light radius is a non-physical construct for efficiency purposes, because otherwise every light affects every fragment in the scene

Simple implementation, has precision problems when using fp16 instead of fp32 see https://google.github.io/filament/Filament.html#listing_speculardfp16

Visibility function (Specular G) V(v,l,a) = G(v,l,α) / { 4 (n⋅v) (n⋅l) } such that f_r becomes f_r(v,l) = D(h,α) V(v,l,α) F(v,h,f0) where V(v,l,α) = 0.5 / { n⋅l sqrt((n⋅v)^2 (1−α2) + α2) + n⋅v sqrt((n⋅l)^2 (1−α2) + α2) } Note the two sqrt’s, that may be slow on mobile, see https://google.github.io/filament/Filament.html#listing_approximatedspecularv

Fresnel function see https://google.github.io/filament/Filament.html#citation-schlick94 F_Schlick(v,h,f_0,f_90) = f_0 + (f_90 − f_0) (1 − v⋅h)^5

Cook-Torrance approximation of the microfacet model integration using Fresnel law F to model f_m f_r(v,l) = { D(h,α) G(v,l,α) F(v,h,f0) } / { 4 (n⋅v) (n⋅l) }

Disney approximation See https://google.github.io/filament/Filament.html#citation-burley12 minimal quality difference

From https://www.unrealengine.com/en-US/blog/physically-based-shading-on-mobile

from https://64.github.io/tonemapping/ reinhard on RGB oversaturates colors

luminance coefficients from Rec. 709. https://en.wikipedia.org/wiki/Rec._709