Šárka Němcová

Optimizing of functional design parameters of kaleidoscopes

Imaging of surface textures requires many combinations of incident illumination angles and detector angles of view. Kaleidoscope is one of the means for measurement of bidirectional texture function of various sample surfaces. An optical system featuring the kaleidoscope is proposed in the paper. Optical parameters of such an imaging system are described and evaluated. We also discuss the optimization process of these parameters which influences the overall imaging performance of a kaleidoscope device. We provide the visualization of various kaleidoscope designs.

Lightdrum—Portable Light Stage for Accurate BTF Measurement on Site

We propose a miniaturised light stage for measuring the bidirectional reflectance distribution function (BRDF) and the bidirectional texture function (BTF) of surfaces on site in real world application scenarios. The main principle of our lightweight BTF acquisition gantry is a compact hemispherical skeleton with cameras along the meridian and with light emitting diode (LED) modules shining light onto a sample surface. The proposed device is portable and achieves a high speed of measurement while maintaining high degree of accuracy. While the positions of the LEDs are fixed on the hemisphere, the cameras allow us to cover the range of the zenith angle from 0∘ to 75∘ and by rotating the cameras along the axis of the hemisphere we can cover all possible camera directions. This allows us to take measurements with almost the same quality as existing stationary BTF gantries. Two degrees of freedom can be set arbitrarily for measurements and the other two degrees of freedom are fixed, which provides a tradeoff between accuracy of measurements and practical applicability. Assuming that a measured sample is locally flat and spatially accessible, we can set the correct perpendicular direction against the measured sample by means of an auto-collimator prior to measuring. Further, we have designed and used a marker sticker method to allow for the easy rectification and alignment of acquired images during data processing. We show the results of our approach by images rendered for 36 measured material samples.

Software simulator for design and optimization of the kaleidoscopes for the surface reflectance measurement

Realistic reproduction of appearance of real-world materials by means of computer graphics requires accurate measurement and reconstruction of surface reflectance properties. We propose an interactive software simulation tool for modeling properties of a kaleidoscopic optical system for surface reflectance measurement. We use ray tracing to obtain fine grain simulation results corresponding to the resolution of a simulated image sensor and computing the reflections inside this system based on planar mirrors. We allow for a simulation of different geometric configurations of a kaleidoscope such as the number of mirrors, the length, and the taper angle. For accelerating the computation and delivering interactivity we use parallel processing of large groups of rays. Apart from the interactive mode our tool also features batch optimization suitable for automatic search for optimized kaleidoscope designs. We discuss the possibilities of the simulation and present some preliminary results obtained by using it in practice.

Optomechanical design of rotary kaleidoscope for bidirectional texture function acquisition

Optical systems are traditionally used for accurate recording and measurement of the real worlds appearance. Present techniques allow us to form a computer-based virtual world, which is used in a variety of technical fields. The crucial issue for future applications of virtual reality is the fidelity of rendered images to real-world objects. This is strongly affected by the appearance of the rendered objects surfaces. Currently, the most applied method of describing a surfaces visual appearance of spatially nonuniform surfaces is bidirectional texture function (BTF). We have designed, optimized, built, and tested a unique portable instrument based on a rotary kaleidoscope principle for BTF acquisition in situ. To the best of our knowledge, such an instrument has never been used before to measure BTF of a surface. We enhanced a common static kaleidoscope by adding rotation, which allows us to get a larger number of images of the sample for more combinations of illumination directions and viewing directions. This results in a higher directional and spatial resolution of measured BTF data. In this paper, we focus on the optomechanical design of the rotary BTF measurement instrument and issues related to its alignment to keep the desired mechanical precision.

