[x3d-public] gltf distance attenuation [was glTF compatibility]

[Andreas Plesch]

https://github.com/x3dom/x3dom/issues/1259#issuecomment-1704585434

has a few plots which show similarities and differences for default
x3d and gltf point light attenuation.

The default parameters for x3d are 1,0,0 and radius=100.
The default parameters for gltf are 0,0,1 and radius=infinite

The plots show attenuation for these default settings with the x3d
math and the gltf math.
For the radius=infinite case the gltf and x3d maths boil down to the
same, so no differences..
For short distances (0 to 10) there are essentially no differences
between the different maths.
For large distances (close to 100), there are differences between gltf
and x3d maths. x3d has a step for 1,0,0 and a small step for 0,0,1.
gltf maths produces smooth falloffs for both 1,0,0 and 0,0,1. The
largest differences occur for 1,0,0 between 60m and 100m distance
where gltf generates significant attenuation and darker scenes, and
x3d does not..

Is the impact of adopting the gltf denominator acceptable for x3d ?

One way to check is to look through the large example archive for
examples with point or spotlights (not too many) with default
attenuation (most of those) and see if any shapes are more than 50m
away from the light. The first part should be possible with some
grepping but the second part is difficult and probably requires a
quick manual check.

Andreas

> > 2. As for attenuation and spot cutoff: Indeed glTF math is a bit different.
> >
> > Personally: I'm OK with
> >
> > - adding fields to change the math to follow glTF.
> >
> > - Or just change X3D spec to follow glTF equations. At these points,
> > glTF equations are just better IMHO, more realistic, and the X3D
> > equations are simpler and just followed what old OpenGL ancient
> > fixed-function pipeline could do. It would indeed change how things
> > look, but I think it would be a change for better.
> >
> > I would not want to have different attenuation / spot behavior for
> > Material versus PhysicalMaterial. Conceptually (when describing it to
> > humans) and technically (when implementing) this seems a bit
> > unexpected. Material versus PhysicalMaterial should not determine
> > light attenuation or spot light beam falloff, they should be
> > independent IMHO (and they are independent now).
>
> Yes, attenuation should not be related to the kind of Material. I was
> thinking about that only as a way to not break anything.
>
> I tend to agree that just changing the spec. for attenuation is
> acceptable. But others may disagree since there are users of
> SpotLight. It may come down to how different the defaults look.
>
> Let's see. For distance attenuation, there could be a combined
> equation which preserves the x3d parameters in the denominator and
> adds the glTF range/radius transition in the numerator. I think this
> could work.