Questions about portals

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Hello everyone,
I have some questions about the portals in Cycles.

1 - If I have a lot of windows in one wall in the scene, is it better to put a portal for each window individually, right inside the frames, or can I put just a big portal that covers all windows?

2 - The portals should be inside or outside, in relation with the windows glasses?

3 - If I have a model of a complete house, with lots of rooms, the portals on rooms I am not rendering can spoil the light in the current room?

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  1. The answer is somewhat scene dependent, but usually 1 big one. It’s possible individual portals might be better, depends on a number of factors like how big the windows are, how far apart they are, etc. Basically, 1 portal is faster than many portals, but you lose efficiency for portal area blocked by the wall. In most cases, 1 big portal is going to be preferable though.

  2. Outside. Should be outside the windowframe/wall as well if that extends out beyond the glass

  3. No, but you may lose render performance with useless samples to the unseen portals. The “light sampling threshold” option under “Sampling” can help mitigate this, but it’s best to have some system to hide the rooms and lights you can’t see, such as by using layers.

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Thank you J_the_Ninja!
Now I am understanding it better, even the right use of the Light Sampling Threshold