Computer Graphics World

OCTOBER 2010

Issue link: https://digital.copcomm.com/i/18322

Contents of this Issue

Navigation

Page 10 of 51

require complicated computation to simulate lighting behavior. Tis revolution in CG, which took place from the mid-1990s to 2000, happened to coincide with the period when computer graphics began to dominate visual effects in Hollywood films. VFX vendors came to feel the necessity of producing a higher level of photorealism in a large amount of shots. Image-based technologies filled this demand. Te result was well received, and rapidly spread in filmmaking. Te most well known exam- ple of this adaptation is Te Matrix series, but there are many Hollywood movies wherein image-based technologies were used. Tis phenomenon resulted in the revision of the original image-based technologies from a practical viewpoint to support the progress of image-based technologies from both the academic and industrial perspectives. Photorealism Comes of Age As we entered the 21st century, various kinds of “bridging” appeared, with the purpose of producing a super-photorealistic look of the specific object. Tis movement was driven by the requirements of the industry, where humanlike characters appear en masse in video games and films. As a result, research related to human expressions began to flourish. For example, in the case of skin or hair, realism is largely affected by the behavior of light, which repeatedly scatters inside skin layers or hair volume. So by incorporating research done in medical optics or cos- metics, CG developed new and efficient physically based models that approximate this complex light behavior. Te modern architecture of hardware rendering, focused on the GPU, allowed the technology applications to run faster. However, in order to increase the realism that physically based models produce, researchers added to the process methods that aimed to derive the mod- el parameters by analyzing photographs of a real human’s skin and hair. Beginning in late 2008, physically based rendering methods became remarkably prevalent in the visual effects field and attracted attention to human expression. Because it is difficult to create such details purely us- ing rules-based methods, techniques that measure detailed realism from photographs were considered more heavily. Image-based technologies contributed to the recovery of the global appearance of digital humans, as well. Consider LightStage, a system in which a set of point-light sources (which can be switched “on” and “off”) are placed along a uniform grid over a dome, and a human sitting in the center of the dome is illuminated by these light sources. A set of photo- graphs taken under each lighting environment (lit by each light source) enables us to capture how the person’s appearance changes according to the changes in lighting and viewing direction. Tis information enables the reproduction of the subject’s appearance from an arbitrary viewing direction under arbitrary lighting conditions. Starting with Spider-Man 2 (2004), this system came to be used in a variety of film projects. Another example is a method called “video motion capture,” which can be thought of as an extension of image-based modeling. Image- based modeling recovers 3D geometry of an object from multiple photographs taken by multiple still cameras from different camera angles. Video motion capture uses multiple video cameras that shoot the performer from different viewing directions; it then recovers the 3D pose of the performer at each frame of the video. Compared to traditional optical or magnetic motion-capture methods, video motion capture has fewer limitations regarding the capture environment (space, costumes, and so forth). Starting with Arthur and the Invisibles (2006), video motion capture (Top) A computer animation of pouring water created using fluid simulation. (Bottom) A spectrogram of sound synthesized using the same graphics fluid simulator. Sharing the same fluid process, synthesized sounds are perfectly synchronized with the computer animation. became actively adopted in Hollywood film projects. From the academic perspective, as seen in recent SIGGRAPH paper sessions (especially since 2008), many ideas for capturing human appearance and motion using im- age-based approaches are published each year. Tis indicates that research in this direction is becoming the trend in image-based technologies. Beauty Factors: Skin and Hair Skin and hair are significant elements in the creation of realistic and believable expressions of human characters. Key to producing a higher degree of realism in these elements is knowing how to accurately and efficiently compute the scattering of light inside of skin and hair—a process known as “volume scattering.” A physically based representation of volume scattering is described by a radiative transfer equation. Solving this equation is computationally expensive, and as a result, various methods that attempted to approxi- mate it emerged and found their way into film and game projects. However, the radiative transfer equation that had been used stands on the assumption that the structure of the scattering media can be approximated by spherical particles, whereby the behavior of the scat- tering media is independent of the direction of the light propagation (that is, it looks the same to light traveling in different directions). Tis assumption limits the physical structure of the scattering media to be- ing isotropic, whereas materials such as skin, hair, and cloth have an anisotropic scattering structure. At SIGGRAPH 2010, Wenzel Jakob presented the paper “A Radiative Transfer Framework for Rendering Materials with Anisotropic Structure,” which aimed at removing such a limitation. Tis paper generalized a radiative transfer equation derived from oriented, non-spherical particles, and then proposed a new vol- ume-scattering model that can represent the anisotropic structure of scattering media with physical accuracy. Te technique may greatly in- crease the realistic appearance of skin, hair, cloth, and other important October 2010 9 Courtesy Doug James.

Articles in this issue

Archives of this issue

view archives of Computer Graphics World - OCTOBER 2010