The LightWorks goniometric light source shader is currently a powerful tool for providing illumination which has a complex spatial distribution. It is capable of reading industry standard photometric distribution files in 4 formats (IESNA, CIE, CIBSE and EULUMDAT). Typically, a goniometric light source is used when an accurate representation of the illumination provided by a complex lighting fixture is needed. The above images show two common applications of goniometric lights. The left-hand image shows a simple room illuminated by a ceiling-mounted light fixture which casts all its light downwards, in a distribution that leaves a fairly dark patch directly under the light. The right-hand figure shows a road scene illuminated by a (UK) car headlight which is modelled as a goniometric light: note how the correct distribution of light from a headlamp is displayed with more light being cast to the left than to the right.

Whilst the goniometric light functionality provided within LightWorks, prior to task 1.2, was already powerful, it did suffer from various limitations. These are summarised in the following list:

• A goniometric light had to have access to a photometric file containing distribution data: if the user had the same data, but in a different form, they needed to write it to one of the supported file formats to be able to use it.
• Even if the shape of a photometric distribution was known, there was no way of passing this information to a goniometric light without first generating a photometric file.
• Goniometric lights were restricted to emitting light from a single point in space. Whilst generally reasonable for illuminating points far from the light, such an approach is innaccurate for points which are near to the light.
• ARCADE partners, using the LightWords ADS as a lighting front end, were unable to conveniently grab the emission profile of a goniometric light source, once it had been created.

The majority of the above limitations of the goniometric light source have been removed by the introduction of Emission Sets and related functionality. A full description of this functionality can be found in the Emission Sets API Document. In summary, the following functionality is now in place for goniometric lights:

• All support for standard photometric files is retained.
• Users can generate their own Emission Set data structure using the new API and pass this to the goniometric light for it to use instead of a data file.
• ARCADE partners can grab the emission profile of any goniometric light source (no matter how generated), as an emission set. This can then be easily imported into their own systems.
• Users can provide their own functional representation of a photometric distribution for a light to use. Such a function can have associated data or no data at all, whatever is appropriate for the lighting distribution that is being modelled.
• If something goes awry when the light attempts to use data in any of the above forms it reverts to its current default behaviour which is to act like a point light source.

Early results are promising, both images at the top of this page were generated using an Emission Set rather than a photometric data file.

Functional representations for the lighting distribution have been used to generate several sample images.

• The left-hand image above shows a point light source (probably the simplest possible) generated using a goniometric light and a functional representation for the distribution which simply returns the value 1 for the luminous intensity emitted in any direction and the value 4PI for the integrated power of the lamp.
• The central image shows the same scene, but illuminated with a standard LightWorks point light source of the same intensity.
• The right-hand image shows a goniometric light with a functional representation that returns a random value for the luminous intensity in any given direction.

AVI QuickTime Mpeg

A more interesting example can be seen above. This shows a combination of 2 "glitter ball" goniometric light sources (one red and one green) illuminating a room. These light sources have functional distributions (again they use no photometric data) which emit light only for small ranges of angles in order to generate the glitter ball effect. The rotation is achieved by simply modifying the light sources' "equator zero" directions for each frame in the animation.

The new area goniometric light source is now complete. Via this shader it is possible to specify a light source which allows emission from an area in space rather than a single point. The area is defined using an appropriate LightWorks primitive. The shape of the photometric distribution for each point on the surface of the area can be defined either empirically or functionally.

Use of this new shader is straightforward; simply create an area light to define the area in space of interest, then create a goniometric light to define the shape of the lighting distribution at each point on the surface of the area. Once this is done create an area goniometric light and pass it pointers to the two previous lights, it then combines the two to provide the desired light source.

Complete documentation on the area goniometric light source shader can be found here.

The following example shows a potential use for the area goniometric shader. Here the images are generated using the light inside the radiosity module (click on each image to see the full-sized version).

• The left-hand image shows a wireframe rendering of the scene: a long, thin area light over a perpendicular, rectangular blocker and the floor.
• The second image shows how this scene would appear (with low radiosity quality settings) using just an area light to illuminate it.
• The third image shows how the scene would be illuminated using a goniometric light over the centre of the floor. This light has an emission set created by hand and shaped so as to provide illumination only at certain angles. The intention was to create an emission set that would be suitable for a long, thin area light which does not cast any light straight down, but does cast light out to the sides. This image shows how it is very hard to model such a light using a point source.
• Finally, the right-hand image shows the area goniometric light represented in space by the area light and with a distribution defined by the goniometric light. The end result is exactly as desired: a long, thin shadow directly under the area light and light being cast out to either side.

Note that the brightness of the image generated with the area goniometric light depends totally on the intensity setting of the area light scaled by the (assumed normalised) goniometric distribution. In this example all the values in the goniometric light's emission set are less than 1 so the final image appears dimmer due to this scaling effect.

The new line goniometric light source is complete. This currently allows emission from a straight line in space with the emission profile for each point on the line being defined via a goniometric light source. Complete documentation on the shader can be found here.

A couple of example images which are based on the same scene and goniometric distribution as the area goniometric light, above, are given below.

• The left-hand image shows the scene with ray-traced shadows and a fairly high level of detail giving a very smoothly illuminated image.
• The second image shows the same scene, but with shadow maps.
• The third image shows a scene with the line goniometric light penetrating the centre of the floor, and
• the right-hand image shows the line goniometric light inside an interacting medium.
  

  
  
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