Help understanding Secondary bounces

[skogzort]

1) Why do we have access to both Primary AND Secondary bounces?

2) Why is there no option to use an Irradiance Map for Secondary bounces?

3) What are the ramifications of using IR map for primary and QMC for secondary V.S. IR map for primary and none for secondary?

Thank you very much for the info

[Paul Oblomov]

1) Simple: as it can be lots faster and it can be used for tweaking lighting (so you setup intensity, overral lighting quality with only first bounce selected and use secondary when you're satisfied to continue tweaking). And you can select which method to use: QMC for brute force (or LC in PPT mode) OR Irrmap + Photons / LC. Every method has its own pro and contras. Choose whatever you like

2) Well, I think it can work (in theory), but this method needs additional information to be able to compute glossy etc., thus, it's faster (and results are better) when there's a smth, that computes this things better

3) You just don't use GlobalIllumination by doing that way.

P.S. It's all IMHO, it can be not as in reality tho

[skogzort]

Thank you for your help!

I think you may be incorrect for # 3. What do you mean by “Global Illumination?”

The VRay documentation defines GI as “Indirect Illumination”. Primary and Secondary bounces are both considered GI, so if you used an IR map for Primary and none for secondary, you would still be using GI. Did you mean something other then "GI" by “Global Illumination”?

The only explanation I have found so far for Secondary Bounces is: “A secondary bounce occurs when a shaded point is used in GI calculations” This does not tell me why I should want to adjust “Secondary” instead of “Primary” bounces, or what I sacrifice by not calculating secondary bounces at all. I understand that adjusting Primary Bounces can make my rendering darker or lighter, but what does Secondary Bounces do?

What do I loose by not calculating them at all?
What do I gain by adjusting them that I can’t do by simply adjusting Primary Bounces?
Why can I use the other GI methods, but not an IR map to calculate them?

Thanks again for the help

[percydaman]

afaik, if you used IR for first bounce and nothing for second, it would be the equivalent of setting up a thousand spotlights in a dome around your scene, and rendering with scanline. Your shadowing would be shadowmaps, simulating the shading occlusion of the skylight.

[Paul Oblomov]

skogzort, you surely need all this stuff in your head ? Maybe you should googlin' a lil ? There's a lot of theoretical stuff...

I understand that adjusting Primary Bounces can make my rendering darker or lighter, but what does Secondary Bounces do?

Imagine you have only 1 light bounce (it strikes from camera view and flies untils it reaches geometry) - when light ray hits the geometry it just won't bounce as it should in real life, instead it will be completely faded (I mean it'll just disappear). This is no GI, just direct lighting. But when you turn on secondary bounces, light will bounce as it should, flying around the scene and VRay will react on every point, where light felt on geometry. Every "incident" is a bright pixel on image. Thousands of 'em - completed render

I can't write such a poems

[skogzort]

Thanks to all for your comments and help. I do appreciate them. What you described for direct light is correct, but what you described for Secondary Bounces actually starts with the Primary Bounce. In fact the Primary Bounce makes up the majority of the GI in the scene. Please let me try to clarify myself. I’m not asking a theoretical question, I’m asking a practical one about using the “Primary and Secondary Multiplier settings”:

What is the relationship between these two distinctly separate methods to affect GI and why would we need both?

Why do we have access to Secondary Bounces, which affect indirect illumination (GI) when we can already affect indirect illumination with Primary Bounces?

I can make the GI in my rendering stronger or weaker by adjusting the "Primary Bounce Multiplier", What can I use the "Secondary Bounce Multiplier" for?

Thank you for your time

[Paul Oblomov]

M8, you really need to read some papers.

Just try to get some color bleeding effect with just primary bounces... and you certanly will understand why we need secondary ones

[skogzort]

Secondary Bounces are used to achieve color bleed.

Got it!
Thank you again. I very much appreciate it. As for reading papers, believe me when I say I would much rather read about it than ask a question in a forum. (Even though the forum does exist for this very reason). I saw upon a closer inspection that this info is indirectly implied in the documentation, my bad for not catching it the first time. I apologize to anyone who thought the question was stupid or lazy.

[RErender]

Well not just color bleed, maybe this will help.

first image is primary bounces only, notice the light stops soon after bouncing off the first wall.



In the second image I've turned on secondaries (in this case the LC). now notice how the light "bounces" all the way to the end of the box.

[jacksc02]

some well drawn arrows there eric!

[RErender]

some well drawn arrows there eric!

those are light waves...


:P

taking donations for a wacom, pm for paypal info, Thanks!

[jacksc02]

light waves...... riiiiiight.........

[ene.xis]

some well drawn arrows there eric!

those are light waves...


:P

taking donations for a wacom, pm for paypal info, Thanks!

[Candelero]

some well drawn arrows there eric!

those are light waves...


:P

taking donations for a wacom, pm for paypal info, Thanks!

I love the ( arrows'waves ) ... they do really look natural thus.

Best regards,
nikki Candelero

[ene.xis]

i remember a long time ago a realyy cientific but easily-understandble( ) description on secoundary bounces....i dont remember where though...