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Comment by rollulus | Hacker News Reader

I'd say that it is rather obvious that ray tracing is not a new thing, since it is simulates how light physically behaves.

I consider this 3d rendering as a spectrum: rasterization requires little computation but has little to do with physics. Ray tracing is what requires a lot of computation and has everything to do with physics. Somewhere in between are hybrid methods: rasterization with ray tracing components added to it, or ray tracing with approximations.

For instance, pure rasterization cannot do shadows. It is approximated by rendering the scene from the viewpoint of a light and test the rasterized scene for occlusions casting shadows. And the other way around: real time ray tracing cannot compute all indirect lighting paths, a subset is only considered at the cost of e.g. variance.

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It's much simpler than that: Both rasterisation and ray tracing are methods to solve visibility. The main difference is that one answers the question "given a primitive, what pixels does it overlap?", the other "given a pixel, what primitives does it overlap?"

Light transport, shading, shadowing are all just implemented on top and not a direct result of the visibility calculation.

> For instance, pure rasterization cannot do shadows.

Well... the technique that you describe (shadow mapping) is actually pure rasterization it just requires more than one rasterization pass. This also ignores the fact that there are other techniques for getting shadows in rasterizing renderers (stencil volumes and other stuff that is rather considered historical today).

I get your point that rasterization doesn't support shadows "naturally" like ray tracing, but in my opinion your wording and the example is rather unfortunate. The same goes for reflections, I would say SSS or caustics probably are better examples since they are really only done with techniques based on ray tracing.

SSS has been done with point clouds for a long time. Only recently have films switched to ray tracing for SSS.

Games these days implement SSS in screen space using rasterisation, no rays tracing either.

Rasterization has just as much to do with physics as raytracing. They can both get you a correct solution to the rendering equation (i.e. be physically correct) if you wait long enough, or get you an approximation of it in a reasonable time scale.

For example if all you need are shadows cast from a point light source, a rasterization technique like stencil shadows will give you the same exact result as simple raytracing, and neither will be physically correct.

It is just that depending on your rendering time budget, some things are better done with rasterization and others are better done with raytracing. Real time engines, with simplified models typically work better with rasterization, while precalculated scenes, with more realistic models work better with raytracing.

Rasterization and raytracing are formally equivalent in a sense. You should be able to algebraically rearrange ray/triangle intersection tests performed in raytracing to get Pineda rasterization. So I don't really see one as more physical than the other. Rather the difference is that rasterization starts with each triangle and determines which rays intersect it, while raytracing starts with each ray and determines which triangles intersect it.

Historically, rasterization has been away of putting triangles onto the screen, maybe with a Z-buffer to determine visibility. It's basically an image space idea, with things like Gouraud shading happening in image space, and though you could extend it put it to use in calculating shadow volumes or shadow maps, it doesn't implicitly deal with light transport. That's the first difference.

Ray tracing, on the other hand, has always been about (forward or backward) rays of light propagating through object space. It wasn't about light transport in the early days (just visibility and shading, based on simple models like Phong) but it is very well-suited to modelling transport, because it addresses the notion of fully-spatial rays in object space.

Writing a physically based (light transport)renderer which was internally based purely on rasterization to rectangular images would be an odd choice, partly because many of the intermediate images would be have to somehow be parameterized to represent locations on a hemisphere, etc.

I'm open to correction on this, but rasterization algorithms are really tied to projections onto a rectilinear grid, orthographic or perspective. Ray-tracing doesn't need to assume/know about this raster grid idea and as a result can be used with other geometries. This makes it strictly more powerful than rasterization. This kind of thing, for example: https://www.glassner.com/computer-graphics/graphics-research... is a very bad fit for polygon rasterizers because each triangle is going to be warped in image space.

You can actually extend the rasterization concept to 3D, as shown in this paper: http://cg.ivd.kit.edu/publications/p2012/3dr/gi2012.pdf

Hmmm, yes, though they do say "In this paper we focus on primary (camera) rays, i.e. rays with a common origin or parallel rays, because only these are also covered by rasterization. We consider secondary rays and efficient global illumination algorithms, such as path tracing or photon mapping, as orthogonal to our approach."

