Tech Explained: Radiosity

How does radiosity create better 3D images?

When creating an image using computer graphics, it is not enough to simply draw the image. In order to create a realistic appearance, the graphic designer also needs to account for the way that light will affect all of the elements in the image. 

In order to create a realistic image of a room filled with furniture, for example, the designer needs to compensate for how a lamp, overhead lighting or a window in the room will affect the appearance of all of the objects in the room. 

One method for doing this is called radiosity. This is a method of creating a computer-generated 3D image (rendering) of the interior of a building, based on an analysis of the way that light reflects off of different surfaces. So, how does radiosity work and why is it so useful?

How radiosity is applied to create a 3D image

Radiosity is a measure of radiant energy — that is, the amount of energy (light) that leaves a surface over a period of time. The theory of radiosity has been used by engineers since the 1950s for solving problems in radiative heat transfer and in calculating the amount of light energy transferred between two surfaces. In 1984, it was introduced as a method for rendering 3D images by researchers at Cornell.

Applying radiosity to graphic design involves first creating a computer model of the image. The surfaces in the model are divided into small areas called patches. An algorithm is then used to calculate a radiosity value for each patch — the amount of energy that is absorbed or reflected by each of the surfaces depicted in the patch. 

This data is then used to produce an image made up of all the patches from a particular viewpoint. In general, patches with high radiosity values are rendered more lightly, which makes them appear brighter, and those with low radiosity values are rendered darker, which makes them appear dimmer. The overall result is to create soft, gradual shadows, as one might see in a real room.

A more realistic image

Radiosity can provide more accurate shadowing and softening of the light than other methods of rendering 3D images. 

By including radiosity calculations in the graphic design process, an added element of realism is leant to the finished scene. This is because of the way the technique mimics real-world phenomena such as colour “bleeding.” 

To imagine how this works, picture a red ball resting on a white table. As light strikes the ball, it casts a shadow. At the same time, a tiny amount of red light from the ball is reflected onto surrounding objects (e.g., the table). This causes the parts of the white table next to the ball to take on a reddish hue. 

Although the effect is subtle, this is what help adds depth and shading to the real world. Duplicating it in an image helps to give the image the illusion of realism.

Uses for radiosity

Radiosity joins recent innovations in 3D rendering, such as imaging the human brain and creating a real-world holodeck. However, while radiosity is used by graphic designers and video game developers to create more realistic images, it is also an important technique for architects, builders, lighting engineers and designers who are modelling interior environments. 

For example, it can help them determine where to place features such as windows, lights and doors to keep rooms cool, or to provide the greatest illumination with the lowest energy expenditure.

There are a number of commercial radiosity programmes available, with most being  incorporated into render engines such as Autodesk 3D Studio Max or Maxwell Renderer by Next Limit. 

Although costly in terms of computer time and memory, radiosity is also the only technique that can reproduce physically-accurate lighting conditions in many common interior scenes.