Min H. Kim

Imperfect shadow maps for efficient computation of indirect illumination

We present a method for interactive computation of indirect illumination in large and fully dynamic scenes based on approximate visibility queries. While the high-frequency nature of direct lighting requires accurate visibility, indirect illumination mostly consists of smooth gradations, which tend to mask errors due to incorrect visibility. We exploit this by approximating visibility for indirect illumination with imperfect shadow maps---low-resolution shadow maps rendered from a crude point-based representation of the scene. These are used in conjunction with a global illumination algorithm based on virtual point lights enabling indirect illumination of dynamic scenes at real-time frame rates. We demonstrate that imperfect shadow maps are a valid approximation to visibility, which makes the simulation of global illumination an order of magnitude faster than using accurate visibility.

Modeling human color perception under extended luminance levels

Display technology is advancing quickly with peak luminance increasing significantly, enabling high-dynamic-range displays. However, perceptual color appearance under extended luminance levels has not been studied, mainly due to the unavailability of psychophysical data. Therefore, we conduct a psychophysical study in order to acquire appearance data for many different luminance levels (up to 16,860 cd/m2) covering most of the dynamic range of the human visual system. These experimental data allow us to quantify human color perception under extended luminance levels, yielding a generalized color appearance model. Our proposed appearance model is efficient, accurate and invertible. It can be used to adapt the tone and color of images to different dynamic ranges for cross-media reproduction while maintaining appearance that is close to human perception.

3D imaging spectroscopy for measuring hyperspectral patterns on solid objects

Sophisticated methods for true spectral rendering have been developed in computer graphics to produce highly accurate images. In addition to traditional applications in visualizing appearance, such methods have potential applications in many areas of scientific study. In particular, we are motivated by the application of studying avian vision and appearance. An obstacle to using graphics in this application is the lack of reliable input data. We introduce an end-to-end measurement system for capturing spectral data on 3D objects. We present the modification of a recently developed hyperspectral imager to make it suitable for acquiring such data in a wide spectral range at high spectral and spatial resolution. We capture four megapixel images, with data at each pixel from the near-ultraviolet (359 nm) to near-infrared (1,003 nm) at 12 nm spectral resolution. We fully characterize the imaging system, and document its accuracy. This imager is integrated into a 3D scanning system to enable the measurement of the diffuse spectral reflectance and fluorescence of specimens. We demonstrate the use of this measurement system in the study of the interplay between the visual capabilities and appearance of birds. We show further the use of the system in gaining insight into artifacts from geology and cultural heritage.

Characterization for high dynamic range imaging

In this paper we present a new practical camera characterization technique to improve color accuracy in high dynamic range (HDR) imaging. Camera characterization refers to the process of mapping device‐dependent signals, such as digital camera RAW images, into a well‐defined color space. This is a well‐understood process for low dynamic range (LDR) imaging and is part of most digital cameras — usually mapping from the raw camera signal to the sRGB or Adobe RGB color space. This paper presents an efficient and accurate characterization method for high dynamic range imaging that extends previous methods originally designed for LDR imaging. We demonstrate that our characterization method is very accurate even in unknown illumination conditions, effectively turning a digital camera into a measurement device that measures physically accurate radiance values — both in terms of luminance and color — rivaling more expensive measurement instruments.

Perceptual influence of approximate visibility in indirect illumination

In this article we evaluate the use of approximate visibility for efficient global illumination. Traditionally, accurate visibility is used in light transport. However, the indirect illumination we perceive on a daily basis is rarely of high-frequency nature, as the most significant aspect of light transport in real-world scenes is diffuse, and thus displays a smooth gradation. This raises the question of whether accurate visibility is perceptually necessary in this case. To answer this question, we conduct a psychophysical study on the perceptual influence of approximate visibility on indirect illumination. This study reveals that accurate visibility is not required and that certain approximations may be introduced.

Insitu: sketching architectural designs in context

Architecture is design in spatial context. The only current methods for representing context involve designing in a heavyweight computer-aided design system, using a full model of existing buildings and landscape, or sketching on a panoramic photo. The former is too cumbersome; the latter is too restrictive in viewpoint and in the handling of occlusions and topography. We introduce a novel approach to presenting context such that it is an integral component in a lightweight conceptual design system. We represent sites through a fusion of data available from different sources. We derive a site model from geographic elevation data, on-site point-to-point distance measurements, and images of the site. To acquire and process the data, we use publicly available data sources, multidimensional scaling techniques and refinements of recent bundle adjustment techniques. We offer a suite of interactive tools to acquire, process, and combine the data into a lightweight stroke and image-billboard representation. We create multiple and linked pop-ups derived from images, forming a lightweight representation of a three-dimensional environment. We implemented our techniques in a stroke-based conceptual design system we call Insitu. We developed our work through continuous interaction with professional designers. We present designs created with our new techniques integrated in a conceptual design system.

Paper3D: bringing casual 3D modeling to a multi-touch interface

A 3D modeling system that provides all-inclusive functionality is generally too demanding for a casual 3D modeler to learn. In recent years, there has been a shift towards developing more approachable systems, with easy-to-learn, intuitive interfaces. However, most modeling systems still employ mouse and keyboard interfaces, despite the ubiquity of tablet devices, and the benefits of multi-touch interfaces applied to 3D modeling. In this paper, we introduce an alternative 3D modeling paradigm for creating developable surfaces, inspired by traditional papercrafting, and implemented as a system designed from the start for a multi-touch tablet. We demonstrate the process of assembling complex 3D scenes from a collection of simpler models, in turn shaped through operations applied to sheets of virtual paper. The modeling and assembling operations mimic familiar, real-world operations performed on paper, allowing users to quickly learn our system with very little guidance. We outline key design decisions made throughout the development process, based on feedback obtained through collaboration with target users. Finally, we include a range of models created in our system.

Edge-aware color appearance

Color perception is recognized to vary with surrounding spatial structure, but the impact of edge smoothness on color has not been studied in color appearance modeling. In this work, we study the appearance of color under different degrees of edge smoothness. A psychophysical experiment was conducted to quantify the change in perceived lightness, colorfulness, and hue with respect to edge smoothness. We confirm that color appearance, in particular lightness, changes noticeably with increased smoothness. Based on our experimental data, we have developed a computational model that predicts this appearance change. The model can be integrated into existing color appearance models. We demonstrate the applicability of our model on a number of examples.

Multiview Image Completion with Space Structure Propagation

We present a multiview image completion method that provides geometric consistency among different views by propagating space structures. Since a user specifies the region to be completed in one of multiview photographs casually taken in a scene, the proposed method enables us to complete the set of photographs with geometric consistency by creating or removing structures on the specified region. The proposed method incorporates photographs to estimate dense depth maps. We initially complete color as well as depth from a view, and then facilitate two stages of structure propagation and structure-guided completion. Structure propagation optimizes space topology in the scene across photographs, while structure-guide completion enhances, and completes local image structure of both depth and color in multiple photographs with structural coherence by searching nearest neighbor fields in relevant views. We demonstrate the effectiveness of the proposed method in completing multiview images.

Multispectral photometric stereo for acquiring high-fidelity surface normals.

Multispectral imaging and photometric stereo are common in 3D imaging but rarely have been combined. Reconstructing a 3D objects shape using photometric stereo is challenging owing to indirect illumination, specular reflection, and self-shadows, and removing interreflection in photometric stereo is problematic. A new multispectral photometric-stereo method removes interreflection on diffuse materials using multispectral-reflectance information and reconstructs 3D shapes with high accuracy. You can integrate this method into photometric-stereo systems by simply substituting the original camera with a multispectral camera.