Faking radiosity with bouncing lights

Here is the render as we basically left it last time.

This light setup is with 5 lights of 15cm diameter each, positionned at the chandelier lights positions, slightly yellowish colored, casting 2 rays each and with 100% darkness for shadows.

This looks quite good already but could we get an even more realistic look all over?

As it is now, there are no indirect lighting coming from the environment. We could use 1 ray per light and thus use the shadow falloff feature to improve the light details under the table and under the chairs. But we would still get black shadows on the back of the chairs.

What would this scene look in reality. It should be easy enough to find out by turning radiosity ON.

The render to the right is what we get from a radiosity render. This was rendered with 1 000 000 photons and 300 Final Gathering samples and took 2h18m to render on a P-IV 2.5mHz Win XP computer.

Although it doesn't look like it, there is actually indirect lighting going on in this render. If you were to increase the gamma correction of this render, you would notice it. But it certainly is not what we could have expected. Why?

Here is a very important set of issues to consider when using radiosity: Radiosity will produce realistic renders only in a scene setup with realistic objects, props and textures, even if those props and objects and texture may not be visible from the camera point of view. This is very different than with simple raytracing where only what is actually visible is important.

In the Thanksgiving scene, for instance, all the walls and the ceiling are red. The red color is composed of Red @ 50%, Green @ 0% and Blue @ 0%. Meaning that all photons hitting the walls and the ceiling have their Green and Blue energy completely absorbed the first time they hit a wall or the ceiling. And the Red energy is absorbed 50% each time it hits a wall or the ceiling. So after just a few bounces around, the photons are left with very little energy. Not much to contribute indirect lighting.

When using Radiosity, all colors should have some of each of the red, green and blue channels in them. Pure colors like this red used on the walls here are almost nonexistant in real life. Every surface reflects at least a little of each channels. For instance, even the black Mackbeth color reflects about 3% on each channels.

The other issue with this scene, is that there are no objects in the scene that could reflect more light back. Because this scene was designed for ray-tracing render, there were no furnitures added apart from those that are directly visible in the camera frame. The room could benefit from drapes around the windows, frames on the wall, etc.

Another quick fix, also would be to set the ceiling color to white. As can be seen, there would be a lot of work to add to get a realistic lighting from radiosity.

The other option is to fake rasiosity. By adding lights in the scene, it is possible to simulate the indirect light coming from the ceiling, the walls and any other objects that would be in the scene.

Here, to the right, is the way I setup the lights in order to fake radiosity. Notice that my plan is not to emulate the radiosity render we just saw. I want a more realistic lighting without the expense of adding a lot of extra models and textures in the scene to reflect back indirect lighting.

There are 10 more lights in the scene. All huge klieg lights 100cm in diameter, 180° cone angle and a falloff at 100cm, intensity at 100%. The kliegs mounted on the walls have the same color as the walls to simulate light bouncing from them and the kliegs mounted in the ceiling have a white color.

The lights are mounted in such a way that they are flush to the walls and the ceiling, just protruding inside by a few millimeters. There are 2 bounce lights on the ceiling and 2 on each walls.

Here, to the right, is the result:

This render already have a more radiosity feel than the real radiosity render. The indirect light is much more visible on the back of the chairs.

Faking radiosity in this way adds the advantage on much more control on the overall look. For instance, I could add white bounce lights to simulate lights bouncing from a white ceiling even though the ceiling is still very red. Even though this scene is very darkly textured,it is nevertheless possible to boost the radiosity look by playing with the bounce light intensities.

For instance, boosting the wall bounce lights to 150% intensity and reducing the ceiling bounce lights intensities to 50% will produce the render to the right. I could adjust the lights intensities and color even further and get a loog very near a real room even though I don't have the ideal conditions in this scene setup.

This example shows very basic and mechanical bounce lights placements. It is very possible and is actually the case in production situation, to add several bounce lights judiciously positionned and oriented to add illumination spots here and there and even add negative intensity lights to add shadows. Artistic use of lighting like that have produced stuningly realistic lighting for years before radiosity became fashionable.