Aditi Majumder

Achieving color uniformity across multi-projector displays

Large area tiled displays are gaining popularity for use in collaborative immersive virtual environments and scientific visualization. While recent work has addressed the issues of geometric registration, rendering architectures, and human interfaces, there has been relatively little work on photometric calibration in general, and photometric non-uniformity in particular. For example, as a result of differences in the photometric characteristics of projectors, the color and intensity of a large area display varies from place to place. Further, the imagery typically appears brighter at the regions of overlap between adjacent projectors. We analyze and classify the causes of photometric non-uniformity in a tiled display. We then propose a methodology for determining corrections designed to achieve uniformity, that can correct for the photometric variations across a tiled projector display in real time using per channel color look-up-tables (LUT).

Color nonuniformity in projection-based displays: analysis and solutions

Large-area displays made up of several projectors show significant variation in color. Here, we identify different projector parameters that cause the color variation and study their effects on the luminance and chrominance characteristics of the display. This work leads to the realization that luminance varies significantly within and across projectors, while chrominance variation is relatively small, especially across projectors of same model. To address this situation, we present a method to achieve luminance matching across all pixels of a multiprojector display that results in photometrically uniform displays. We use a camera as a measurement device for this purpose. Our method comprises a one-time calibration step that generates a per channel per projector luminance attenuation map (LAM), which is then used to correct any image projected on the display at interactive rates on commodity graphics hardware. To the best of our knowledge, this is the first effort to match luminance across all the pixels of a multiprojector display.

Perceptual photometric seamlessness in projection-based tiled displays

Arguably, the most vexing problem remaining for multi-projector displays is that of photometric (brightness) seamlessness within and across different projectors. Researchers have strived for <i>strict photometric uniformity</i> that achieves identical response at every pixel of the display. However, this goal typically results in displays with severely compressed dynamic range and poor image quality. In this article, we show that strict photometric uniformity is not a requirement for achieving photometric seamlessness. We introduce a general goal for photometric seamlessness by defining it as an optimization problem, balancing <i>perceptual uniformity</i> with <i>display quality</i>. Based on this goal, we present a new method to achieve <i>perceptually seamless high quality displays</i>. We first derive a model that describes the photometric response of projection-based displays. Then we estimate the model parameters and modify them using perception-driven criteria. Finally, we use the graphics hardware to reproject the image computed using the modified model parameters by manipulating only the projector inputs at interactive rates. Our method has been successfully demonstrated on three different practical display systems at Argonne National Laboratory, made of 2 × 2 array of four projectors, 2 × 3 array of six, projectors, and 3 × 5 array of fifteen projectors. Our approach is efficient, automatic and scalable---requiring only a digital camera and a photometer. To the best of our knowledge, this is the first approach and system that addresses the photometric variation problem from a perceptual stand point and generates truly seamless displays with high dynamic range.

Fast high-resolution appearance editing using superimposed projections

We present a system that superimposes multiple projections onto an object of arbitrary shape and color to produce high-resolution appearance changes. Our system produces appearances at an improved resolution compared to prior works and can change appearances at near interactive rates. Three main components are central to our system. First, the problem of computing compensation images is formulated as a constrained optimization which yields high-resolution appearances. Second, decomposition of the target appearance into base and scale images enables fast swapping of appearances on the object by requiring the constrained optimization to be computed only once per object. Finally, to make high-quality appearance edits practical, an elliptical Gaussian is used to model projector pixels and their interaction between projectors. To the best of our knowledge, we build the first system that achieves high-resolution and high-quality appearance edits using multiple superimposed projectors on complex nonplanar colored objects. We demonstrate several appearance edits including specular lighting, subsurface scattering, inter-reflections, and color, texture, and geometry changes on objects with different shapes and colors.

Perception-based contrast enhancement of images

Study of contrast sensitivity of the human eye shows that our suprathreshold contrast sensitivity follows the Weber Law and, hence, increases proportionally with the increase in the mean local luminance. In this paper, we effectively apply this fact to design a contrast-enhancement method for images that improves the local image contrast by controlling the local image gradient with a single parameter. Unlike previous methods, we achieve this without explicit segmentation of the image, either in the spatial (multiscale) or frequency (multiresolution) domain. We pose the contrast enhancement as an optimization problem that maximizes the average local contrast of an image strictly constrained by a perceptual constraint derived directly from the Weber Law. We then propose a greedy heuristic, controlled by a single parameter, to approximate this optimization problem.

