Retrace threshold - understanding the slider's value

[Fredrik Averpil]

Hi,

I believe I understand what the retrace threshold option does, but I don't quite understand how a value of 0.1 vs 0.9 differs in theory.

Is it correct to assume that a value of 0.9-1 will retrace pretty much everywhere V-Ray believes the Light cache is insufficient (slower to render), and a value of 0-0.1 will retrace less (faster to render)?
Is it also correct to assume that the retrace threshold option will retrace in glossy reflections (where V-Ray believes that the Light cache is not accurate enough) and not only in geometrically dense areas (such as corners, hair etc)?
Is it safe to use a low value for retrace threshold (such as 0.2-0.4) for animation or could this become a source of flicker?

[tashko.zashev]

Hi Fredrik,

Activating the LC retrace threshold option will force V-ray to use Brute force method for some areas. It is similar to the Detail enhancement option.
Low Retrace values (0,3) will be faster to render, but may be not enough to prevent light leaks. High values will increase the render time but the GI will be more precise. In my tests values between 0,4 - 0,8 work fine, but it depends on the scene i think.

I made a little example.



Hope this is helpful.

[Fredrik Averpil]

Hi Tashko,

Thanks, that helps.

So, since it initiates Brute Force, I am guessing it improves sampling of GI, lights, reflections, refractions, shaders (everything being secondary samples and not just the GI), am I right?
And the threshold value itself determines whether an area is problematic or not, and if Brute Force should get initiated there or not, am I right?

[vlado]

So, since it initiates Brute Force, I am guessing it improves sampling of GI, lights, reflections, refractions, shaders (everything being secondary samples and not just the GI), am I right?

It only affects cases where the light cache is used.

And the threshold value itself determines whether an area is problematic or not, and if Brute Force should get initiated there or not, am I right?

This is correct, yes.

Best regards,
Vlado

[vlado]

Here are some more examples, I should really put these in the help index:

http://forums.chaosgroup.com/showthr...441#post523441

Best regards,
Vlado

[Fredrik Averpil]

It only affects cases where the light cache is used.
This is correct, yes.

Okay, so for example, if I've got a case of bad Light Cache accuracy somewhere where my material is a not so glossy and reflective material (brushed aluminium for example), not only will the GI get retraced by Brute but also the glossy reflections will get Brute'd, right?

[vlado]

Okay, so for example, if I've got a case of bad Light Cache accuracy somewhere where my material is a not so glossy and reflective material (brushed aluminium for example), not only will the GI get retraced by Brute but also the glossy reflections will get Brute'd, right?

In general, yes, especially if you use the "Use light cache for glossy rays" option.

Best regards,
Vlado

[Fredrik Averpil]

Ahhhhhhh... now I get it.

The examples you posted were really good... but just one last question, heh heh. If my primary engine is not Brute Force, but instead I'm using Irradiance map as primary engine -- will the areas which are not as accurate get Brute Forced or will the Irradiance Map be used instead?

[sharktacos]

This has all been very helpful. What I'm still a bit confused by is the term "threshold." In the AA image sampler (color threshold), and in the DMC sampler (adaptive threshold), threshold is the sensitivity. So a lower threshold means a higher quality render and slower render times. With this it seems to be the opposite: a higher retrace threshold means a higher quality render and slower render times. With that in mind I'm wondering if "threshold" is not the best word to use here, and instead if it would be more clear if it was called the "retrace amount" instead?

[vlado]

The examples you posted were really good... but just one last question, heh heh. If my primary engine is not Brute Force, but instead I'm using Irradiance map as primary engine -- will the areas which are not as accurate get Brute Forced or will the Irradiance Map be used instead?

The irradiance map will be used for the primary GI bounces seen in glossy reflections and refractions.

Best regards,
Vlado

[vlado]

This has all been very helpful. What I'm still a bit confused by is the term "threshold." In the AA image sampler (color threshold), and in the DMC sampler (adaptive threshold), threshold is the sensitivity. So a lower threshold means a higher quality render and slower render times. With this it seems to be the opposite: a higher retrace threshold means a higher quality render and slower render times. With that in mind I'm wondering if "threshold" is not the best word to use here, and instead if it would be more clear if it was called the "retrace amount" instead?

Well, strictly speaking a threshold is just a value that divides two things... but I agree that "amount" might have been clearer.

Best regards,
Vlado

[sharktacos]

Haha, fair enough Vlado

Another question: I know that for "Use light cache for glossy rays" option it does not work to save out the LC map (for walkthrough animations) since the information is not saved to the LC map. Is the same true for the retrace threshold or is it possible to use this with a "from file" LC map in order to avoid light leaks?

[vlado]

It is possible to use both options for fly-throughs, actually.

Best regards,
Vlado

[sharktacos]

Can you explain what you mean Vlado?

Here's what I was referring to:
"We are not using glossy reflections in this scene, but if you are using them and not using the Use light cache for glossy rays option at the same time, then you can set the Secondary GI engine to None for the final animation. This is because you won't need the light cache and turning it off will save memory." (source)

and also
"Light Cache for glossy rays is not saved neither Irradiance Map nor in LC map - this option could only be used when LC is calculated on the fly but it has impact even if IM is loaded from file. So if you want to use it LC for secondary bounce should be always ON even if you already have it saved in IM. This means LC is calculate twice - once for IM and another once again for glossy rays but since in both cases LC speeds up the calculations a lot the final rendering of scenes with a lot of glossy effects will be much faster than if you have it calculated only for IM." (source)


Based on that, here's the scenario: I have an animation with no moving objects. I render a single frame LC/LC in flythrough mode and save the LC map. I set the LC to "from file" and render another frame IM/LC and save the IM map. I now set the GI to IM/none (secondary GI off) with the IM "from file" and render the animation. So since LC is off when I render the animation the options for "use glossy" and "threshold" obviously would be disabled. I don't think that info gets saved to the IM (or LC) maps. So the only thing I could imagine you might mean is that you could render the whole animation with IM/LC (with the LC set to "fly through" and not "from file") so that the LC is calculated for every frame (in addition to the LC that was already baked into the saved IM map) and the "use glossy" and "threshold" options would then work. Is that right?

The reason I'd prefer not to have the LC on for every frame is that it is a lot faster to just render IM/none than it is to render IM/LC, and it seems like it defeats the purpose of saving the LC map if I'm just going to have it on in the end anyway. Unless of course there is some magic where it could use the saved LC map (which is not baked into the IM map) and use that for GI stuff and use the new LC stuff just to figure out the blurry reflections and light bleed stuff without doubling up the render times. That would be cool.

[vlado]

Well, the "use light cache for glossy rays" and the "retrace" options are only used if the light cache is enabled. If the secondary engine is not the light cache, then these options have no effect.

Best regards,
Vlado