GI Settings for huge images

[Art48]

Hi,

We're about to render a huge image (10.000px x 10.000px), consisting of 4 big industry machines (think: Boxes open on one side, we're looking in in a detailed "interior" of said machines (but not highdetail hoses and stuff, just some screws)) in a huge abstract room, heavily backlit (mystic) but still well reckognizable on their front which is looking at the camera.

If I would have to guess I would say:

About 50% of the image (Yes, that much) is consisting of a flat, simple wall with a simple shader
Another 30% of the image consists of a fairly detailed floor shader, complex procedural maps, glossy, bumpy, cracks. Grungy concrete floor.
Another 20% of the image are the remaining 4 machines.

That makes each machine covering about 1k to 3k pixels width (Some standing in the front, rest in the back)

We have about 25 lights in the scene in total so there is enough direct light. They consist of 30% standard Spot Lights and rest VRayLights (Plane)

Now my gut feeling totally drops to zero on a "feeling" for neccassary GI settings at that resolution.
The thing is we work with reflective/refractive (yes, both) GI caustics and the lighting relies on them working properly and looking good.
So I am fairly sure that we need very good GI Settings for that image. Now I have no clue where "very good" lies at and at which point it becomes "overkill".

Sadly I can't post a picture due to NDA reasons, just a screenshot of the layout.

(Light comes from the middle grey strip in the background walls)

Can anyone either tell me or show me some resources where I can start to dig in on what GI settings would be acceptable for such a scenario? Are there specific formulas or something?
Brute force is no option. I would like to stick with IM/LC.

[Art48]

Any Pointers?

[Morbid Angel]

Does this machinery of yours move at all? If no, then I recall people saying that every time you double the resolution the GI can be reduce by half in the prepass. I think its a common practice to also calculate the irradiance map at a much lower size and then render full size image.

What are the exact settings would be hard to say without looking at your actual scene of course.

[Neilg]

my first thought is to kill the caustic in the base render and do it as a separate pass, you'll have a lot more freedom. you can render the caustics in a simplified scene or with other methods then.
Maybe this is worth a look - http://i.imgur.com/J2bcYpZ.jpg

When I posted it vlado said to "not render several frames and instead set the max. density for the caustics to a small number (2-3 cm perhaps). Then the render will take some RAM, but because nearby photons will be merged instead of simply added to the photon map, it will typically not run out of RAM".

[Art48]

I don't mean ACTUAL caustics, just the GI caustics.

[super gnu]

GI caustics are a nightmare.. the only way to get them clean is ..overkill gi settings.. especially with reflective gi caustics. thats why its usually off.

id find an area where you have lots of reflective gi caustics, set it to BF /lc and do region renders upping the quality until the lighting /caustics look fine

then sit back and render the bad boy on as many machines as you can.

obviously imap/lc will be way faster, and when doing images that size i usually go for min of -10 and say -2 or -1 for max.. you still want the small details preserved after all..

however ive never seen imap deal with reflective gi caustics properly.. just end up with a few bright splotches across the image.


wrt rendering the imap at a lower res first.. i never understood where this idea came from.. afaik you may as well just use a lower min and max. it has the same effect.

[3LP]

my first thought is to kill the caustic in the base render and do it as a separate pass, you'll have a lot more freedom. you can render the caustics in a simplified scene or with other methods then.
Maybe this is worth a look - http://i.imgur.com/J2bcYpZ.jpg

When I posted it vlado said to "not render several frames and instead set the max. density for the caustics to a small number (2-3 cm perhaps). Then the render will take some RAM, but because nearby photons will be merged instead of simply added to the photon map, it will typically not run out of RAM".

Dude, this is great, but what a nightmare to handle!
If you have several revisions it's just a pain to work like that.
Isn't there any other way to get this done simply with the progressive render in Vray 3?
I rather have a pass rendering for 10h and take 2 minutes to setup than having 10 renders of 3 minutes and spend 2h in PS or max to preps the scene...

Such a peaty, as caustics are great to bring that little realism to create even more beautiful images easily.
Render engines like Arion does seem to have that covered
But Vray really seems to lack them.

Stan

[Neilg]

That wasn't done on every version of the image either - once the furn & camera was locked down (after 10 rounds of changes) I rendered it once, they saw it, asked for it to be toned down and I dropped the opacity of the layers a bit. the pass in the final image was the first one rendered. like 15 minutes in photoshop.