Secondary bounces before primary, how?

[fAEkE]

So, just wondering, how can vray calculate Light cache before IrrMap, should it not be the other way around? Both LC and IM calculates rays from the camera, yes?

Look at this image from 3dspot manual:



Can someone explain this to me? Is it LC that decides the primary bounces that the IM is using aswell? I'm obviously missing something.. hopefully some kind soul can help me out?

[joconnell]

As far as I know, the light cache does actually trace a full lighting solution for the scene, it just isn't totally accurate, especially for the highly detailed first bounce. Vray just trims off the amount of light from the very first bounce in the light cache and replaces it with the more accurate light from the primary bounce. Perhaps the quicker secondary light cache solution is being used as a guide for the primary as an acceleration method?

[samuel_bubat]

As far as I understand it, but I might be wrong !
The lightcache wich is raycasting creates a rough illumination of the scene first, so the Irradiance map wich is raytracing knows where light is.
The primary is basically a "beauty" pass that refines the rough GI from the secondary pass. I think it´s similar when using MR with Photon Mapping
and than you add all the fine details only in your view with Final Gathering.

So secondary pass is not the second bounce as you labeled it in your drawing. Both are independent Gi calculations, Lightcache to light your scene as a whole
and Irradiance Map to add details in your view.

[fAEkE]

Right, but does IM use the exact same primary rays as the LC? Hmm, maybe Vlado can help us

[samuel_bubat]

Vlado for sure can answer this in detail.
But I would say, no they don´t use the same rays. While the LC scatters rays randomly all over your scene
The IM uses pretty optimized samples generated from your IM passes and tresholds , too shoot rays.

[joconnell]

It would make sense for it to use the first bounce of the lc as a guide since the direction would the correct, but IRmap is tracing far far more rays overall so it might not be all that useful.

[vlado]

Right, but does IM use the exact same primary rays as the LC? Hmm, maybe Vlado can help us

No, they are not exactly the same rays.

Best regards,
Vlado

[joconnell]

Three questions on this vlado if they're quick to answer!

1. Does the secondary in the lc do a full gi solution (including a low quality primary bounce) but then replace the low quality lc first bounce with the light solution from the full quality Primary gi solution?

2. With the light cache doing almost infinite bounces it's trying to go from the camera back to a light source and recording the energy loss at each stage. With samples that never hit a light source or bounce off into space (like photon mapping leaks) does vray just pretty much feed in a value of 0 light at the start of that particular path's contribution? Similar to a glossy object reflecting black objects just adding nothing to the reflective component of a render?

3. This might be a trade secret but out of interest how does it figure out when to give up on a path being traced too? If it was being fired from the light you'd at least know when the light loss was bringing a paths contribution down to an amount that makes no difference but since LC is going the other way does it just cut off when it intersects with another paths already solved sample?

[samuel_bubat]

Three questions on this vlado if they're quick to answer!

1. Does the secondary in the lc do a full gi solution (including a low quality primary bounce) but then replace the low quality lc first bounce with the light solution from the full quality Primary gi solution?

Well, yes sort of. you can switch your primary and secondary to LC to see how your secondary would look like. I use this quite often to determine the quality of my secondary.

[fAEkE]

What do you look for when determine if the quality is good enough for LC? I mean, it doesn't look very pretty no matter how high settings you have

[joconnell]

That's true - if you use the normal light cache as a primary and secondary it'll never be good enough to capture the detail needed in the lighting unless you use it in progressive path tracing mode, which is a totally different story. Generally LC is good enough unless you see things like light leaks or big splotches on your walls. The secondary bounce of GI is very very soft with almost no sharp detail in it so once it's smooth, has enough bounces to fill out the room you're lighting and has no blotches / glitches then it's good enough.

[fAEkE]

Thanks, this will help me quite a bit