[x3d-public] Physically Based Materials (PBR) in X3D

[Michalis Kamburelis]

2018-03-16 14:05 GMT+01:00 Don Brutzman <brutzman at nps.edu>:
> Regarding backwards compatibility.  It would be interesting to consider
> whether, instead of adding new nodes, we might go ahead and completely
> extend existing Material node and other lighting-related capabilities.  This
> has potential benefit of keeping the X3D v4 scene graph more closely
> consistent for X3D v3 and VRML97 content, allowing superior rendering of all
> content as the default rendering mode in X3D v4.  A backwards-compatibility
> rendering mode also might be possible for when authors explicitly want it...

To my knowledge, such upgrade path (to just take old X3D 3 content and
render it with PBR) is not possible, in the general case. Not without
significantly changing the look of X3D 3 content, in a way that is not
necessarily better (when the conversion from "Phong parameters->PBR
parameters" was done automatically).

The parameters for PBR (albedo, roughness) are really different than
non-PBR Phong (diffuse, specular). The material specification is
different, and the light specification is also different (lights in
PBR need an "environment map", like ImageCubeMapTexture, to make these
cool reflections you see on the screenshots from PBR renderers).

IOW, the materials require different preparations by the graphic
artist / content creator. The PBR workflow for graphic artists (and
how it differs from the workflow for Phong lighting model) is nicely
described on https://www.marmoset.co/posts/physically-based-rendering-and-you-can-too/
. You cannot just toggle a boolean flag in the renderer to say "render
this with PBR".

Unity3d had the same choice, and they decided to *not* upgrade
materials in existing projects. They keep a large set of shaders doing
Phong lighting, and the existing materials use these Phong shaders.
The Phong shaders can be even still used in new projects. If someone
wants to convert his/her project to PBR, then (s)he must walk through
all the materials and lights, and upgrade them, setting suitable
parameter values, and creating new textures as necessary. The
resulting look with be unavoidably different anyway.

That's because, as far as the math is concerned, the PBR equations are
not a "more general form" of non-PBR equations. PBR equations impose
some limits that the non-PBR equations did not.

Regards,
Michalis