Indirect lighting is a great new feature in Poser8. It allows us to do directly some things we had to fake before, like global illumination and ambient occlusion.
Keep in mind that AO in Poser is a way to fake and speed occlusion in a render. IDL (Indirect Lighting) does the real thing.
So TAKE AWAY ALL AO NODES in your materials before to use IDL, or the render will take a huge amount of time to complete. AO nodes are not compatible with IDL, except the rare occurances where the AO node is used to drive a function to give a special effect. AI in light is desactivated when using IDL.

Here i will explain how i set up a scene to have an outdoor lighting using IDL and only one infinite light in Poser8. Outdoor lighting is one of the hardest thing to do in Poser in a realistic way. It's no more the case thanks to IDL.
Before Poser8 we had to use IBL to fake the illumination. We also had to make great use of AO nodes to give the effect of occluded surfaces. It gave unrealistic results as the AO effect was present even when the surface was directly lit by a light source. IDL will give you a more natural result, correct illumination and realistic AO. Of course better results lead to higher render times. Everything has a price. But the results well worth the extra render time. And as P8 renders faster than previous Poser versions due to the newly distributed buckets, the render time is just a little higher than with ancient versions.

I will use UrbanFuture2 by Stonemason as this set has a lot of details that will demonstrate the effect of IDL.
I will also use Bagginsbill's Environment Sphere available here to illuminate the scene. IDL will 'project' the image map of the EnvSphere on your scene. If something is between the EnvSphere and an object of your scene, the object will not be illuminated, except if you set up enough IDL bounces for the light to reach it.
I will use only one light to simulate the sun. One infinite light with raytraced shadows (you can use depth mapped shadows if you prefer, it will render faster).

First of all, i load the Environment Sphere and load a map of a sky on it. You must use an equirectangular map in a high size enough to avoid pixelisation. The one i will use is in jpg format, but you can use HDRI as it will give you better results. The color and brightness of the map will decide of the color and brightness of the illumination. The shader Bagginsbill built on this prop already makes it self lit, so it will participate in the IDL calculation.
Then i add one infinite light with raytraced shadows set to 2 as blur radius. I lower the intensity of the light to 85%. Leaving the light to 100% intensity in addition to the IDL will give you an oversaturated render. Always lower the intensity of your lights when using IDL.
After having loaded the Stonemason set, i go to the material room to delete all the AO nodes. It can take some time to be sure to get rid of all of them, but it's really important. Leaving them will give you higher render times and an extra AO effect that we do not want here.
I've also added some ships to populate the scene. Remember to delete any AO nodes in their materials too.
Now set up the camera. For this render i will use a 24mm focal lens on my camera to have a great perspective and better depth. Never hesitate to change the default 55mm focal, it can make a lot of difference.


For your test renders you can disable IDL.

Now comes all the fun, the IDL settings.

These settings are found with the render settings. It consists of a check box to tick to enable Indirect Lighting and a slider for the IDL quality.
Though i have to recommend you to use the script by Dimension3D included with P8 that gives much more control over IDL. This script allows you to set up the IDL bounces independantly from the raytraced bounces, to set up the IDL irradiance cache, IDL intensity and more.
You'll find it from the top menu in Scripts/Partners/Dimension3D/Render FireFly. You'll have the classic render settings plus the advanced ones in the same panel.


Keep in mind that the settings i will use are not the perfect ones, i'm still experimenting them. On top of that, you will not have ideal IDL settings that will fit all scenes. Each scene will need its own settings according to the effect you want, the geometries present in the scene etc. I just used some that worked and gave me the result i was after.

To better understand the difference IDL makes in a scene, here is a render without IDL, only the infinite light.


And now, i turn the IDL on and here comes the magic.


The settings used here are the default ones. Of course we can do A LOT better, it was just to demonstrate the difference only one check box can do.

To understand the different settings found on the D3D panel, i report you to the explanation of WhichWay in a Renderosity forum i will summarize here:

"First (raytrace) Bounces: Same as Bounces used to be - limits the number of Reflective/Refractive/Transparent surfaces hit. Needs to be at least 1 for IDL. Higher numbers give more accurate optics, but at increased render time. I haven't needed more than 1 so far.

