Physically Accurate Lighting Simulation in Computer Graphics Software

A.B. Khodulev, E.A. Kopylov


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2. SPECTER SYSTEM

We refer here to Specter User Manual.

2.1 Comprehensiveness of local model

Light sources

Specter supports self-luminous objects of arbitrary shape but with Lambertian directional distribution of emitted light. In addition, several types of standard light sources are available: point, linear, rectangular, circular, for which a goniometrical diagram can be assigned that describes directional distribution of emitted light. Parallel light source is available too.

Material attributes

In addition to simple specular/diffuse reflectivity and transparency Specter provides direction-dependent reflectivity and transparency (both specular and diffuse) that can be characterized by Bi-directional Reflectance Distribution Function (BRDF, ref. to ch.3.2.1). One standard BRDF is permitted that describes light reflectivity/transparency of smooth dielectric boundary (Fresnel formula). A general BRDF (for isotropic surfaces) can be specified in tabular form as well. It allows to describe not only reflectivity variations but also color variation in dependence on direction of incoming light and viewing direction. The restrictions of general BRDF are:

Texture assignment is supported.

The above attributes are assigned to surface. There are also volume attributes. Specter provides the most common ones: refraction index and attenuation coefficient.

Medium

As opposed to the other two systems Specter permits to set and control medium (transmittance and index of refraction), background (color, luminance, texture, sunset / dawn mode), atmospheric phenomena (fog, haze, depth), and glare effect. The glare effect, supported by Specter, provides for generation of more convincing images of night scenes at rainy days. It simulates physical phenomena of scattering the light in water drops around the light sources (ref. to ch.3.2, ch.3.6.7, and ch.5.4.2).

2.2 Completeness of global light propagation model

The Bi-Directional ray tracing scheme used by Specter provides complete global light propagation model for scenes composed of specular and diffuse objects including dielectric boundaries (Fresnel objects) and self-luminous objects. This is a very important property as it allows to obtain accurate simulation results for models involving arbitrary long sequences of specular and diffuse interreflections, which is typical for some technical devices such as plane light emitters, and user even have not to bother about the complexity of light paths in a model in question.

For objects described by general BRDF the global model is not complete; BRDF-to-diffuse and diffuse-to-BRDF light transfer is not supported.

2.3 Accuracy of simulation

Specter provides tools for mesh subdivision. A finer mesh results in a more accurate illumination map (i-map). Inside a single mesh element a linear function is used to represent actual illuminance distribution. As opposed to LVS, during global lighting analysis Specter does not use adaptive subdivision. Specter uses term "ideal i-map" to denote the best in some sense illumination map on a given mesh. An actual i-map produced as a result of global lighting analysis (done by means of forward Monte Carlo ray tracing) can differ from ideal i-map. This is error of simulation method.

A unique feature of Specter is that it provides an estimate for this error; moreover, user can set a desired level of the error and system proceeds till this level is reached (monitoring the current accuracy level achieved and displaying intermediate results if user requested it; of course, premature interruption is possible).

The main drawback of Specter approach to accuracy control is that it uses only global error estimate: an average value for the whole scene or for some areas of interest specified by user. Specter cannot provide more accurate simulation in one place and less accurate in others, or even a uniform error distribution over the scene (this is hardly achievable in frames of forward ray tracing scheme). Nevertheless, it should be mentioned that other systems have no means to estimate real accuracy of the simulation at all.

2.4 User interface features

Output result presentation

In addition to ordinary images and images in color fringes, Specter provides various other formats of result presentation: numerical tables of luminance / illuminance distribution, graphs of different types, and interactive "Pick" action to extract luminance / illuminance at the specified point in an image. Graphs and images are available interactively, so that user can immediately observe change of results in course of simulation and as a feedback to changes in scene.

To provide a realistic image for various display types Specter supports gamma correction as well as a more general correction mode based on monitor calibration data or Stevens-Marsden formulae [MNM84]. Specter also provides the monitor calibration tool [KK94].

System interactivity

Specter is a highly interactive system. Any attribute of material or light sources can be changed interactively. Camera can be set interactively too. Camera path can be stored to provide animated walk-through.

Specter produces incremental output of simulation results: user may specify time interval and the system automatically visualizes results (images or graph) with a specified periodicity, so that user just observes the changing picture and stops the simulation manually when the results appear satisfactory.

On the other hand, as opposed to LVS, incremental recalculation after scene change is not supported.


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Copyright 1996 Andrei B. Khodulev, Edward A. Kopylov.- All Rights Reserved [Home]