Just what that "orthogonal" means is a bit mysterious, but their project seems to be to generalize rasterization further than they've got in this paper: "we aim for further generalization, in particular, a parameterization which allows for incremental computation, not only for the ray direction, but also the ray origin"

Even ray tracing does not capture all physics of electromagnetism and only works at the level of geometrical optics: any effect, like diffraction or iridescence, that arises due to the wave-like nature of light still need to be implemented in ad-hoc way in a ray tracing algorithm. But fully simulating Maxwell's equations (or QFT) to keep track of those minor effects would be insanely expansive.

> since it is simulates how light physically behaves.

I think ray tracing is at best an ad-hoc model for light that produces nice results, but physically based it isn't.

It's physically based, it's just not a complete simulation of all the physics involved. It's at least as physically based as most rigid body, cloth or fluid physics simulation.

Radiosity would qualify better as `based on physics` than ray tracing, but can't do things like mirrors. I could live with classifying ray tracing as `geometry based`, like Newton studied mirrors.

(You could do something that uses ray tracing to determine what's visible and what isn't and radiosity to determine the colour, but that's an entirely different story).

All rendering is an attempt to solve the rendering equation which is very much physically based. Whitted ray tracing is doing a lot of simplification but path tracing (which is closer to what people are talking about doing in real time with new hardware) is a pretty principled attempt to solve the rendering equation via Monte Carlo methods. I don't see how that's not physically based. The definitive book on this stuff is literally called "Physically Based Rendering" https://pbrt.org/

What is your qualm with calling ray-tracing physically based? Don't most models even include transmissivity and refraction?

I mean, unless you're expecting them to calculate absorption/re-emission at every bounce...

It still seems pretty "physical" to me.

Here you're using geometrical optics which models is a narrow beam (ray) which is idealized as a line. It all becomes simple vector math form there onwards.

However, Physics knows since the end of the 18th century light is a wave.

https://en.wikipedia.org/wiki/Young%27s_interference_experim...

It's not that the model breaks down only in extreme conditions (like Newton's laws of mechanices), but in day to day situations as well.

I think that's the essence of my qualm.

Where in day-to-day situations, like walking around outside or in a building do you see diffractive phenomena where a ray approximation breaks down far enough for you to notice?

Usually all of that is smoothed over by light sources being extended sources not points, so the interference contrast is lost by infinitely many interference patterns being overlaid incoherently. Also, almost all light sources (except for rlasers) have microseconds of coherent emission, so the pattern changes so fast it blurs into a regular blurry edge of shadow.

I can only think of some very special situations where some blinds select a very narrow angular range of sunlight and then you see interference fringes in the shadow.

Or when you look into a puddle with an oil film or at some sort of diffraction grating or holographic film (which can be predicted with ray-based methods, like Wigner-distribution based ray-tracing, though that still comes with some error at large angles).

Even in laser optics, 95% of the optics design is done with geometrical optics methods, because the rays you use can be related to the phase profile of the radiation in the system. You can then integrate (with rays) the diffraction pattern (but not as well in the shadow of apertures ofc).

plenty of examples, but https://en.wikipedia.org/wiki/Glory_(optical_phenomenon)

is a pretty one.

The wave-like nature of photons does not exclude the particle behavior. Light is both a particle and a wave.

Physicists still rely on snells law. Optics courses still includes path tracing when studying refraction and dielectrics.

Excluding the particle behavior of light just because the wave nature exists, is not something a physicist would do.

True, but the concept of ray used here is neither particle nor wave. The whole thing is way more geometry than physics.

I mean, you're arguing a term that's used in physics isn't physical. Raycasting is often used when solving EM equations.

Are you purporting that photon streams don't follow a ray? And when they interact with a surface they don't obey Snells law? And when you look at the interface between mediums a percentage of the intensity is transmitted and the remainder is reflected (the ratio of which is determined by the angle of incidence?)

Would you say the same thing about rigid body physics simulations? There's no such thing as an idealized rigid body in reality but such simulations use lots of useful approximations the are also used in "real" physics.

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