Interior rendering in Solid Angle Arnold

I've heard about frustration going from people trying to render an interior like scenes in Arnold. Especially it's related to the ones who use approximation techniques like Vray or Mray has. Arnold is a different kind of thing. If you will act as if it is Vray for example and try to light a simple enclosed scene through the some window  - you will get much noise. To get rid of that noise you have to sample it more and more which leads to long render times with a still visible noise. Arnold is "Fully Brutforced", Noise, in most cases, Is Main your Enemy.
Today I will try to get rid of the noise caused by probably the most noisiest thing in Arnold - diffuse bounces.
I will use Softimage and SitoA.

 

So, we have a simple interior like scene with area light in window and everything with standard material. The settings are:

 Lets render it:

...notice the noise

this noise is caused by the high energy areas lit by light in window and to eliminate it we must somehow change the way light acts. We must lower that high energy areas, in other words "to flat that light" as much as possible. So, the Light Decay Filter comes to mind, and the settings are:

render:

...notice, the noise is far lower from the previous and we Lost All contrast. Ok, we have another problem --- overall contrast. Don't panic, lets duplicate our light, turn off the Light Decay Filter and set bounces to 0. Don't let that contrast light be taken into account by diffuse bounces. Now you have 2 lights in you scene: one is for diffuse bouses with Decay Filter (aka GI Light)  and one is for overall contrast with zero bounses and no Decay Filter (aka Direct Light). Lets render again:

compare with first image

now we have contrasts and much-more lower noise.
Of course, we are losing contrast in GI bounces, but you have to sacrifice every time in CG in order to get result in a reasonable amount of time and money. I think it 's a good kind of back door  that we can use.
You can further investigate this approach. For example, set  1 shadow sample for each of that light, experiment with Decay Filter settings or even turn on falof filter in "Direct Light".

Hope this trick will help you! Cheers!