Relationship between photons/N-value/finalgather/splotches.

[silverlw]

I will try to explain The relationship between photons/N-value/finalgather/splotches. These are the most important things (only things) you need to understand. im not the most pedagogic person but will try my best

Here i have a simple room wich i intend to light with lightmaping. The skylight wich is a HDRI comes from the right via two windows. I have also added a directional "sunlight" and lit the two lamps in the room. This is how the irradiancemap look like with standard 20.000 photons and N-value 200.
Awfull. The light is big yellowish splotches and it's hard to tell where the light comes from.

[silverlw]

I change the number of photons to 200.000 instead to get a more detailed irradiance wich gives me this result. More detailed but still alot of highintensity speckles that comes from the highintensity HDRI. The number of "cells" in the irradiancemap is now up to 34.000. We must increase accuracy of these photons so we raise the N-value. N-value tells how many photons that should be grouped together and their sum will defince the final colour. This irradiancemap will produce heavy splotches nomatter how high values of FG you use.

[silverlw]

This is the result of 200.000 photons and N-value 2000. Much smoother but still with some strange bright/dark patterns where it shouldnt be any. When we raised the N-value the numbers of "cells" for the irradiancemap drastically dropped to 5000. This is because we have taken the 200.000 photons and divided them into groups (cells) of 2000 each.

[silverlw]

This is still 200.000 photons but now with N-value 8000. The numbers of cells is rather low (1500) but the light solutions is drastically improved and better reflect the true lighting conditions.

[silverlw]

I did a rendertest with the above irradiancemap (photonmapping + FG 800 ) and the endresult seem to render quite fast and without to much splotches. There is still artifacts in the red circle wich needs some higher FG values and i still think we need a bit more detailed Irradiancemap. "After i did this little tutorial i found out it was bad geometry" I will increase the numbers of photons so we get more than 1500 cells.

[silverlw]

I shoot some more photons, 8 millions and filtered with n-value 8000. This produced about 43.000 cells wich i think is ok. (took me about 6 minutes to generate) We can now even start to see the sunlights shape in the irradiancemap. The splotches start to come back and i should probably use even higher n-value.
It's the high contrast hdri causing it and another way to go is to filter the hdri a bit but im stubborn and will use this irradiance in final render anyway. I have to use quite high FG to get rid of the splotches now.

[silverlw]

Final! for this little tutorial anyway. Thanx for being patient with me.
I upped FG to 4000 and rendered with undersampling 2 fullscreenAAx8 - 46minutes.