Implementing a Physically Based Shading without locking yourself in

Posted on 2020-04-15 by Julien Guertault

Over the last few months I have been trying to push my understanding of Physically Based Shading, by actively exploring every corner and turning over every stone, to uncover any area where I lack knowledge. Although this is still an ongoing process and I still have a lot to do, I thought I could already share some of what I have learned in the process.

Last weekend the Easter demoparty event Revision took place, as an online version due to the current pandemic situation. There, I presented a talk on Physically Based Shading, in which I went into electromagnetism, existing models, and an brief overview of a prototype I am working on.

The presentation goes into a lot of detail about interaction of light with matter from a physics point of view, then builds its way up to the Cook-Torrance specular BRDF model. The diffuse BRDF and the Image Based Lighting were skipped due to time constraints. I am considering doing a Part 2 to address those topics, but I haven’t decided anything yet.

In the mean time, please leave a comment or contact me if you notice any mistake or inaccuracy.

Abstract

How do you implement a Physically Based Shading for your demos yet keep the possibility to try something completely different without having to rewrite everything?
In this talk we will first get an intuitive understanding of what makes matter look the way it looks, with as much detail as we can given the time we have. We will then see how this is modeled by a BRDF (Bidirectional Reflectance Distribution Function) and review some of the available models.
We will also see what makes it challenging for design and for real-time implementation. Finally we will discuss a possible implementation that allows to experiment with different models, can work in a variety of cases, and remains compatible with size coding constraints.

Slides

Here are the slides, together with the text of the talk and the link to the references:
Implementing a Physically Based Shading model without locking yourself in.

Video

And finally here is the recording of the talk, including a quick demonstration of the prototype:

Interference shader

Here is the shader used during the presentation to illustrate light interaction at the interface between to media:

Acknowledgements

Thanks again to Alan Wolfe for reviewing the text, Alkama for the motivation and questions upfront and help in the video department, Scoup and the Revision crew for organizing the seminars, Ronny and Siana for the help in the sound department, and everyone who provided feedback on my previous article on Physically Based Shading.

Addendum

Following the publication of this article, Nathan Reed gave several comments on Twitter:

FWIW – I think the model of refraction by the electromagnetic field causing electrons to oscillate is the better one. This explains not only refraction but reflection as well, and even total internal reflection. Feynman does out the wave calculations: https://feynmanlectures.caltech.edu/II_33.html

It also explains better IMO why a light wave keeps its direction in a material. If an atom absorbs and re-emits the photon there is no reason why it should be going in the same direction as before (conservation of momentum is maintained if the atom recoils). Besides which, the lifetime of an excited atomic state is many orders of magnitude longer than the time needed for a light wave to propagate across the diameter of the atom (even at an IOR-reduced speed).

Moreover, in the comments of the shader above, CG researcher Fabrice Neyret mentioned a presentation of his from 2019, which lists interactions of light with matter: Colors of the universe.
Quoting his summarized comment:

In short: the notion of photons (and their speed) in matter is a macroscopic deceiving representation, since it’s about interference between incident and reactive fields (reemitted by the dipoles, at least for dielectrics).

From Maxwell’s equations to Fresnel’s equations

Posted on 2020-03-22 by Julien Guertault

This series of short videos shows how to derive Maxwell’s equations all the way to Fresnel’s equations. Each one is about 10 to 15mn long.

The first four videos show how to use boundary conditions to deduce the relationship between the electromagnetic field on both sides of a surface (or interface between two different media).

The next four videos use the previous results to obtain the Fresnel equations, for S-polarized and P-polarized cases.

The rest of the series then dives into other topics like thin film interference.

The series assumes the viewer to be already familiar with the Maxwell equations, so it can be helpful to first see the explanation by Grant Sanderson of 3Blue1Brown on Maxwell’s equations.

The physics of cloaking

Posted on 2015-12-06 by Julien Guertault

In this seminar, Dr. Greg Gbur presents the current state of research on cloaking devices, the differences between science fiction and what seem to actually be possible, and different applications beyond invisibility, like protection from thermal radiation or earthquakes.

GDC 2015 presentations

Posted on 2015-03-09 by Julien Guertault

The Game Developers Conference took place last week in San Francisco. As I am starting to see more speakers publish their slides, I am creating this post to keep track of some them (this list is not meant to be exhaustive).

Advanced VR Rendering, by Alex Vlachos (Valve).
Code Clinic 2015: How to Write Code the Compiler Can Actually Optimize, by Mike Acton (Insomniac Games).
Doing Math with RGB (and A) Correctly, by Jim Van Verth (Google).
Great Management of Technical Leads, by Mike Acton (Insomniac Games).
Parallelizing the Naughty Dog engine using fibers, by Christian Gyrling (Naughty Dog).
Physics for Game Programmers : Numerical Methods, by Erin Catto (Blizzard Entertainment).
Rendering the World of Far Cry 4, by Steve McAulley (Ubisoft Montréal).
VR Direct: How NVIDIA Technology is Improving the VR Experience, by Nathan Reed (NVidia).

For a more extensive list, Cédric Guillemet has been garnering links to GDC 2015 papers on his blog.

History explained

Posted on 2012-07-05 by Julien Guertault

So yesterday was the announcement of a massive breakthrough: CERN have discovered a new particle thought to be a Higgs boson. This is a milestone discovery future generations will have in their textbooks. Maybe it is a good time to remind the excellent animation Jorge Cham published a few months ago, explaining in simple words what the Higgs boson is. (it is also a good time to have a look at the photos taken soon before the launch of the Large Hadron Collider)

Looking Around Corners using Femto-Photography

Posted on 2012-03-21 by Julien Guertault

The MIT Media Lab, that I mentioned previously when they published their experiment consisting in filming the very propagation of light, strikes again by using their device to take pictures of objects hidden from the line of sight.

Update: there is now a TED talk presenting femto photography.

Filming the propagation of light

Posted on 2011-12-13 by Julien Guertault

The MIT Media lab published some jaw dropping videos of experiments consisting in filming the very propagation of light. By repeatedly capturing a very short beam of light, they reconstruct images of its propagation.