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Ray threshold setting
Posted: 16 August 2017 10:53 AM   [ Ignore ]  
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Joined  2012-03-21

I’m confused about the wording in the help file regarding Ray Threshold.  On one hand, it seems to say that setting it higher would mean that less rays should contribute since more would fall under that number in terms of energy.  But then it goes on to say that setting it higher means longer render times.  I just want to know:

Does a higher number or a lower number mean longer renders?
What am I sacrificing by getting a shorter render time?

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Posted: 16 August 2017 03:11 PM   [ Ignore ]   [ # 1 ]  
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Hi gvzmedia,

I assume that you refer to this text:

Ray Threshold [0..100%]

This value helps to optimize render time. With complex scenes, particularly those containing many reflective and transparent surfaces, 90% of the processed rays contribute less than 10% to the general picture brightness and color. With a threshold value of, for instance, 15%, the rays stop their movement from the camera into the scene as soon as their brightness falls below this value.

For reflections/transparencies, this generally means that the higher the value, the greater the degree to which very small reflections/transparencies will be taken into consideration - with correspondingly longer render times.


Very simplified said, it is the information produced from a given surface for example, based on its transparency or reflection. Think of many mirrors set up so the one that is “calculated’ can see another mirror, of course this mirror reflects something, and so on. This “chain” of visibility based on mirrors is explored by that mirror. Since nothing really reflects 100%, after a while the mirror on the chain’s end will be not given a huge change anymore. This change is then measured by the threshold. The ray stops here. Two mirrors set up parallel could reflect for ever and crash the system perhaps, but since each reflection will naturally have a little loss, if not set to an un-natural value of 100% or even more (yes it’s possible), this threshold will stop it.

.
Yes it is confusing to use the “value” here without specifying if the value means the threshold or the value of the brightness. Small reflections in the image can have a high value, and even very small, based on its high value, it is included, even if the object has only the size of a pixel. Typically the term parameter is used for entries and value is used for a given appearance in the scene, based on a parameter, perhaps that old rule is not really applied clearly. All the three “depth” parameter work in the same way, but with with a fixed number of “iterations” instead of “signal power”.

I hope the function of this parameter is clear now as I think you understood it correctly from the start.

All the best

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Posted: 17 August 2017 09:23 AM   [ Ignore ]   [ # 2 ]  
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It’s still confusing.  Currently the default setting is .1%.  I raised it to 15% and I think I saw improvements in rendering time. Does that mean that before it was waiting until the energy of the rays dropped below .1% to stop their bounce?

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Posted: 17 August 2017 09:34 AM   [ Ignore ]   [ # 3 ]  
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My quick test file shows me that at 100% there are no reflections at all.  If I drop it to 50, then 10, then finally .1%, I get better and better reflections but the render times go up.  So it would seem to me, that the number indicates at what level of energy the rays stop bouncing.

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Posted: 17 August 2017 02:35 PM   [ Ignore ]   [ # 4 ]  
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Hi gvzmedia,

Yes, if the result of any given scene element combination leads to a lower value in the image than the threshold, the ray will stop exploring the scene.

You certainly have the Ray Depth and Reflection Depth set to the highest amount, otherwise this would limit your exploration.

.

Threshold 100%, if the reflection “Brightness” nor the Illumination is set higher than 100%, in other words below 100%, then typically one would not see any reflection. The render-time based on reflection will be very short.

Threshold 50%, if the next reflection value (again that would be the value in the image) will not be higher than 50% then the reflection algorithm will stop here, and the render time will not increase for this reflection.

Threshold 10%, anything that results in a higher reflection value in the image will be rendered, anything below will not, the render-time will be longer than the 50% but shorter than the 0.1%

Threshold 0.1%, as for all other calculations above, this value is dependent on the values in the scene, like the reflectivity and the, e.g., the Illumination [Material] of an object, which can be 1000% for example. From all four examples, given the same scene and both depth values high, this will result in the longest render time of all four.

All the best

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Dr. Sassi V. Sassmannshausen Ph.D.
Cinema 4D Mentor since 2004
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Posted: 17 August 2017 03:13 PM   [ Ignore ]   [ # 5 ]  
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P.S.: Please have a look at this scene file

https://www.amazon.com/clouddrive/share/o3iQYyXhWuKJjiIvcAV7YlmT7kx4ihRlVvfaIRkfwBu

Go to takes, use Render all Takes, and I hope that the scene itself will explain everything.

If you increase the Reflection brightness and/or the Luminance over 100%, the results will change.

Digging deep to understand each part in the application is certainly a good way to feel very comfortable at one point, keep going!

My best wises,

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Dr. Sassi V. Sassmannshausen Ph.D.
Cinema 4D Mentor since 2004
Maxon Master Trainer, VES, DCS

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