vec2(0.1591, 0.3183);

vec2 SampleSphericalMap(vec3 v)
{
    vec2 uv = vec2(atan(v.z, v.x), asin(v.y));
    uv *= invAtan;
    uv += 0.5;

    return uv;
}

It's supposed to take in a world space vector, but when I pass it the face-normal vector of the mesh in the fragment shader, the mapping is insanely distorted...

Has anyone ever done this before?