The method will first be explained for simple ray tracing without interreflections. This is certainly not a physically plausible case, but due to its low computational cost, it is often used in real time applications (e.g. virtual reality) and it is widely spread in commercially available software. Generally, the diffusors can be placed in the scene using a regular grid on the projection plane. This grid resolution can be significantly coarser than the image resolution. The first intersections with scene objects are computed from the view point, and at these intersection points the diffusors are placed, with the same normal as the intersected object's surface. Diffusors are white, perfectly diffuse surface elements, used only for metering purposes, and are not visible in the final image.
If no interreflections are taken into account, the irradiances are
influenced only by direct illumination from the light sources
(sometimes, when an "ambient term" is added, it should be added to
irradiance values also, simply by adding a constant to all irradiance
values). According to (7.1) irradiance can be expressed as
where M is the number of point light sources,
is the power of the light source k,
is the distance between the light source k and the diffusor,
is the incident angle,
can be estimated from the average reflectivity.
It can be shown that a view dependent ambient term can also be used for non diffuse BRDFs, this is the topic of forthcoming work.
The irradiances are determined for a very low resolution image, which is not displayed. Of course, it is possible to render the "irradiance image" in high resolution to illustrate the irradiance distribution in the scene. For illustration purposes, instead of using white metering elements, all scene surfaces are set to medium gray (albedo = 0.5) Lambertian surfaces instead of using their original BRDFs. We call the result of rendering such a scene a cement image (see Color plates 11b, 15a and 15b). Note that this cement image is completely independent of the original BRDFs, so it is the same for various scene attribute settings. It depends only on direct lighting, as interreflections are not taken into consideration, yet.