[x3d-public] Model Profile: Lights or Not

[Leonard Daly]

In the X3D WG call today there was a discussion of usefulness of lights 
with models. Some participants contend that certain models have lights 
and those need to be included with the model. An example of this would 
be building lights, running lights on vehicles, or emergency lights.

*Summary: *Including lighting in models is not the way the topic is 
taught in artistic educational institutions throughout the world. Model 
lights reduces the models usefulness -- it cannot be printed and take 
additional work to use in other applications/environments.


It is my contention that lighting is not the purview of the completed 
(static or animated) model. This position is explained and justified 
below based on industry development, limitations of use, and alternatives.

First I wish to define a few terms:

 1. Geometry - the collection of quads, triangles, lines, points, and
    other surfaces that comprise all of the geometry elements of the
    object.
 2. Textures - the collection of all surface or volume coloring. This
    include (flat) material, image and volume textures, physically-based
    rendering (PBR), and specialty materials usually expressed as shaders
 3. Rigging - the process of defining a skeleton and attaching the
    surface to the skeleton for animation
 4. Animation - The process of defining all motions and positions that
    the object may assume. This includes predefined motions (such as
    walking, running, etc.) and procedural ones.

In the Model profile I explicitly excluded lighting because lights are 
part of the scene. A light interacts with multiple models and the scene 
environment. The scene environment also interacts with lights (e.g., fog 
or mirrors). It might be argued that certain lights that only interact 
with the model could be used. Those lights would not interact with 
anything else -- either another model or the scene environment. All of 
the examples listed in the start of this post could interact with the 
scene -- building lights with other buildings, running lights on wet 
pavement, emergency lights with everything within range.

Note that even small dim lighting like dash lights would interact with 
anyone who gets in the vehicle, and the result would be different 
depending on the texture of the person.

Keeping lights out of models allow models to be used in many different 
situations. The WG looked at the TurboSquid site. None of the models 
that were examined included (model) lights. One person brought up the 
page on 11 Helpful Do's and Don'ts 
(https://blog.turbosquid.com/2017/07/13/selling-3d-models-online/). #2 
on that list is saying "DO" prepare your file (model) with lights and 
cameras. These are referring to external lighting and static viewpoints, 
especially those used to construct the model previews; these are not 
lights intrinsic to the model.

A (static) model with lights cannot be printed. The lighting changes how 
various parts of the surface appear and needs to include other changes 
to the environment, none of which is available to the printer. If the 
interior of a model needs to be lit, just bake the lighting into the 
texture. This goes for selective/localized lights (e.g., spot lights) or 
diffuse/panel lights (building windows).

Existing lighting equations are just approximations to what happens in 
the real world. Different types of systems (ray trace, Phong reflection, 
sub-surface scattering, etc.) are used for different levels of results. 
The closer to "reality" one needs, the more complex (and time-consuming) 
the processing. In nearly every situation, the lighting equation is out 
of control of the modeler. An assumption of a particular kind of 
lighting system for rendering can severely limit the use of the model.

Lighting is baked into the texture. This can be accomplished in several 
different ways depending on the model requirement. It can be done as a 
material with flat geometry and vertex colors, an image texture applied 
to the surface, PBR (if available), or highly-specialized materials 
implemented with shader code. I have not encountered a single example 
where a model absolutely must contain lights.


-- 
*Leonard Daly*
3D Systems & Cloud Consultant
LA ACM SIGGRAPH Past Chair
President, Daly Realism - /Creating the Future/
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