How to light a room, with ceiling and wall light models?

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  11 November 2013
Question How to light a room, with ceiling and wall light models?


I am trying to render a night scene in Mental Ray (3ds Max), which has light (models) on the wall and ceiling. Usually with a daylight system lighting the room I can use very dim lights and self-illumination etc to create the right effect, but now the light sources actually have to provide all the illumination for the room I am stuck.

The image attached shows three things I'm having trouble with:

1. The shadow of the support
2. The bright spot on the ceiling right above it
3. The odd diffusion (or lack of) through the shade

Each of these I could fix on their own. For example making the light shape a sphere or setting the support to not cast a shadow, setting self-illumination on the shade and then moving the actual light further away, ect.

But I wonder if there is a proper way to do this that I am missing since it must be very common thing, and these 'tricks' kind of go against the idea of photometric lighting don't they?

How do I configure a powerful light, to interact realistically with geometry close by?
Attached Images
File Type: jpg CeilingLight.JPG (20.1 KB, 22 views)
  11 November 2013
For the light on the ceiling itself, you either need much softer raytraced shadows (so there's no hard-edged shadow of the shade) and the center part set not to cast shadows, or else you just want a spot light with no shadows aimed at the ceiling, with a slightly soft edge to its cone. The spotlight will be about as controllable an approach to that circle of light on the ceiling as you could ask for.

You could let GI carry all the illumination from that circle of light down to the rest of the room, but often a better starting point is to place a large area light up above the ceiling aimed down, not lighting or shadowing the ceiling or light fixture, but just illuminating and casting soft shadows from everything else in the scene. Even live-action features do this kind of cheat -- they use extra "area lights" from above the ceiling, instead of all the light coming from the light fixtures themselves. Look at this behind-the-scenes video for reference:

Having a big area light coming down through the ceiling provides such soft illumination that it will resemble light diffused through a large lampshade, or light bounced off the ceiling, in terms of casting very soft shadows of everything in the scene. You don't need the light fixture or ceiling to cast shadows in this light, but it does need raytraced soft shadows for everything else.

Use a quadratic decay (inverse square) for realistic light falloff. As long as you are using a linear workflow, this will be the most correct and natural looking possible setting. (If you didn't use a linear workflow, then quadratic decays would look too harsh and global illumination would not work realistically either.)

Jeremy Birn
Author, Digital Lighting & Rendering, 3rd Edition
  11 November 2013
Hi jeremybirn,

I experimented with both the spotlight and area light and ended up using a light with a large disc shape below the actual fitting (which was configured not to cast shadows). Setting the distance to the ceiling correctly gives the kind of illumination around the light I wanted, and the GI looks good everywhere else too.

Thanks for answering - the problem was not easy to describe so I know the question must have been hard to discern but your response really helped!
  11 November 2013
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