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<p>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.</p>
<p><b>Summary: </b>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.<br>
</p>
<p><br>
</p>
<p>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.<br>
</p>
<p>First I wish to define a few terms:</p>
<ol>
<li>Geometry - the collection of quads, triangles, lines, points,
and other surfaces that comprise all of the geometry elements of
the object. </li>
<li>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</li>
<li>Rigging - the process of defining a skeleton and attaching the
surface to the skeleton for animation</li>
<li>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. </li>
</ol>
<p>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.</p>
<p>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.</p>
<p>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
(<a class="moz-txt-link-freetext" href="https://blog.turbosquid.com/2017/07/13/selling-3d-models-online/">https://blog.turbosquid.com/2017/07/13/selling-3d-models-online/</a>).
#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.</p>
<p>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).</p>
<p>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.<br>
</p>
<p>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.<br>
</p>
<p><br>
</p>
<div class="moz-signature">-- <br>
<font class="tahoma,arial,helvetica san serif" color="#333366">
<font size="+1"><b>Leonard Daly</b></font><br>
3D Systems & Cloud Consultant<br>
LA ACM SIGGRAPH Past Chair<br>
President, Daly Realism - <i>Creating the Future</i>
</font></div>
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