First (AO) Irradiance Cache: Used for AO and disabled for IDL. If you're doing IDL, leave this at 0.

Irradiance Sample Size: Globally reduces the maximum distance allowed between IDL evaluation points. (I'd guess it does something similar for AO when IDL is off.) Lower numbers mean more evaluation points, smoother IDL effects and substanially more render time. I've always kept it as the top end of 20 so far. My reccomendation would be to lower this from 20 only as a last resort.

Intensity: Set the global balance between IDL and direct/specular lighting. I've always used 1 so far.

Now we hit the most important ones for IDL.

Bounces: Sets the number of bounces off surfaces used to sample where IDL is coming from. Depends on how complex the scene is. Higher numbers allow light to sneak under tables and things. Luckily, because of the magic of the Irradiance Caching algorithm, the render time does not depend exponentially on this number. Starting with a value of 5 seems plausible to me and don't be too afraid of even higher numbers.

Samples: This sets the number of random rays sent out after the last bounce to figure out how much light is arriving at the last bounce point. It has to be pretty high for two reasons - one, it is a statistical sample, so the error in the result goes down rather slowly with the number of samples, and two, the rays have to sample a complete hemisphere at the last bounce point. If there are too few rays sent out, they might miss important nearby geometry. Expect this number to be several hundred, if not thousand.

Second (IDL) Irradiance Cache: This is the most critical number. It effectively sets the accuracy in IDL that you're willing to accept and will cause the render to churn until it gets there. On the other hand it is the source of much magic as it forces more IDL evaluation points precisely in those places where the geometry is changing quickly and fewer where not much is happening. The actual maximum error in percent = 100 - Irradiance Cache. This means several things. First, high accuracy is to high numbers, or to the right. The usable numbers must be less that 100; 100 leads to no caching and unbearable render times. For high accuracy, the slider range is very compressed at the top end. E.g., to go from 5% maximum error to 2.5%, the number goes from 99.5 to 99.75. This is hard to manipulate with the slider and you probably need to type it in. Also, for each cut in the error by half, the render time could as much as quadruple.

These last two settings are the heart of IDL and they are somewhat coupled. For example, setting (IDL) Irradiance Cache high for high accuracy will fail if Samples is not also high enough to get reliable IDL estimates at the IDL evaluation points and vice versa. Fortunately, the test renders I was doing seem to be very demanding since they have such flat, untextured surfaces. For most renders, quite a bit sloppier calculation of IDL is fine. For starters, try (IDL) Irradiance Cache at 50 and work your way up carefully."

So finding the right settings to have a balance between quality and acceptable rendering time is a matter of experimentation.
For this scene, i had good results with the following settings:


And you can see in the final render how the occlusion is naturally distributed, only in the shadowed areas and not in the directly lit ones. It's especially obvious in the shadowed parts under the decks.


To enhance the realistic feeling that gave IDL, i do a z depth pass using Semidieu Advanced Render Settings script, and i add it in Photoshop using DOFPro with a slight grain. Et voila!


Click to see full size picture (1785x2000px)

Remember that you do NOT NEED IDL in every render. But for outdoor scenes it's a wonderful addition!

Here is as an example another render using IDL that would be really hard if not impossible to do without it.


Click to see full size picture (2000x1552px)

I used the famous Sponza model by Marko Dabrovic.
The only way for light to reach the interior of the model is by the opening on the above. Here i set 12 IDL bounces for the light to be distributed all along the sides arches.
As for the first scene, only one infinite light for the sun, Bagginsbill's Environment Sphere and IDL on.


Bagginsbill free Environment Sphere
Environment Sphere links
Semidieu Advanced Render Settings
HDRShop to convert pictures in equirectangular format (Download v1 on top menu)
Sponza model by Marko Dabrovic (click 'Download in the upright corner, use the 3ds version to use in Poser)