Frontiers of Engineering Reports on Leading-Edge Engineering from the 2004 NAE Symposium on Frontiers of Engineering (2005) / Chapter Skim
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Capturing and Simulating Physically Accurate Illumination in Computer Graphics--Paul Debevec
Capturing and Simulating Physically Accurate Illumination in Computer Graphics--Paul Debevec
Pages 99-112
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From page 99... ...
Computer-generated images are two-dimensional arrays of computed pixel values, with each pixel coordinate having three numbers indicating the amount of red, green, and blue light coming from the corresponding direction in the scene. Figuring out what these numbers should be for a scene is not trivial, because each pixel's color is based both on how the object at that pixel reflects light and the light that illuminates it.
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From page 100... ...
In early computer graphics, interreflections of light between surfaces in a scene were poorly modeled. Light falling on each surface was computed solely as a function of the light coming directly from light sources, with perhaps a FIGURE 1 A simulation of indirect illumination in a scene, known as the "Cornell box." The white wall at the back picks up color from the red and blue walls at the sides.
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From page 101... ...
, path tracing generates images by tracing rays from a camera to surfaces in the scene, then tracing rays out from these surfaces to determine the incidental illumination on the surfaces. In path tracing, rays are traced not only in the direction of light sources, but also randomly in all directions to account for indirect light from the rest of the scene.
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From page 102... ...
And also, very importantly, light in the real world is interesting because typically there are different colors and intensities of light coming from every direction, which dramatically and subtly shape the appearance of the forms in a scene. Computer-generated scenes, when constructed from simple shapes, textured with ideal plastic and metallic reflectance properties, and illuminated by simple point and area light sources, lack "realism" no matter how accurate or computationally intensive the lighting simulation.
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From page 103... ...
One limitation of environment mapping is that it cannot reproduce the effects of object surfaces shadowing themselves or of light reflecting between surfaces. The reason for this limitation is that the lighting environment is applied directly to the object surface according to its surface orientation, regardless of the degree of visibility of each surface in the environment.
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From page 104... ...
In a typical scene, directly visible light sources are usually hundreds or thousands of times brighter than indirect illumination from the rest of the scene, and both types of illumination must be captured to represent the lighting accurately. This wide dynamic range typically exceeds the dynamic range of both digital and film cameras, which are designed to capture a range of brightness values of just a few hundred to one.
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From page 105... ...
. Photographs are taken using a series of varying exposure settings on the camera; brightly exposed images record indirect light from the surfaces in the scene, and dimly exposed images record the direct illumination from the light sources without clipping.
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From page 106... ...
We later used our lighting capture techniques to record the light in St. Peter's Basilica, which allowed us to add virtual tumbling monoliths and gleaming spheres to the basilica's interior for our SIGGRAPH 99 film Fiat Lux (Figure 4, bottom)
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From page 107... ...
To accomplish this we use a series of light stages to measure directly how an object transforms incidental environmental illumination into reflected radiance, a data set we call the reflectance field of an object. The first version of the light stage consisted of a spotlight attached to a twobar rotation mechanism that rotated the light in a spherical spiral about a person's face in approximately one minute (Debevec et al., 2000)
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From page 108... ...
. Mark Sagar and his colleagues at Sony Pictures Imageworks used related techniques to create the digital stunt actors of Tobey Maguire and Alfred Molina from light stage data sets for the film Spider-Man 2.
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From page 109... ...
A REMAINING FRONTIER: DIGITIZING REFLECTANCE PROPERTIES Significant challenges remain in the capture and simulation of physically accurate illumination in computer graphics. Although techniques for capturing object geometry and lighting are maturing, techniques for capturing object reflectance properties -- the way the surfaces of a real-world object respond to light -- are still weak.
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From page 110... ...
Pp. 189198 of the Proceedings of the Association for Computing Machinery Special Interest Group on Computer Graphics and Interactive Techniques 98 International Conference.
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From page 111... ...
FIGURE 7 a. A digital model of a digitized illuminated manuscript lit by a captured lighting environment.
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From page 112... ...
1983. Association for Computing Machinery Special Inter est Group on Computer Graphics and Interactive Techniques Computer Graphics 17(3)
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