[x3d-public] [x3d] X3D minutes 4 OCT 2019: X3D-glTF review, defer Annotations, Layout component, #X3Dv4 implementations/issues update

[Michalis Kamburelis]

Brutzman, Donald (Don) (CIV) <brutzman at nps.edu> wrote:
> a. glTF Lighting Extension defines three lights, Directional Point and Spot, which only define color.  These are simply defined, e.g. geometric aspects but only a single color value.
>         https://github.com/KhronosGroup/glTF/tree/master/extensions/2.0/Khronos/KHR_lights_punctual
>
> b. Meanwhile Physically Based Rendering (PBR) requires more sophisticated parameters, which are included in core glTF material properties but do not have corresponding light assets defined.  It seems that glTF implementations are allowed to have custom lighting parameters.  This explains why some rendering comparisons of glTF models appear to be rendered differently by different tools.

Point b. here is not entirely correct, or omits some crucial info:

- There *is* a glTF extension to define image-based lights, aka
"environment lights". See
https://github.com/KhronosGroup/glTF/tree/master/extensions/2.0/Vendor/EXT_lights_image_based
. It's basically a cubemap + some parameters. We should look at it
closely when designing `EnvironmentLight` node (as well as at
`PhysicalEnvironmentLight` from Timo Sturm paper).

- So, while glTF *core* specification does not define any lights, but
there are glTF extensions to define "punctual" (point / directional /
spot, quite similar to X3D) and "image-based" (aka environment)
lights.

- Both "punctual" and "image-based" lights can work with PBR
materials. They also all can work with Phong materials (i.e. existing
X3D Material nodes). All the lights answer a question "how much light
(of what color) comes from a given direction". It's just that
"image-based" lights answer this question using quite different data
input.

- If you want your scenes to look pretty, then you will probably want
to use some "image-based" lights. But "image-based" lights are not a
necessity of PBR. You can use "punctual" lights (aka "classic X3D
lights") with PBR materials as well.

- More: See my PBR notes on
https://github.com/michaliskambi/x3d-tests/wiki/How-to-add-PBR-to-X3D%3F#what-about-lights
, section "What about lights?". Disclaimer: I did not yet implement
environment lights in CGE, my knowledge is based on what I read and
observed.

Regards,
Michalis