What actualy generates GI?

[loofer]

I've been using Max and Vray for years and always assumed that GI had to be generated specifically, either by the GI environment overide in Vray, or by an HDR image in Max's normal environment slot or by a big soft area light like a Vray dome. As this was my belief, this was how I always set my scenes, a GI light and a sunlight of some sort and never really played to much with it.

Now, by accident I was rendering a scene and turned off all my GI and soft lights but noticed under my render elements there was still GI being generated even though there was only 1 direct target light in the scene. So my question is, if GI is switched on does Vray generate GI automatically from normal scene lights, even a simple single target direct? I was almost sure in the past I would get jet black shadows doing this unless I used a secondary light source.

[Morbid Angel]

I've been using Max and Vray for years and always assumed that GI had to be generated specifically, either by the GI environment overide in Vray, or by an HDR image in Max's normal environment slot or by a big soft area light like a Vray dome. As this was my belief, this was how I always set my scenes, a GI light and a sunlight of some sort and never really played to much with it.

Now, by accident I was rendering a scene and turned off all my GI and soft lights but noticed under my render elements there was still GI being generated even though there was only 1 direct target light in the scene. So my question is, if GI is switched on does Vray generate GI automatically from normal scene lights, even a simple single target direct? I was almost sure in the past I would get jet black shadows doing this unless I used a secondary light source.

I think you have incorrectly interpreted GI term in vray. GI - Global illumination simply means light that's coming from everywhere bouncing around illuminating the world/objects in world. Of course in real world there is a main light source - the sun without it it would be pitch black (if we didn't have other sources of light).

Now having said that, in vray gi means light that bounces from objects, or in other terms indirect illumination. Where direct illumination is lighting coming from sources like lights, f.e. sun, or vray light, indirect is lighting that is bouncing from one object to another. Indirect is a complex lighting in its own, because by bouncing light it also colors the light from surface it has bounced from, and makes the bounced light dimmer eventually making it vanish.

In your case a dome light emits light from hdr however its not considered indirect illumination / gi. Its still considered a direct light source. Same goes for sun/sky system. Except that sky requires to have gi on since its not really a light. But if you take vray sky and place it in to a dome light it will illuminate the scene without gi just the same.

One reason you could have gotten dark shadows before is depending on your scene, you could have had not enough geometry to bounce the light around like in space. If you take any real object in space far away from any planet then the shadow part will be pure black.

With GI on in your scene the lighting is split out in two different components - direct and indirect lighting. Direct lighting is for the most part the same as if gi is off. And indirect lighting will just add more bounce light from objects.

But

[Vizioen]

edit: Morbid was faster and explained it better

[loofer]

I think you have incorrectly interpreted GI term in vray. GI - Global illumination simply means light that's coming from everywhere bouncing around illuminating the world/objects in world. Of course in real world there is a main light source - the sun without it it would be pitch black (if we didn't have other sources of light).

Now having said that, in vray gi means light that bounces from objects, or in other terms indirect illumination. Where direct illumination is lighting coming from sources like lights, f.e. sun, or vray light, indirect is lighting that is bouncing from one object to another. Indirect is a complex lighting in its own, because by bouncing light it also colors the light from surface it has bounced from, and makes the bounced light dimmer eventually making it vanish.

In your case a dome light emits light from hdr however its not considered indirect illumination / gi. Its still considered a direct light source. Same goes for sun/sky system. Except that sky requires to have gi on since its not really a light. But if you take vray sky and place it in to a dome light it will illuminate the scene without gi just the same.

One reason you could have gotten dark shadows before is depending on your scene, you could have had not enough geometry to bounce the light around like in space. If you take any real object in space far away from any planet then the shadow part will be pure black.

With GI on in your scene the lighting is split out in two different components - direct and indirect lighting. Direct lighting is for the most part the same as if gi is off. And indirect lighting will just add more bounce light from objects.

But

Thanks very much for such a comprehensive reply, I think this hits the nail on the head. When I was testing GI in the beginning to understnd it I was using a simple cube on a plane and with one direct light I was getting really dark shadows. That's what convinced me I always needed a soft fill light but as you say, it was just a lack of objects bouncing light into the dark areas that caused that, with complex scenes this isn't the case.

Mystery solved!

Cheers

[Morbid Angel]

Thanks very much for such a comprehensive reply, I think this hits the nail on the head. When I was testing GI in the beginning to understnd it I was using a simple cube on a plane and with one direct light I was getting really dark shadows. That's what convinced me I always needed a soft fill light but as you say, it was just a lack of objects bouncing light into the dark areas that caused that, with complex scenes this isn't the case.

Mystery solved!

Cheers

To be quite honest, in terms of photography / stage lighting, its normal to use additional lights to "shape" things better. Usually two three lights are always used, some of them very soft. So nothing wrong with doing that, I do that all the time in my renders.

First its quicker to render then using pure gi, more control over lighting. Also in terms of compositing, to have lights there means speculars will be controlled on per light level, often that's desired in finaling stage.

[loofer]

Yeah specular is a big reason for me using multiple lights. I probably won't be relying on pure GI as it's too incosistent, pillars in a courtyard with no structures near cast much darker shadows than those on the buildings for example. It's more that I like to understng the logic behind the system and when the image contradicts my understanding of thing I start to worry.....

[Morbid Angel]

I agree with what you said. But actually imho the gi and reflection are very complex effects. We as cg artists often get them wrong. For example being limited by hardware we often limit the number of bounces, number of samples for glossy/gi, interpolate etc, all that affects the look/quality/photorealism.

A long time ago there was a major thread on one forum about maxwell being super photorealistic and people were saying no way vray could have matched that. I took on that challenge and matched the look 1 to 1 without any fancy tricks. This goes to show that for the most part all renderers are the same in terms of how they treat lighting/shading etc. Its often users who make errors which in turn affects the look of the image.

With experience, if you had enough time and data you could measure a single image and reproduce its lighting value very accurately, but who has the time?

[loofer]

It depends on the type of work you do, I'm purely architectural and if you're matching into an existing photo then real world simulation is important. Most of what I do is promotional though and the last thing peope want is realism because it's bloody dull to look at, it took me years to figure out that I should be lighting my scenes like a movie set and not like the real world.