Seeing the light
18 November 2003
How can we tell how things would have looked to people in the past? This simple question led Kate Devlin, Veronica Sundstedt and Alan Chalmers from the University's Computer Science Department to become involved in realistic graphics and archaeology.
Computer-generated images are becoming a popular way of presenting archaeological information. To date, however, the emphasis has been on using these images for display purposes, with interpretation and research taking second place to the demand for a visually stunning presentation. In realistic computer graphics, the main goal is the creation of images that are perceptually equivalent to a real scene. So for computer graphics to benefit the archaeological community they must offer the archaeologist the chance to extend or enhance their analysis of a site or artefact. Furthermore, the accuracy of the images produced should be quantifiable – the archaeologist must be confident that what they see in the generated image is comparable to what would have been seen originally.
Initial work in this area began in 1994 and in 2001 Dr Alan Chalmers and Duncan Brown (Southampton City Heritage Services) won Bristol University’s first Business Plan Competition with ‘ArchLight’, a system for producing highly realistic, computer-generated environments. Since then the research of the computer graphics group has focused on one area of realistic simulations that is often neglected: original lighting. Light cannot be captured in the archaeological record and consequently its importance is not always considered in interpretations of past environments. The ways in which we view, perceive and understand objects is governed by our current lighting methods – steady, bright electric light or large windows – but in order to understand how an environment and its contents were viewed in the past we must consider how they were illuminated.
Standard three-dimensional modeling software tends to base the lighting conditions on daylight, fluorescent light or filament bulbs, and not the lamp and candlelight used in past. Realistic lighting simulation must address both the physical interaction of light in a scene (how light is distributed) and the spectral profile of the light source (the colour of the light). With control over this, an accurately lit representation of an environment can be achieved and the virtual version of an original site or artefact can be manipulated without having to physically touch or harm the real version.
The archaeologist must be confident that what they see in the generated image is comparable to what would have been seen originally
Once an archaeological site or artefact has been modelled in a threedimensional modelling package it must be rendered. That is, the colours, textures, light and shading are computed thus producing the final images from the three-dimensional geometry. In order to obtain an approximation of the original lighting in an archaeological representation, two factors must be addressed in the rendering process. First, the spectral composition of the light – the colour of the light given off by the burning fuel – must match that of the fuel type that would have been used in a specific archaeological instance. Second, the distribution of this light – the path it takes around a scene and the reflections and inter-reflections that occur – must mimic the behaviour of light in the real world.
The only trace of light in the archaeological record is the methods used to provide it, be they hearths, candles, lamps or windows. In preindustrial societies, daylight was the regulating factor of the working hours. If we compare that to conditions today, sunlight is far less relevant to how we work. The evidence from architecture tells us the most about lighting – a lack of glass and a need for security often meant smaller windows, therefore dimmer interiors. Going further back in time, the unyielding darkness of a deep cave would require some form of artificial light for navigation purposes alone. It seems plausible that objects and environments were affected by the limitations of lighting, and this influence may have extended into their design. By recreating the means of illumination for a given environment and simulating it accurately, the archaeologist may (literally) find new ways of viewing things.
The initial step in recreating the light source involves experimental archaeology. In consultation with the Department of Archaeology, various types of light sources were physically recreated. These included tallow candles and reeds, an animal fat lamp, beeswax candles and olive oil lamps. Each of these fuels produces a different colour when burnt. To obtain this unique spectral profile for each fuel, detailed data were gathered using a spectroradiometer, a device that measures the absolute value of the spectral characteristics without making physical contact with the flame. The values obtained from the original light sources can then be used as lighting values for a computer-generated model, so that a scene can be rendered under its appropriate lighting conditions.
One of the most recent projects undertaken was Veronica Sundstedt’s MSc dissertation on the ancient Egyptian temple of Kalabsha. In 1963 the temple, which dates back to 30 BC, was moved to a new site in order to save it from the rising waters of Lake Nasser. Working closely with Egyptologists it has been possible to use computer graphics and information from the archaeological record to recreate the temple, place it back to its original location and orientation, and illuminate it as it may have appeared some 2,000 years ago. An animation was created showing how sunlight would have affected the perception of the temple over a ten-hour period in January 30 BC. A comparison of this animation and the video footage taken at the site in January 2003 enables the Egyptologists to compare just how the new location and orientation of the temple, in addition to the damage, has altered its appearance. The interior of the temple was also modelled in detail. This allows the Egyptologists to see how the hieroglyphics would have been perceived in the past – not as they are seen today, unpainted (the paint having long since faded) and under modern lighting – but brightly painted under the illumination of sesame oil lamps.
Overall, this research into colour and light has shown how easy it is for our own preconceptions to intrude into the ways we view archaeological objects or sites, but a definitive explanation should never be expected in archaeology. Visualising a past environment is fraught with difficulties from the outset, so a means of validating computer-generated representations provides an exciting opportunity to explore and test new ideas, with computer graphics becoming as beneficial to the archaeologist as they are to the public.