Global Illumination

The first and foremost question that people ask about Rayfront or radiosity renderers is "Why bother?". They take much longer to calculate their illumination results than standard computer graphic renderers. There are two answers: the scientific one (for architects as well as building scientists need to know about light!) which is that we need this level of calculation to work out realistic values for lighting in a space; the second is the artisitc one (and building scientists reading this are as responsible for the poetry of lighting for people as any architect!): the subtle variations of light and shade in the pictures possible through global illumination is a fantastic start on the assessment of the quality of lighting in our buildings.

To quote from the introduction to the manual for Lightscape an early (1996-7) renderer that addressed physically based rendering: "The final illumination of the room is determined by the interaction between the surfaces and the billions
of photons that are emitted from the light source. At any given point on a surface, it is possible that photons have arrived directly from the light source (direct illumination) or else indirectly through one or more bounces off some other surfaces (indirect illumination).

" If you were standing in the room, a very small number of the total photons in the room would enter your eye and stimulate the rods and cones of your retina. This stimulation would, in effect, form an image that is perceived by your brain. Computers replace the rods and cones of a retina with the pixels of the computer screen. One goal of a global illumination algorithm is to recreate, as accurately as possible, what you would see if you were standing in a real environment. A second goal is to accomplish this task as quickly as possible, ideally in real time (30 images per second). There is currently no single global illumination algorithm that can accomplish both of these goals."

Revit includes both a radiosity renderer and a raytrace renderer. Together they combine to provide illumination quality that combines the best of both approaches.

Raytracing:
"One of the first global illumination algorithms to be developed is known as ray tracing. In ray tracing, it is recognized that while there may be billions of photons traveling about the room, the photons you primarily care about are the ones that enter the eye. The algorithm works by tracing rays backward, from each pixel on the screen into the 3D model. In this
way, it computes only the information needed to construct the image. To create an image using ray
tracing, do the following procedure for each pixel on the computer screen:

Radiosity:
"To address some of the shortcomings of the ray tracing algorithm, researchers began investigating alternative techniques for calculating global illumination. In the early 1960s, thermal engineers developed methods for simulating the radiative heat transfer between surfaces. Their goal was to determine how their designs would perform in various applications such as furnaces and engines. In the mid-1980s, computer graphics researchers began investigating the application of these techniques for simulating light propagation. Radiosity, as this technique is called in the computer graphics world, differs fundamentally from ray tracing. Rather than determining the color for each pixel on a screen, radiosity calculates the intensity for discrete points in the environment. Radiosity accomplishes this by first dividing the original surfaces into a mesh of smaller surfaces known as elements. The radiosity process calculates the amount of light distributed from each mesh element to every other mesh element. It then stores the final radiosity values for each element of the mesh.

Unfortunately, even though these approaches are physically based and can therefore be seen as representing the actual lighting in a space reasonably accurately, the software does not provide us with the ability to read from it the lighting levels. We will be doing this in the course, so we need to EXPORT our model to Rayfront/Radiance and then to run Rayfront for a cloudy sky condition to represent in false colour the light levels we attain.