Why is lighting a scene so difficult in Iray?

Fishtales

I must be one of the few that don't have too much bother lighting scenes. As an ex Electrician who has forgotten a lot about watts, lumen and foot candles Google is your friend. I am also a long time photographer, I have handled cameras for 60+ years, my Dad was an Amateur Photographer, so know a fair bit about the settings and use this as my starting point for all images.

I set up this scene a number of years ago to test all the lights that come with DAZ. It is indoor using Scene Only.

2021-09-28 22:26:58.342 Total Rendering Time: 45 minutes 19.24 seconds

A) Linear Point Light  - 3500 Lumen - Sphere 10x10 - 2750 K

B) Point Light - 5500 Lumen - Sphere 10x10 - 2750 K

C) Linear Point Light - 4500 Lumen - Sphere 10x10 - 2750 K

D) Point Light - 2500 Lumen - Sphere 10x10 - 2750 K

E) Spot Light - 1500 Lumen - Sphere 10x10 - 2750 K - Spread 60 Beam Exponent 4.0

Distant Light through windows behind camera - 10000 lumen - 4500 K

Tone Mapping 30/4.0/400

Click on image for full size.

This one is outside with Environment set at Midnight on the 21st December in the Northern Hemisphere, so dark, with a Spotlight in front and the flame for the match set to Emissive at 6 Lumen. I also set up the DOF on it.

2021-09-28 10:42:41.452 Total Rendering Time: 1 minutes 42.94 seconds

Spot Light - 200 Lumen - Rectangle 100x100 - Spread Angle 38.5 - Beam Exponent 4.0 - 6500 K

Tone Mapping 30/2.0/400

Camera DOF Settings

Focal Length 80mm

Focal Distance 100

F/Stop 22

DOF ON

Click on image for full size.

Richard Haseltine

Do note that a Linear Point Light is just a Point Light for Iray - the falloff adjustments are 3Delight-only.

Leonides02

Fauvist said:

Leonides02 said:

 

I read somewhere that one reason for that is that the renderer is tracing light beams that bounce endlessly around that enclosed space, getting weaker and weaker - beyond the point where they'd have a noticable effect, but still requiring calculations to be made on them. If you have somewhere - a hole in the wall out of shot, say - for rays to escape to, it'll speed up your renders. Another approach is to have a black plane with 0% glossiness somewhere out of shot to soak up stray light beams.

I can't say I've noticed much difference myself, but it's someting you might like to try.

Default is zero, which is infinite.

The default, I believe, is "-1" which I have read is actually 32 bounces, not infinite.

Also, one thing to note about Ghost Lights is that they don't create any specular highlights. Therefore, they can make a scene look very flat and unrealistic.

There are no specular highlights on TV sitcoms either.  If you want specular highlights with ghost lights, use some highly reflective shaders in the scene. 

True. But TV sitcomes aren't exactly known for their beautiful lighting. Most of us are going for something more cinematic. 

Ghost lights are good for some things, but they won't make the scene pop if that is all you use. 

Also, I'm pretty sure that no matter how specular the shaders, they won't create highlights with ghost lights. There is nothing to reflect.

Fauvist

Leonides02 said:

Fauvist said:

Leonides02 said:

 

I read somewhere that one reason for that is that the renderer is tracing light beams that bounce endlessly around that enclosed space, getting weaker and weaker - beyond the point where they'd have a noticable effect, but still requiring calculations to be made on them. If you have somewhere - a hole in the wall out of shot, say - for rays to escape to, it'll speed up your renders. Another approach is to have a black plane with 0% glossiness somewhere out of shot to soak up stray light beams.

I can't say I've noticed much difference myself, but it's someting you might like to try.

Default is zero, which is infinite.

The default, I believe, is "-1" which I have read is actually 32 bounces, not infinite.

Also, one thing to note about Ghost Lights is that they don't create any specular highlights. Therefore, they can make a scene look very flat and unrealistic.

There are no specular highlights on TV sitcoms either.  If you want specular highlights with ghost lights, use some highly reflective shaders in the scene. 

True. But TV sitcomes aren't exactly known for their beautiful lighting. Most of us are going for something more cinematic. 

Ghost lights are good for some things, but they won't make the scene pop if that is all you use. 

Also, I'm pretty sure that no matter how specular the shaders, they won't create highlights with ghost lights. There is nothing to reflect.

