Once I know the color of atmosphere, I create an omni light as the main ambient light in the scene and enable "ambient only". Place the omni in the middle of the room then adjust the attenuation range. The "start" of the far attenuation should be fit to the room and the "end" of the attenuation is out of the room as shown in picture. I use uniform scale to scale down the Omni to fit to the height of the room so that the ratio between start and end should be around 1:2 or 3:5.

I usually make 2 layers of shadow for my interior scenes. The first layer is for the shape of the shadow from light source and the second layer is for soft shadows from indirect light and area light. The different look between soft and sharp shadows makes the scene look more fuzzy and realistic.

Basically, I always have one free direct under the ceiling and make the multiplier as small as possible. A typical value might be 0.1-0.2 and the color as dark as possible. What I want is only the soft shadow of every object in the room on the floor. Make the shadow color light grey and change the shadow density to a negative number like -1, -2 or -3 with a sample range as large as possible ie. 28-50. The negative shadow seems to be the easiet way for me to fake the area shadow. The advantage of the negative shadow is that it saves time during rendering. The negative shadow will appear darker or brighter depending on the color of the ambient light, so if the shadow appears very dark, you can reduce either the color or shadow parameters.

Casting these types of shadows is not limited to the floor. Sometimes I also place this kind of direct light to cast the shadow onto the ceiling or even on the wall.

3. Add specular and diffuse pass from direct light

3.1. Creating skylight

When the skylight come into the room from opening ie. windows or doors, I consider those openings that the sky light came through as area light sources. Put new omnis as blue skylight outside every opening and adjust the far attenuation ranges to fit in the opening area. You may use cast shadow from these lights and the shadow should be soft as well.

The sunlight itseft is not an arealight as the sun is only a small light spot in the sky, compared with the sky. (The sky is considered as a big dome of an area light) The sunlight shadow has a sharp edge. Sunlight is also considered as a directional light source.

Place a target directional light as a sun outside of the room, and set the shadow to a shadow map or raytrace and change the edge of the shadow to be sharp.

Note: Sometimes, I finish the rendering without the sunlight and this may make the atmosphere look cooler. With the sun, the interior might look hot and look like it was in the middle of the afternoon.

4. Indirect light and illumination

Take a look around the whole scene again. I notice that there are 2-3 main indirect illuminations that we need to add.

4.1 Illumination from the big orange wall. I place an orange omni light behind the orange wall and make the attenuation as big as the wall size. Use ambient only. The ambient only light will also make the orange wall illuminte but if we put normal specular and diffuse light, it will illuminate only the rest of the wall, floor and ceiling. The advantage of ambient only is it will affect the opposite side of an object.

4.2 Illumination on the floor, affected by sunlight. When the sunlight touchs the floor, it will bounce and scatter to its surroundings. I put a color of blue (floor's color) + a bit of yellow tint omni (ambient only) under the floor and set the attenuation size to fit into the sunlit area. If you see the scene is too full of ambient only light, it will make the scene lack dimension. You may change the omni to diffuse and specular instead. This method is flexible, it depends on your sharp eyes.

In a bigger interior scene that is more complex and has more indirect illumination, we might not put indirect light on all the illumination. Just placing a few main indirect illumination lights is enough as we are not the machine that can calculate all the illumination correctly. The look of the scene is based primarily on your sharp eyes. I also compare light to a brush that we are playing with on our artwork.