Game Engines 2021W Lecture 19

Resources

Ray tracing

• Sarah Abraham's Computer Graphics lecture notes, especially her notes on basic ray tracing and ray tracing pseudocode.
• Scratapixel's introduction to ray tracing and implementing ray tracing.
• Photorealistic rendering and the ray tracing algorithm from Physically Based Rendering: From Theory To Implementation by Matt Pharr, Wenzel Jakob, and Greg Humphreys.

Godot's SDFGI & related tech

• Juan Linietsky, Godot 4.0 gets SDF-based real time global illumination (2020)
• Daniel Wright, Dynamic Occlusion with Signed Distance Fields (SIGGRAPH 2015)
• Jamie Wong, Ray Marching and Signed Distance Functions (2016)
• Jinkai Hu et al, Signed Distance Fields Dynamic Diffuse Global Illumination (2020)

Notes

Ray Tracing


Key questions:
 - how does it work?
 - why is it so slow?

How does light work, really?

"Ground truth" for how light works is Quantum Electrodynamics
 - i.e., quantum mechanics for electromagnetism


QED
 - you have photons, and you have electrons
 - photons are absorbed by electrons, and then they are emitted by electrons
   - when absorbed, electrons go to "higher" orbitals (higher energy)
   - when an electron moves to a lower energy orbital, it emits a photon

so, to model light, ultimately we're modeling how electrons and photons interact

Where does color come in?
 - frequency, i.e. energy, of photons
   - have to be in a specific frequency band to be visible light photons

So, for rendering, why does this matter?
 - how things look is determined by how photons bounce around
   - their energy, hence their color, changes as they bounce around
   - brightness is how many photons you get

Light is painful to model entirely because
 - there are LOTS of photons
 - they bounce EVERYWHERE
 - and this all affects how things look


Perceived color is contextual
 - you don't know the color of anything until you know what its surroundings look like
 - human color perception is all about correcting for this 

physicists will say that photons have specific colors, i.e., their frequency
 - but actually, we perceive light based on the color of objects

Something that is "red" will not emit red light (unless it is a light source).  Instead, our 
brains model the scene and figure out it is red based on context
 - because different energy (color) photons will be emitted depending on
   lighting conditions

Dim light, different color light (indoor vs sunlight), all affect the "energy" of photons but 
we can tell the difference, we see the same color.

So even a pure physics-based model isn't enough, have to take into account human perception

This is all to say that ray tracing has its benefits, but is far from a completely general solution
  - we'll discuss this more

Ray tracing, we start with a received "photon", and we trace back its path to a light source
 - optimization, as we don't care about light that can't be perceived

rays won't follow a direct path
 - interesting lighting effects come from how the rays bounce around
 - have to account for much (but not all) of this bouncing

Part of the complexity is the recursive nature of the ray tracing algorithm

The other part is how rays interact with surfaces
 - they are diffracted and reflected

Earlier CGI tends to focus on reflections
 - think Terminator 2
 - a fully reflective surface is "easiest", i.e., liquid metal
   - because we just have to model the surface as rays bouncing off at
     angles defined by the surface

The real world is filled with surfaces that "capture" photons
 - they bounce around inside the object before releasing the photons

Movies use ray tracing with complex material models

But then, how are we doing "real time" ray tracing?
 - we're fast enough to do a lot of this recursion and support
   non-trivial surface models

Regular lighting algorithms are not recursive at all
 - We at most consider one bounce
 - we often model surfaces as "self lighting"


Godot does not support ray tracing currently
 - but this isn't that much of a disadvantage now, since it is rarely used
   in practice, and only in limited ways