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
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 
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
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.
skogzort, you surely need all this stuff in your head ? :shock: 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
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
M8, you really need to read some papers. :roll:
Just try to get some color bleeding effect with just primary bounces… and you certanly will understand why we need secondary ones
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.
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.
some well drawn arrows there eric!
those are light waves… :roll:
taking donations for a wacom, pm for paypal info, Thanks!
light waves… riiiiiight…
:lol: :lol:
I love the ( arrows’waves ) … they do really look natural thus.
Best regards,
nikki Candelero
i remember a long time ago a realyy cientific but easily-understandble( :lol: ) description on secoundary bounces…i dont remember where though…
well not as scientific as my memory recall it…but it might help:
http://www.chaosgroup.com/forum/phpBB2/viewtopic.php?t=13747&highlight=secondary+bounces
Wow – That post did have some good info. Thank you for adding it. The color bleed answer and RErender’s illustration have shown me why I would want to have secondary bounces turned on in VRay. This is good enough for me. I’m grateful for the info and ready to move on.
For the sake of explaining my original confusion it occurs to me after reading through all of these posts again that I should have worded my questions differently. I have always understood what primary and secondary bounces are. What I didn’t understand is the practical reason why Vray has a separate multiplier for each.
Consider:
If you increase the Primary Bounce multiplier, then obviously the intensity of secondary bounces will be increased. If you decrease the Primary Bounce Multiplier, then obviously the intensity of secondary bounces will be decreased. Since the primary bounce multiplier effectively controls both, I could not understand what practical reason you would use a separate secondary bounce multiplier for. It’s like having two volume dials on a stereo. It seems like it would make more sense to have a simple on/off check box for secondary bounces.
Much thanks again for everyone who participated.
ur absolutely right…from a scientific point of view…
Nevertheless the ability of tweaking this 2 values provides gr8ter control to achieve the look u need…instead of working ur math untill u get to it!
freedom! :lol:
u gotta LUV Vray!
Another reason for the primary / secondary bounce settings is that the primary or first bounce of light as mentioned earlier contributes a hell of a lot to the image and in certain cases such as light hitting off a bright surface, it can actually be quite detailed and sharp so you need a really high quality method of calculating it to get an accurate result. For secondary bounces, the light rays have already bounced off 2 surfaces (the direct light hit, then the first GI bounce) and unless you’re in a perfect environment where everything is a mirror like surface or flawlessly smooth white objects, the light will generally start to spread out and diffuse - the net results of this is that there aren’t as many defined details in the lighting as there were in the first bounce or the direct light.
As you can see in the picture above, theres a distinct line at the corners of the white box where the light is bouncing off it so there’s quite fine detail that we need to get in our orimary bounce.
Now on to vray itself, we have a choice of 4 different gi methods, each with different benefits. QMC and PPT are both very high quality, raytracing type solutions where each pixel in the image is sampled so you get extremely detailed but unfortunately grainy images because of the level of detail being returned in the global illumination. Irmap and lightcache are approximated methods so they aren’t as realistic as QMC or PPT, they do on the other hand have the benefit of speed seeing as they both take shortcuts - they aren’t sampling each pixel individually, they’re taking a broader view of the GI solution.
So what does that mean in terms of primary and secondary? Well for primary we want to capture all of the detail in the lighting we can - seeing as the primary bounce contains most of the light in the GI solution we want to capture this as accurately as possible so we need a high quality method such as QMC or irradiance mapping with its settings turned up quite high. For the secondaries, we dont need anywhere near the same quality the main reason being that the quality and detail probably doesnt exist in the light in the first place - the more light bounces around, the more its chances of hitting an uneven surface which will scatter the light and diffuse it - the secondary bounce tends to be a lot softer and more blurry and if we were to use a high quality method like QMC then we’d possibly introduce grain due to the way it works and we’d en up turning up our samples to end up with a clean soft secondary bounce. The alternative to that is to use an approximating method like light cache or photon mapping which will do a less accurate lighting solution and take shortcuts - the good thing about that is there isn’t any sharp details in the secondary bounces anyway so the lower quality methods blurry results actually suit our purpose perfectly. Since they end up using blurring techniques to smooth out their results its giving us a result that we want without having to resort to really high samples as we’d need for something like qmc.
Vray is pretty much giving you the option of splitting your GI methods so that you get really high quality in the primary where it’s needed and speedy, clean results in the secondaries where you can cheat a little to bring render times down.
Ooooh, nice arrows joconnel!