View Full Version : Need for decay when using GI?

01 January 2006, 10:00 AM
Im attempting a photorealistic rendering of a room, and Im using inverse square falloff on the lights since I have understod that is the way real light behaves.

But controlling the light intensity across the scene seem to be a real problem. Some areas apear to bright (overexposed) and some to dark. Ive been trying to correct this by experimenting with the multiplier and decay start values for the lights. But this is a pain since testrenders take so much time. Overall my sceen apear to bright and lacking contrast. Just plain and dull.

So my question is if there is any need to use any decay on the lights when Im using GI? GI alone might suply enough variation in the shading?

The room has three flouresent lights in the celling, a small light in the window, and three small lights on a shellf in the room illuminating the shellf. Im thinking about not using any falloff on the flouresent tubes, thereby making the main illumination easier to control, and use decay on the small lights to provide variation in the illumination in those local areas.

What do you guys think?

01 January 2006, 03:05 PM
I don't know if you'd necessarily want to take off the decay from an interior light source. Although GI does provide a gradient look for secondary bounces, I think if you had no decay on your light source it would blow out too many areas. Are you rendering out your lights in seperate layers? You may want to consider this not only for the sake of controlling your render times, but it will allow you to adjust intensities quickly in post work.

Ian Jones
01 January 2006, 09:51 PM
Don't forget that controlling the number of bounces has an good deal to do with controlling the spread of illumination.

01 January 2006, 07:54 AM

Thanks. Im pretty new to this whole thing, how does rendering lights in layers work?


Ok, thanks. Ill look into that.

Do you guys know if theres a right way to set the lights? The flouresent lights have a multiplier of 8, inverse square decay and a start value about 1 meter. Does that sound good? Are there any guidelines to follow when setting up these kinds of scenes or do I have to rely on my own judgement?

01 January 2006, 10:57 AM
Are you doing a gamma lookup on the image when you view it? This can drastically change the apparent falloff of lights.

You should be viewing the image with a gamma of 2.2

This will mean you'll have to gamma-convert your textures, there's a couple of tutorials linked in threads here about working in linear colour space. Try searching for them.

CGTalk Moderation
01 January 2006, 10:58 AM
This thread has been automatically closed as it remained inactive for 12 months. If you wish to continue the discussion, please create a new thread in the appropriate forum.