Here is a little experiment that demonstrates the photon shooting of yafray (not to confuse with the photon lamp for caustics!)
Ok, so here is the scene with emit=1 on the red and green walls but emit=0 everywhere else and NO LIGHTS:
All you can see are the walls, so lets turn on GI with “full”, “best” and set the depth to 1:
Explanation: The red and green walls are emitting light into our scene and illuminating the white parts. The depth means that only one bounce of light is used. Using radiance cache speeds up the calculations (and makes it “smoother”) but you don’t have to use it.
Experiment #1: Lets add an area lamp the size of the wall to the red side (in the GI settings, make sure photons are OFF):
It acts like a normal area light - yay.
Experiment #2: Go to the GI settings and turn on “photons”:
Oh no! All of a sudden our area lamp doesn’t work?
Explanation: When photons is enabled, yafray will shoot photons from all of your lamps as a helper for the GI! Notice how the last photo is “brighter” than the second one? That is because the photons were shot and helped the GI calculations to hit the emitting walls as opposed to the non-emitting surfaces.
Hope that helps someone