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View Full Version : Yet another one of those pesky "Volumetric Lighting with MR"-questions


hellspawned
10-08-2010, 07:03 AM
First and foremost: I'm using Maya 2011.

I have this scene I'm working on, which among other things contain a couple of furnaces with a grate at the front and a few slits along the sides of the body, covered with glass (Steam-punky thing). What im aiming at achieving is beams of light, with a fog effect, coming out through all of these openings in the furnace body. Rendering is to be done with Mental Ray, as I'll need FG for the lighting as well.

So, now i need to add the actual lights to these furnaces.

Point lights seemed appropriate, as it emits equal amounts of light in all directions, and the source of light (the presumed fire inside the furnace) is supposedly roughly at the middle of it (and this also makes lighting a simpler job).

So I turn on Light Fog for it Raytrace Shadows, add lightfog effects to the lights, and... nothing. The light cast on surrounding objects work fine, there's just no fog.

I've read up on different methods of achieving volumetric lighting, and the commonly propagated method seems to be through using Parti_Volume. But I can't for the life of me find an up-to-date tutorial on this, all of them seems to be too old and they don't work.

Additionaly, all of them uses spotlights for their sample scenes, which is not what I need to use. Is it possible to achieve a nice volumetric effect with point lights at all?

If so, if anyone around here could be bothered to make a mini-tut, a quick runthrough of how I'd go about managing to get the volumtric effect I desire, for example in a scene where a few objects are put around a point light, or even better link me to a complete, up-to-date tutorial on how to do this, would be greatly appreciated!

Cheers!

lightcache
10-08-2010, 12:34 PM
try this

http://www.digitaltutors.com/09/training.php?cid=3&vid=2991

hellspawned
10-08-2010, 06:38 PM
I'll give it a look when I've got some cash, don't really have money to spend on this right now :/

In the meanwhile, if anyone has a free tutorial on the matter, gimme! :bounce:

InfernalDarkness
10-08-2010, 07:47 PM
I would try it with three spotlights in each furnace. Keep your shadow rays low at first to avoid horrific rendertimes, but keep in mind that parti_volume will increase your rendertimes drastically no matter how you do it. It can be very fickle, so I'd try it all in a separate scene first. You may have to overlay your volumetric render in the end, as opposed to rendering it all in one single pass.

Parti_volume is a total pain, you'll get it working then it'll "break", or it will never work, and it can be frustrating. Keep at it - the results are worth it!

Galakgorr
10-08-2010, 08:12 PM
mental ray will definitely render simple light fog from spot lights, it just takes a little work to get it to behave.

make a spot light, set falloff to quadratic, give it a high intensity. create light fog like you normally would in software. then grab the renderCone (coneShape1) node that's created. turn on volume samples override, set it to maybe 12 for a start, and turn on depth jitter. you can map a 3d texture to the spotLightShape's fogIntensity parameter for some noise if you want.

i attached a test image with those settings.

hellspawned
10-09-2010, 04:09 PM
mental ray will definitely render simple light fog from spot lights, it just takes a little work to get it to behave.

make a spot light, set falloff to quadratic, give it a high intensity. create light fog like you normally would in software. then grab the renderCone (coneShape1) node that's created. turn on volume samples override, set it to maybe 12 for a start, and turn on depth jitter. you can map a 3d texture to the spotLightShape's fogIntensity parameter for some noise if you want.

i attached a test image with those settings.

Ah, together with InfernalDarkness'es 3-spotlight-idea, that would work out. Cheers :)

hellspawned
10-11-2010, 07:56 AM
Another question: How do i control the falloff of the fog? I've managed to produce fog now, but at the moment its a solid cone-shaped block of fog with an even density, and slight falloff towards the edges, but still its uniform inside the coneshape.

I guess its something about mapping a black-to-white ramp to something, right? I just havent managed to work out which type of ramp, and to which attribute, so if anyone could clue me in on this I'd really appreciate it.

InfernalDarkness
10-11-2010, 09:05 AM
Had some time tonight so I pounded out a rough scene showing how parti_volume would work. Granted, it's a rough model and speedy render, but you get the idea. One could texture the volume or lights or whatever and have much better results, and obviously a better model would help.

http://img535.imageshack.us/img535/4232/fireplacemrparti1.jpg

hellspawned
10-11-2010, 11:15 AM
For the moment, I think I prefer using mayas lightfog, seems more reliable than parti_volume, from what people have said here and in other threads - No risk of it suddenly, randomly not working.

I do have my previous question though, how to control the falloff so that it looks good. Any idea?

Galakgorr
10-11-2010, 06:59 PM
for the falloff you could adjust the falloff of the light itself. or, use a 3d texture map for the fog density.

hellspawned
10-11-2010, 07:31 PM
What 3d-map should I use to control falloff, so that its strong and visible close to the source, but completely invisible towards the end of the cone?

Galakgorr
10-11-2010, 07:48 PM
you could project a 2d ramp with a 3d projection node.

make a ramp. make a projector. make a 3d placement node.

connect ramp.outColor -> projector.image
connect place3dTexture.worldInverseMatrix -> projector.placementMatrix

adjust the projector type and move the placement node around until it's where you want it.

granted i don't know how well this would work with complex camera moves, but if you can't control the falloff just using the light intensity, this is the only other method i can think of. i don't know if light decay regions affect maya light fog, but that's worth a shot too.

hellspawned
10-11-2010, 09:52 PM
What and where exactly do I connect the projector node to?

Galakgorr
10-11-2010, 09:55 PM
f1 would be a big help here probably

a projector node can output straight to your shader or property of an object, just like a texture. the projector is just taking an input texture and literally projecting it onto a surface or volume based on the projection type rather than using UVs.

so just connect the output of the projector to the fog density or whatever property you want the texture to affect.

hellspawned
10-11-2010, 10:10 PM
Ah, sorted it out now. Popped a luminance node inbetween the projector and the lightfog node, cause it was being pissy about connecting a color property (outcolor) to an alpha-based property (density). First it wasnt visible at all, had to bump intensity up to 50, but now it works, thanks a bunch :)

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