Hi I just watched the google video from the 06 Blender Conference of @ndy and Matt talking about setting up lights.
At one point they had a too strong light on the floor under suzanne and to fix this they used another light under the floor.
For the life of me I can’t understand what they are doing there.
How does a light underneath shine through the floor and diminish it?
I missed something critical I think. Can someone please explain this to me.
Thanks
Perhaps they used the turned on the ‘negative’ setting on the lamp beneath the floor?
Aligorith
well they did talk about using negative lights a bit later on in the presentation for fake AO and localised shadows…
but if they work from behind planes to affect the front side as well that is news to me…perhaps one of them will see the post and reply 
Not sure if this is the part you are on about, but at one point the (strong) overhead lamp he had placed earlier in the demo was interfering with what he was showing at the time -so he just dragged it under the floor to get it out of the way.
I can’t remember exactly which part you’re referring to there, but if you give me the time code when it happens, I’ll tell you.
cheers
I haven’t seen the video so this might be irrelevant but if they’re not ray traced (or if shadow is “off”) then lamp light passes through surfaces.
Larry: Andy added the lamp especially and then moved it under the floor
Matt: thanks it is at 22m56s in the video
Andy: ah… could be… the demo was about avoiding raytracing to save render time and get soft shadows
Ok, it doesn’t diminish the above light, but I guess you can say it diminishes some of the contrasty visual effect. It looks strange if you have a very strong light, but the underside of suzanne is completely dark. In real life (or a renderer that does diffuse GI bounces the light coming from above will also bounce up off the floor, take some of the colour of the floor, and go back at suzanne.
So to simulate that effect, you can just take another lamp to be the ‘bounce lamp’ similar in philosophy to a photographer with a white bounce card holding it underneath a model to reflect light back up. Since that lamp doesn’t have shadows on, the light passes straight through the plane and illuminates the underside of suzanne. There’s also ‘No Specular’ on, since you don’t want it to look like a directional light source, but just the results of diffuse light bouncing around.
Does this help?
Yes it does. Thank you very much Matt
Its a pity they only recorded the first half because I really learned things from this presentation.
I hope there will be a full section on lighting in the upcoming book…which I will be sure to buy of course 
Colin’s nodes presentation was very informative too. Thanks Colin
No worries! Glad you enjoyed it
Where is the video? I can’t see any conference vids listed on the program at blender.org. I remember seeing them at blendernation but the only options were bitTorrent and Google 
EDIT: Found them but they’re all torrent. Are there any straight MOV downloads anywhere or should I find a torrent client for Mac?
EDIT AGAIN: Don’t worry. I’ve got BitTorrent now and the video in question too.
So when @ndy said “The floor is really bright” he didn’t mean that he would take some light out of it, he meant that being so bright it should be reflecting light up under Suzanne. So, as Matt said above, the added lamp provided the bounce light to take the blackness out of Suzanne’s underside.
One interesting point is that apparently Pixar did not raytrace their movies until Cars!
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