Several years ago, I mentioned the Italian company CoeLux, which specializes in making artificial windows: light fixtures that look like sunlight in a clear blue sky.
The price of their products is apparently in the range of several tens of thousands of dollars (I’ve heard prices like $20k to 50k), which makes it out of reach for most individuals. Not many details about their invention are available either (from the promotion material: LED powered, several hundred watts of electrical power, a solid diffuse material, and a thickness around 1 meter), and I was left wondering what was the secret sauce to their intriguing technology.
The window in this photo is in fact an electrical light fixture.
The YouTube channel DIY Perks has been working on day light projects for a while now, improving at each iteration. Yesterday they published a video explaining how to build a light that seems to give very similar results as CoeLux’s product, from some basic materials that are fairly simple to find. Since their solution takes roughly the same volume, it’s tempting to think it uses the same technique
It’s extremely satisfying to finally see how this works and, despite the practical aspects, quite tempting to try if only to see how it looks in real life.
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.
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.
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).
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 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.
In this 2014 talk, one of the designers of the original ARM processor, Sophie Wilson, gives an overview of the history of processors and what to expect in future.
The presentation covers in layman’s terms topics like Moore’s law (obviously), pipelining, parallelism, power consumption, heat dissipation, processor specialization and cost of production among other things. As explained, all those aspects are facing difficult challenges that are likely to shape the future of microprocessors, which in turns impacts both hardware and software engineers.
On March 7th, route of flight Alaska Airlines 870 was slightly adjusted to meet the path of the solar eclipse happening on that day. On board was astronomer Mike Kentrianakis, who recorded the following video where the eclipse is visible from begin to end.
So that’s my job in a sense: search other worlds for alien life.
So when I’m on a long plane flight, like coming over here, and the guy sitting next to me says: “So what do you do?”. Chatty fellow. I say: “Well I search other worlds for alien life.”. And then, he leaves me alone for the rest of the flight, I can get some sleep. It’s a great job description, I like it.
This excerpt is part of the introduction of the following lecture by NASA scientist Dr. Christopher McKay, on the search of life beyond Earth. He talks about Mars exploration, a potential mission to Enceladus, the challenges of field research in such environment, how to detect life (without accidentally destroying it in the attempt), how to avoid contamination and what would be some practical consequences to finding life. It’s a great insight into the current state of the field, delivered with an entertaining tone.
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.