Ghost lights do produce highlights.  Dome and sky and sun are OFF.  Nothing in the scene except a neutral grey background, the chair with the textures that come with it, and 1 ghost light.  The "something" that is reflected is the light.

Richard Haseltine

A ghost light is more-or-less invisible, highlights in Iray are the result of reflecting the light source - an invisible light source gives nothing to reflect.

Hylas

Richard Haseltine said:

A ghost light is more-or-less invisible, highlights in Iray are the result of reflecting the light source - an invisible light source gives nothing to reflect.

If it´s not visible, then how can it be light, though?

Something I´ve always wondered about ghost lights.

margrave

Hylas said:

If it´s not visible, then how can it be light, though?

Something I´ve always wondered about ghost lights.

Ectoplasm. 

Fauvist

Richard Haseltine said:

A ghost light is more-or-less invisible, highlights in Iray are the result of reflecting the light source - an invisible light source gives nothing to reflect.

In the 2 images of the throne I posted above, you can see the highlights - the ghost light reflecting off the silver. See - there's darker shaded areas, and middle tone areas, and pure white areas - those are highlights.  What type of highlight is missing from the chair?  If you mean a reflection of an object, then no, there's no reflection of an object.

Fauvist

Richard Haseltine said:

A ghost light is more-or-less invisible, highlights in Iray are the result of reflecting the light source - an invisible light source gives nothing to reflect.

You're right, and I'm wrong.  The highlight that is missing is a mirror-like highlight.  I faced a yellowish light-bulb colored Ghost Light at an art deco mirror with nothing in the scene except a grey background, and you're right.  The mirror reflects nothing and appears almost totally black.

Fauvist

Ok, now I'm really confused.  This image has the Ghost Light off, and I added a photometric spot light pointing at the mirror.  The totally black rectangle that covers part of the mirror is the area I spot-rendered.  Not only can you not see anything reflected in the mirror, you can't see the wall either.

Richard Haseltine

I used thre planes to make a nook, sat a sphere in it 9and made it shiny), then added a smaller plane as a light. The renders are with cutout opacity at 0.001 and 1 - only the latter has a highlight from the plane.

This is of coruse physically impossible, it's a cheat, but it's a very useful one.

Fauvist

Richard Haseltine said:

I used thre planes to make a nook, sat a sphere in it 9and made it shiny), then added a smaller plane as a light. The renders are with cutout opacity at 0.001 and 1 - only the latter has a highlight from the plane.

This is of coruse physically impossible, it's a cheat, but it's a very useful one.

 Sorry, I don't understand.  "cutout opacity at 0.001 and 1"  What surface did you set the cutout opacity on?  The plane you made into a light?

jestmart

I worry that ghost lights may be a bug in Iray and may some day get fixed so I wont be using them any more.

Leonides02

Richard Haseltine said:

I used thre planes to make a nook, sat a sphere in it 9and made it shiny), then added a smaller plane as a light. The renders are with cutout opacity at 0.001 and 1 - only the latter has a highlight from the plane.

This is of coruse physically impossible, it's a cheat, but it's a very useful one.

Please correct me if I'm wrong, but any "ghost light" with an opacity greater than 0 is no longer a ghost light, right? I mean, it's visible in the render as a light as usual...?

apocalypse_cow

If you set an emissive surface to a very low cutout opacity (like 0.001) it's a ghost light. It emits light but is not really visible anymore. Because it's not visible anymore it produces no more reflections (like a vampire) and since specular highlights are basically just reflections it produces no specular highlights. It still produces glossy highlights just fine.

Depending on you scene that might not be a problem at all. For a normal human character for example, the only really (specular) reflective surfaces are the eyes. So you won't get catch lights in the eyes but otherwíse a ghostlight works like any other emissive light. Not necessarily any flatter at all.

Where it might be different is if you have objects with highly reflective surfaces - shiny metal objects for example, or cars. in that case the light might look flat indeed.

Richard Haseltine

Fauvist said:

Richard Haseltine said:

I used thre planes to make a nook, sat a sphere in it 9and made it shiny), then added a smaller plane as a light. The renders are with cutout opacity at 0.001 and 1 - only the latter has a highlight from the plane.

This is of coruse physically impossible, it's a cheat, but it's a very useful one.

 Sorry, I don't understand.  "cutout opacity at 0.001 and 1"  What surface did you set the cutout opacity on?  The plane you made into a light?

On the ghost light, yes.