QMC | Internal illumination Type

Let's compare a scene that was lit by object illumination to a scene that was lit by conventional arealightsources. First one being lit by object illumination:

GI Bounces: 3, GI-Gamma 2.2, Geometry AA
Scenario: Internal illumination only (object illumination)

click to enlarge


The second picture is a scene lit by conventional lightsources. Before you can make a comparable render containing conventional lightsources only you'll have to turn your GI bounces down by 1. So in his case from 3 to 2.

This is because areas that are directly lit by a conventional lightsource will be interpreted by the GI as illuminated.

So which settings do we need to also adjust to make the two concepts comparable?
Area shadows have to be turned on and the lights' falloff have to be inverse square. Also we have to use a lot more samples on the lights' detail attributes in order to to get a nice continuous lightstroke from the ring shaped light on the tablesurface and an even lightdefinition around the bigger Sphere on the wall. I also disabled Speculars for the lightsources.


GI Bounces: 2, GI-Gamma 2.2, Geometry AA
Scenario: Internal illumination only (conventional area lightsources)

click to enlarge 
Even with one bounce less the conventional lightsources were a lot slower to render, that's why I only took it up to 1024.

You could gain some speed if you turn down the settings of the lightsources e.g. area shadow quality but I just went for the default for the rest of the values.
We have a problem here with these conspicious bright pixels that are scattered around.
As you see they get fewer but more prominent when using higher settings. When does this occur?
It happens when you render a QMC GI scene with conventional lightsources (as area light) that have the natural inverse square falloff and are very near, on or overlapping with geometry. Best antialiasing can soften it to a certain extend but it's faster to either change to Irradiance Cache GI or move the lights further away from geometry or set it to another falloff type.
I did the latter and set it to inverse square clamped for rerendering:


GI Bounces: 2, GI-Gamma 2.2, Geometry AA
Scenario: Internal illumination only (conventional area lightsources)

click to enlarge

Now (with the highest lightvalues clipped off) the overblown areas and sampleblips were surpressed resulting also in a slightly darker picture.

Note how low amounts of samples can give us already a pretty nice result!
Quality/ time wise it can now even outperform the (still yet unoptimized!) rendertimes of object illuminated lighting.

BUT:
Even if you can achieve comparable quality in less time, be aware that you need to set up the lightsources properly. The freedom of having different falloffs, shadowtypes, samplesettings etc. can lead to some inconsistencies within the picture like different levels of grain in certain spots or unrealistic decay for some lights in the scene.
Lighting with illuminated objects in QMC GI appeals more like a monolithic system.

Besides there is a big speed advantage to come for object illumination, we're going to talk about that when we get to optimization. Yepp, there's hope - even for brute force. ;-)


But let's keep the area lights in the back of our mind for later when we deal with Irradiance Cache.
Area shadows might help us later to maintain detail for the interpolated, blurred world of IR...