Immersive teleconferencing: a new algorithm to generate seamless panoramic video imagery

This paper presents a new algorithm for immersive teleconferencing, which addresses the problem of registering and blending multiple images together to create a single seamless panorama. In the immersive teleconference paradigm, one frame of the teleconference is a panorama that is constructed from a compound-image sensing device. These frames are rendered at the remote site on a projection surface that surrounds the user, creating an immersive feeling of presence and participation in the teleconference. Our algorithm efficiently creates panoramic frames for a teleconference session that are both geometrically registered and intensity blended. We demonstrate a prototype that is able to capture images from a compound-image sensor, register them into a seamless panoramic frame, and render those panoramic frames on a projection surface at 30 frames per second.

Asynchronous Distributed Calibration for Scalable and Reconfigurable Multi-Projector Displays

Centralized techniques have been used until now when automatically calibrating (both geometrically and photometrically) large high-resolution displays created by tiling multiple projectors in a 2D array. A centralized server managed all the projectors and also the camera(s) used to calibrate the display. In this paper, we propose an asynchronous distributed calibration methodology via a display unit called the plug-and-play projector (PPP). The PPP consists of a projector, camera, computation and communication unit, thus creating a self-sufficient module that enables an asynchronous distributed architecture for multi-projector displays. We present a single-program-multiple-data (SPMD) calibration algorithm that runs on each PPP and achieves a truly scalable and reconfigurable display without any input from the user. It instruments novel capabilities like adding/removing PPPs from the display dynamically, detecting faults, and reshaping the display to a reasonable rectangular shape to react to the addition/removal/faults. To the best of our knowledge, this is the first attempt to realize a completely asynchronous and distributed calibration architecture and methodology for multi-projector displays

Color Seamlessness in Multi-Projector Displays Using Constrained Gamut Morphing

Multi-projector displays show significant spatial variation in 3D color gamut due to variation in the chromaticity gamuts across the projectors, vignetting effect of each projector and also overlap across adjacent projectors. In this paper we present a new constrained gamut morphing algorithm that removes all these variations and results in true color seamlessness across tiled multi-projector displays. Our color morphing algorithm adjusts the intensities of light from each pixel of each projector precisely to achieve a smooth morphing from one projectors gamut to the others through the overlap region. This morphing is achieved by imposing precise constraints on the perceptual difference between the gamuts of two adjacent pixels. In addition, our gamut morphing assures a C1 continuity yielding visually pleasing appearance across the entire display. We demonstrate our method successfully on a planar and a curved display using both low and high-end projectors. Our approach is completely scalable, efficient and automatic. We also demonstrate the real-time performance of our image correction algorithm on GPUs for interactive applications. To the best of our knowledge, this is the first work that presents a scalable method with a strong foundation in perception and realizes, for the first time, a truly seamless display where the number of projectors cannot be deciphered.

Hardware accelerated real time charcoal rendering

In this paper, we present simple rendering techniques implemented using traditional graphics hardware to achieve the effects of charcoal drawing. The effects include characteristics of charcoal drawings like broad grainy strokes and smooth tonal variations that are achieved by smudging the charcoal by hand. Further, we also generate the closure effect that is used by artists at times to avoid hard silhouette edges. All these effects are achieved using contrast enhancement operators on textures and/or colors of the 3D model.Our contribution lies in unifying the methods to achieve these effects under the common framework of contrast enhancement operators. Further, since the effects have been implemented using traditional graphics hardware, a single rendering pass is sufficient to create different effects. Hence, we can render highly complex models with large number of triangles at interactive rates. Thus, our method is especially suited for applications like scientific visualization and preliminary sketches/animations.

Registration Techniques for Using Imperfect and Par tially Calibrated Devices in Planar Multi-Projector Displays

Multi-projector displays today are automatically registered, both geometrically and photometrically, using cameras. Existing registration techniques assume pre-calibrated projectors and cameras that are devoid of imperfections such as lens distortion. In practice, however, these devices are usually imperfect and uncalibrated. Registration of each of these devices is often more challenging than the multi-projector display registration itself. To make tiled projection-based displays accessible to a layman user we should allow the use of uncalibrated inexpensive devices that are prone to imperfections. In this paper, we make two important advances in this direction. First, we present a new geometric registration technique that can achieve geometric alignment in the presence of severe projector lens distortion using a relatively inexpensive low-resolution camera. This is achieved via a closed-form model that relates the projectors to cameras, in planar multi-projector displays, using rational Bezier patches. This enables us to geometrically calibrate a 3000 times 2500 resolution planar multi-projector display made of 3 times 3 array of nine severely distorted projectors using a low resolution (640 times 480) VGA camera. Second, we present a photometric self-calibration technique for a projector-camera pair. This allows us to photometrically calibrate the same display made of nine projectors using a photometrically uncalibrated camera. To the best of our knowledge, this is the first work that allows geometrically imperfect projectors and photometrically uncalibrated cameras in calibrating multi-projector displays.