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In Cycles Render Properties > Light Paths > Max Bounces there is the Total field. What does Total mean?

If Total is set to 12, does that mean if any the other values (Diffuse, Glossy, etc.) is greater than 12, then it's clamped to 12?

Or does Total mean the sum of the other values, i.e., if the sum of the other values is greater than 12, then the sum is clamped to 12?

Or does it mean something else? I can't find a definitive explanation. Any suggestions? Thanks. Blender 3.5.1, Windows 11

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  • $\begingroup$ After a bit of brainstorming, I guess I could try to give you some guesses, just give me a bit of time to type my answer. $\endgroup$
    – mqbaka mqbaka
    Commented Jun 13, 2023 at 6:09

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Indeed, the official documentation for it does not give a lot of explanation about how it works : enter image description here

But after having thought about it for a while, I think I can try to give some sort of answer.

The word 'bounce' reffers to a ray that hits a surface (at least a second time) before reaching a light source. When a ray is cast from the camera, it's going to bounce around untill eventually reaching a light source. The total max bounces reffers to the number of bounces a ray can make before it is 'canceled'. The first hit is not considered a bounce for it's considered to be a direct hit (that's why when the number of bounces is set to 0, it results in direct only).

I put bounce in quotes because, it doesn't have to be a litteral bounce, the ray can be penetrating some transparent surface, or entering a volume etc... And that's where those values kick in :

Values for the max bounces

Those values above, for instance, mean that a ray can bounce at most once on a diffuse surface, at most 4 times on a glossy one, go through a reffractive surface at most 8 times, enter a volume at most 2 times and traverse a transparent surface 8 times at most. At the moment the ray exeeds one of those numbers, the ray is going to be killed. Atop of that, the total numbers of the bounces it makes is not allowed to exeed 8 which means, after 8 bounces, the ray will be also be killed.

As an example, let's consider two rays R1, R2 and R3 cast from the camera and the bounces they make in order :

R1 : camera -> diffuse -> transparent -> glossy -> transparent -> glossy -> reffractive -> light source.

R2 : camera -> transparent -> glossy -> glossy -> transparent -> glossy -> diffuse -> light source.

R3 : camera -> transparent -> glossy -> diffuse -> glossy -> diffuse.

In this case, only R1 and R2 are going to make it to the light source as R3 already exeeded the allowed amount of diffuse bounces, even though it is still under the max 8 bounces so it gets terminated before it could make it to the light source.

You can think of it as 'quotas'. Each ray has 8 bounces that can be whatever type they are as long as each bounce type stays under its 'quota'. If, after the 8th bounce, the ray still haven't reached a light source, it will be terminated.

The keypoint in understanding it is that the number of bounces the ray makes is computed on-the-go and the ray is going to be killed as soon as it exeeds one of its bounce quota so there is no clamping going on. It's like using a credit card where the user is just not allowed to spend beyond the card's limit and if he reaches the limit, the card is immediatly disabled.

I don't know if I made it clear enough but that's how I understand how it works. Anyway you can just comment out if some point of the explanation are fuzzy or inacurate.

I hope I helped.

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  • $\begingroup$ Thank you for your thoughtful response. Much appreciated! I think I understand the purpose of defining bounces/quotas for Diffuse, Glossy, etc., however, I still don't understand the role of Total and how its value influences the other values. $\endgroup$
    – WilburPost
    Commented Jun 13, 2023 at 16:20
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    $\begingroup$ As I explained in the answer, it doesn't influence the other values, it influences the render. Max bounces means that every ray that exeeds that number of bounce before reaching a light source is killed. $\endgroup$
    – mqbaka mqbaka
    Commented Jun 14, 2023 at 10:43
  • $\begingroup$ Thank you again for your response. I'm afraid I still don't understand. If, for example, Total is 12 while all the other categories (Diffuse, Glossy, etc.) are 24, does that mean all those other categories get their 24 bounces? If yes, then what is the purpose of Total? If no and Total restricts those values to 12, then isn't that clamping? $\endgroup$
    – WilburPost
    Commented Jun 14, 2023 at 16:34
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    $\begingroup$ As I told you before, it's not merely clamping. If total is 12 then the ray is killed after 12 bounces, that's all. Those 12 bounces may be all 12 diffuse,12 glossy or 6 diffuse and 6 glossy. Total bounces doesn't care about the type of bounce, it just kills the ray after 12 bounces. $\endgroup$
    – mqbaka mqbaka
    Commented Jun 15, 2023 at 5:25
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    $\begingroup$ If total is 12, no category is ever going to have 24 bounces as the ray is killed after 12 bounces. $\endgroup$
    – mqbaka mqbaka
    Commented Jun 15, 2023 at 6:01

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