Vray atmosphere - 'god' rays or 'dust' rays

[tricky]

I have been messing around with an interior image, experimenting with my methods, and really trying to get my head around DMC sampling. Mixed success so far - works great with a greyscale but adding textures really slows things down beyond what are acceptable. I'm working on this...

With this same scene, I am trying to use vray atmosphere to get subtle rays of sunlight coming through the windows and perforations on the left hand side. Ordinarily, we'd fake it in post, but I wondered if vray atmosphere is a viable alternative. I've been messing around with settings for an hour or two now and just cannot get definition in the atmosphere. I just end up with a general 'greyness'.

With a scene like this should I be able to create this kind of subtle effect or is this unlikely?

I am using a gizmo for the fog and at the moment, my camera is outside of this gizmo. I have scattergi ticked (32 bounces) and subdivs are set to 512. My distance (at the moment and for this render) is 50000mm. Step size is 5mm. Max steps 10000. I am just using the daylight system (which is a vray sky and vray sun).

[Morne]

Remember to set your VRaySun shadow bias to 0.2mm so you can catch shadows from small objects. (With system units of meter, it makes the default shadow bias of the sun 200mm and then it "doesnt see" small objects for shadows)

[tricky]

Yep - already done. I generally set lights to zero by default these days.

[devin]

It looks like your gizmo extends out further than just the room, I'd say the sunlight is being filtered through all that fog and by the time it reaches the room it's very muted. I'd isolate the fog to the room you want the God rays in, that will make a big difference.

[Neilg]

I keep the gizmo's very close to the area you want, very rarely use scatter GI and turn up the intensity of the sun by 2-4x. If you still want some GI scatter, dropping the sky intensity so that the sun rays pierce through more is also worth a look. Would probably just disable those ceiling lights entirely, they'll just add haze - whereas you want beams of sunlight for this kind of image.
If your camera is in direct light that can make it very hazy - i'd build non renderable/shadow casting boxes just off to the side so the rays dont start until some ways inside the scene.

[tricky]

Good tips. Will give it a try.

[trixian]

I have had issues with this as well.
In the end I have found that using a separate directional light for the haze/fog works best, since one ideally needs to render this as a separate pass since chaos still have not enabled us to only output the envfog to a render element and not show in the "beauty".
I generally find that to get the amount of light streaks one wants, the fog density needs to be so high that it would completely cloud the shaded areas when rendered directly for your output.
Real haze/fog seems to catch the light better at much lower density levels imho.
This seems to affect stuff like muffy water too (shader-fog). I always have trouble getting visible light shafts in water while still retaining some clarity.

[vlado]

Real haze/fog seems to catch the light better at much lower density levels imho.

Can you show me photos of that? The references that I could find are indeed quite foggy:











Best regards,
Vlado

[tricky]

Are they your photos Vlado? They're great. Definitely rays from God.

[vlado]

Are they your photos Vlado? They're great. Definitely rays from God.

Hehe, no, I just googled them. Here is a bit more interesting read: http://photo.stackexchange.com/quest...-of-light-rays - apparently it's not that easy to photograph the thing either:

Depending on the range of light in the scene you might be able get both the light and the room within tolerable limits, but it is likely that the room will be very dark. If that's the case you could use a tripod and take two exposures, for the room and light ray, and blend them in post processing.

Best regards,
Vlado

[Undersky]

This is why it would kick ass to have the option on a per-light basis aswell =) Just enable and play around with density and so on =)

[vlado]

This is why it would kick ass to have the option on a per-light basis aswell =) Just enable and play around with density and so on =)

I don't see how that will help you. The sun light is everywhere anyways, so you'll come up against the same issue. What would help however, is the ability to boost the fog illumination separately from its density - maybe we just need to add a lighting multiplier for "artistic" purposes.

Best regards,
Vlado

[Morne]

Just make vraylights work with max volume fog and lens effects like we did in the old days with standard lights before VRay.
Is it that hard?

I get that VRay is physically correct and that's fine for most part of my work, but then there are these odd times when we need it to be NOT physical

[Mokiki]

Having a seperate slider for the illumination would definately be a plus. But I agree with morne and undersky even if not physcally correct sometimes clients just want those rays and not the side effects of those. Thats why I proposed in the vray 3.0 forum being able to produce those rays per lightsource without the darkening of the scene through fog. Like you are able in Arnold.
Vlado already mentioned that there might be a problem with the way MAX would handle this scenario and he is the person with the understanding of the technology. I'm just happy that vray provides me with pretty pictures :P

[vlado]

Thats why I proposed in the vray 3.0 forum being able to produce those rays per lightsource without the darkening of the scene through fog.

This is exactly what the said option would do (minus the light source options, you'll still have to set these up in the environment fog).

Best regards,
Vlado