# Can someone explain why raytracing isn't able to make Global Illumination?

I've watched a few videos about raytracing vs pathtracing, and one thing that consistently pops up is the idea that pathtracing seems to be able to do global illumination, whilst raytracing is not. Here's an example of what I mean (timestamp linked): https://youtu.be/LAsnQoBUG4Q?t=320

If the rays in Raytracing still bounces, why does it not create Global Illumination like Pathtracing does?

• The person in the video is talking about Whitted-style ray-tracing, which is limited to bounce rays only off ideal specular surfaces (reflection/refraction) and considers only Dirac delta light sources. Whether you choose to simulate the remaining bounces is up to you, and the process still uses ray-tracing. Commented Jun 15, 2021 at 20:29

"Path tracing is a form of ray tracing that can produce soft shadows, depth of field, motion blur, caustics, ambient occlusion, and indirect lighting."

So a path tracer is a ray tracer, but not all ray tracers are path tracers.

• It's also worth noting that (pre-Whitted) ray tracing was originally invented as a hidden surface algorithm, something that we now usually call "ray casting". But it's still a kind of ray tracing. Commented Jun 16, 2021 at 5:51

This question is the result of the modern era using the blanket word ray-tracing to denote every similar technique as it's based off of it. Just wanted to give a more historical answer if anybody is interested.

As lightxbulb said, historically, Turner Whitted was the first person to come up with the raytracing technique but it only had a limited number of rays. Reflection/Refraction and these secondary rays got terminated when they hit a diffuse surface since no GI rays were shot.

The problem of Global Illumination is recursive and as you might've guessed it requires many many rays. The math wasn't that complex and sophisticated in pure raytracing. Then later on, Radiosity method was introduced by Goral which accounted for GI but only for diffuse surfaces using the Finite element method. This was a major step ahead and helped in laying the foundation for pathtracing.

After that Pathtracing got introduced by Kajiya which accounted for all types of surfaces, He used Monte-Carlo Integration to solve the recursive integral related to the GI problem.

As time went by, the word raytracing somehow overshadowed the other words in this field. And recently, with NVIDIA dishing out their RTX technology pretty much sealed the fact. So raytracing in today's era doesn't necessarily mean whitted-style raytracing which was the original raytracing.

• I would refrain from attributing ray-tracing to Whitted. Appel's paper is from 1968 for example, and the idea is much older. Ray-tracing is called this way because you trace rays. In fact in the production of lenses they also use ray-tracing, but not in the context of Whitted-style ray-tracing or path tracing. This is the reason it is an umbrella term - because it simply describes processes that use the tracing of rays, regardless of the framework on top. If one wants to be truly precise, then path tracing is just numerical integration for the series arising from the rendering equation. Commented Jun 16, 2021 at 9:10
• I know that, used to be called ray casting way back then, but didn't want it to become a wikipedia historical article so I've dumbed down the answer. The original raytracing used to produce all these photorealistic images was infact given by Turner Whitted so no harm attributing the technique to him. Commented Jun 16, 2021 at 13:58
• Dumbing down the answer in a manner that makes it incorrect is not recommended. Whitted-style ray-tracing can be attributed to Whitted, ray-tracing in general cannot be. The usage of the term ray-tracing in literature is for any process that involves the tracing of rays. If one wants to be specific about the technique, they can be (i.e. Whitted, Kajiya, Cook, etc.). Then again, Kajiya's work subsumes Cook's work, which subsumes Whitted's work. From a scientific perspective both Cook's and Whiited's work are trivial, and wouldn't even be worth mentioning in most other fields. Commented Jun 16, 2021 at 14:06
• Like I said, the you are referring to the core idea of casting rays which was known as ray-casting when Appel used it. Ray-tracing got popular because of the whole algorithm of Turner whitted to produce photorealistic images. So when you say ray-tracing, you aren't talking about just casting rays generally. It was named Whitted-Style afterwards when the same technique started to be used in many other similar algorithms to differentiate it. Nevertheless, you don't like the answer just downvote it? don't wanna argue with you in the comments anymore, too troublesome. Commented Jun 16, 2021 at 14:13
• I don't plan to argue with you either, your opinion simply disagrees with the usual usage in literature. I will also leave this here: en.wikipedia.org/wiki/Ray_casting en.wikipedia.org/wiki/Ray_tracing_(graphics) "The idea of ray tracing comes from as early as the 16th century when it was described by Albrecht Dürer, who is credited for its invention.[6]" Commented Jun 16, 2021 at 14:19