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After creating an animation, I decided to try and optimize my render settings so I could get the final result much faster. (I am using cycles) A lot of people advised me to lower the bounce count, so I did just that. In order to make sure I wasn't losing detail, I decided to run test renders with various bounce counts.

During this testing, I encountered something strange.

Inside my scene, I have a somewhat shiny object with low roughness (Principled BSDF, nothing else). I also have an object with an emission shader. When rendered, you can see the reflection of the emissive object in the shiny one. However, when I set the bounce count to 0, this reflection is still visible, even though it should not be because, well, its a reflection and 0 bounces should mean there are no reflections and the scene would just be shrouded in complete darkness unless the ray hits a light source.

Why is that so?

Unfortunately, I cannot share any images because I have saved them in OpenEXR format.

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Light paths inherit the proprety after the first bounce

Short answer is that the bounces number is not referring to the occoured light bounces but only to the "diffuse/glossy/transparency reflection bounces" as said in the tooltips:

enter image description here

The manual define the light paths as the following:

Reflection and transmission rays can further have these properties:

  • Diffuse: the ray is generated by a diffuse reflection or transmission (translucency).
  • Glossy: the ray is generated by a glossy specular reflection or transmission.

Imagine the following situation (path tracing actually works the other way around, but let's forget that for a moment):

The photon is emitted by light source, notice that at this moment the ray associated with the photon is "neutral", it hasn't hit any surface so it can't be defined as "diffuse" or "glossy", it's just a ray.

Now the photon hits a surface. If the surface is diffusive it generate a Diffuse ray, if it's reflective it generate a Glossy ray. Now, if the Diffuse/Glossy ray is captured from the camera, how times the Diffuse/Glossy ray bounced?

None. It has just been generated. It is visible by the camera, but has bounced none.

The question those numbers are answering can be better phrased as:

How many times are the Diffuse rays allowed to bounce in my scene?

enter image description here

If you picture yourself that the Diffuse rays are the little arrows and not the long one, you'll have an easier time in understanding the values.


Disclaimer: I dunno if it's the whole truth, but it makes sense and I thought it whort sharing my point of view

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    $\begingroup$ This is such a good explanation, thanks :). $\endgroup$ – Jachym Michal Jul 3 at 8:35

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