Retrace threshold parameter

[Karonte]

Hi Vlado.
I think Retrace threshold parameter is a good tool to solve some problems.
The fact is that my render are slow ..sometimes twice.
so i ask...what is the meaning of the value "1" in this parameter?

thanks

[Da_elf]

how can you say you think it would be a good tool (meaning its not there) then ask what a value of 1 means?

[Karonte]

It is a good Help with "use LC for glossy rays" but i don't know if i have to' use "1" or i can change this value . This value is related to' ?
Thanks

[Da_elf]

where is Retrace threshold?

[absolute3d]

You can look there, example 4

Ciao

[JohnVK]

Yes, but it still doesn't explain the impact of changing the retrace value. The impact of 0 (no retrace) and 1 is clear from the examples. But what about 0.5? What does that do and how does this impact quality / IR map rendering time / etc.

This is what Karonte is asking. I would like to know as well

[Da_elf]

ooooooooo. its new to 2.0. sorry still on 1.5 thats why i didnt see it

[zeronove]

bump to push some further infos of retrace threshold values,
what are related to?
is it the same of the photon mapping?
what are the purpose of values over 1?

[Morne]

still no answer here? Any way to speed up the retrace? Towards 0 speed up but less quality, towards 1 slow down A LOT but better quality? Is that about the just of it?

[vlado]

Sorry for the late reply, I missed this thread. Think of it as controlling the length of a ray after which the light cache is used. So if it is 0, the light cache is always used; as the retrace amount gets bigger, the primary engine will be used closer to the start of the ray.

I've tried to make a sketch here, hopefully it will make things clearer.



Best regards,
Vlado

[bardo]

so..is it correct to ask you to display the unit used?? As, for example, in the world DE option where, if set, it displays the unit measure used.

Thanks for the sketch above, now it's more clear.

[Morne]

Sorry for the late reply, I missed this thread. Think of it as controlling the length of a ray after which the light cache is used. So if it is 0, the light cache is always used; as the retrace amount gets bigger, the primary engine will be used closer to the start of the ray.

I've tried to make a sketch here, hopefully it will make things clearer.

[ATTACH=CONFIG]7005[/ATTACH]

Best regards,
Vlado

So then before this feature came along, which GI was used for a ray?
Just asking because I'm also getting double rendering times with a value of 1

[vlado]

So then before this feature came along, which GI was used for a ray?

In the context of the picture, always the secondary engine (the light cache) - as though the dotted line is right at the beginning of the secondary ray.

Best regards,
Vlado

[vlado]

so..is it correct to ask you to display the unit used?? As, for example, in the world DE option where, if set, it displays the unit measure used.

It does not have a fixed unit; it's a coefficient that takes into account the light cache sample size and the properties of the surface that spawned the secondary ray (i.e. how glossy it is etc) so that you get an optimal result. You can only be sure that smaller values will generally move the line closer to the start of the ray (and thus make things faster) than larger values (which will be slower, but more accurate). You can sort of visualize this if, for example, you compute the light cache with a red global light tint, save it, return the globa light tint to white, and render a normal image. Then parts that are red will show you areas where the light cache is used, and where not - something like this.

The light cache with red lighting.


The image with normal lighting, brute force GI for primary engine, light cache with the red file, "Use light cache for glossy rays" enabled, but retrace is disabled. You can see that the light cache is used everywhere for the glossy reflections.


Same as before, but retrace is enabled with the default value 1.0. You can see that the primary engine (brute force) is used for reflections close to the mirror, and also around the contact areas of the box with the ground.


Same as before but with retrace threshold set to 5. This expands the area where the brute force engine is used, which makes the rendering also slower.


Same as before but with retrace threshold 0.5. This shrinks the area where the brute force engine is used and makes rendering faster.


Best regards,
Vlado

[vlado]

Here are some more examples:

Different glossy values; for materials with high glossiness, the brute force method is used more (otherwise there are artifacts when you see the light cache).


For materials with low glossiness, the light cache is used more, since it is blurred anyway, so no need for a higher precision.


Different light cache sample sizes. For large sample sizes, the brute force method must be used more often to hide the artifacts from the large samples.


For smaller light cache sample sizes, we can use the light cache more, as the artifacts would be smaller.


Best regards,
Vlado