Compensating for colour artefacts in the design of technical kaleidoscopes

In computer graphics and related fields, bidirectional texture function (BTF) is used for realistic and predictive rendering. BTF allows for the capture of fine appearance effects such as self-shadowing, inter-reflection and subsurface scattering needed for true realism when used in rendering algorithms. The goal of current research is to get a surface representation indistinguishable from the real world. We developed, produced and tested a portable instrument for BTF acquisition based on kaleidoscopic imaging. Here we discuss the colour issues we experienced after the initial tests. We show that the same colour balance cannot be applied to the whole picture as the spectral response of the instrument varies with the position in the image. All optical elements were inspected for their contributions to the spectral behaviour of the instrument. The off-the-shelf parts were either measured or the manufacturer’s data were considered. The custom made mirrors’ spectral reflectivity was simulated. The mathematical model of the instrument was made. We found a way how to implement all these contributions to the image processing pipeline. In this way, a correct white balance for each individual pixel in the image is found and applied, allowing for a more faithful colour representation. Also proposed is an optimized dielectric protective layer for the kaleidoscope’s mirrors.

Geometric calibration of rotational kaleidoscopic instrument

The measurement of spatially varying surface reflectance is required for faithful reproduction of real world to allow for predictive look of computer generated images. One such proposed method uses a rotational kaleidoscopic imaging, where illumination and imaging paths are realized by subimages on kaleidoscopic mirrors and illumination is carried out by a DLP projector. We describe a novel geometric calibration method for a rotational kaleidoscope that is necessary to get aligned and accurate data from measurement. The calibration has two stages. The first stage mechanically adjusts the camera, the projector, and the autocollimator against the kaleidoscope mirrors. The second stage is based on the software. By random perturbation of camera and projector in corresponding mathematical model of the kaleidoscope we estimate better real positions of camera and projector in a physical setup, comparing the computed images from the software simulator and the acquired images from the physical setup.

Birefringence test for basic load case

Birefringence can be a crucial problem for many optical instruments operating with laser beams. Even for birefringence free optical elements, birefringence can be introduced due to mounting forces and torques. Highly sensitive polarimeters need to be used to reveal the limit values of linear or circular retardance introduced into the optical system. The correct assessment of polarimeter’s performance needs to be done with an appropriate test sample. We propose a birefringence test sample based on a basic load case of a circular plate loaded with torsion stress. Such a test sample has many advantages for a birefringence measurement testing. There is always present a zero level of stress in the middle of the circular sample. The stress level linearly increases towards the perimeter and its slope can be set arbitrarily. There is no change of either the sample volume or its shape under torsion stress. In the paper we evaluate the birefringence of a sample under torsion stress with Jones matrix calculus. We used a torsion stressed sample to verify reliability of our proposed setup for birefringence measurement based on polarizing Mach-Zehnder interferometer.

Alignment Issues for any Sample Direction BTF Measurement

Measurement of the bidirectional texture function (BTF) of the surfaces with portable instruments allowing on-site measurement outside the laboratory conditions is a newly emerging technique enabling data acquisition of the real objects without the necessity of their extraction from the environment. A practical issue of the sample measurement is the instrument alignment regarding the measured surface. The aim is to measure a sample surface under any orientation in space and to align the measurement instrument to the surface so to measure surface reflectance of the sample surface. This paper describes the design of two setups allowing adjustment of a portable BTF measurement instrument regarding the sample surface at any position. The final instrument alignment is performed with a help of feedback information provided by a simple laser autocollimator.

Lightdrum: surface reflectance measurement on site

We present a portable instrument for on site measuring of surface reflectance represented by the bidirectional texture function (BTF) and the bidirectional reflectance distribution function (BRDF). Our device allows for measurement application scenarios outside the laboratory without the necessity to extract the measured sample from its environment because the instrument is taken to the measured sample. The concept is a rotational lightweight light stage with a compact hemispherical dome and cameras along the meridian and light emitting diode (LED) modules illuminating the sample surface. The LED modules are fixed on the hemisphere and the six cameras can move along the arc in the range of the elevation angle from 0 to 75 degrees. By rotating the hemispherical dome along its axis we can set all possible camera directions to a measured sample. We use an auto-collimator to adjust the correct perpendicular direction of the instrument against the sample. The proposed instrument is portable and fast while maintaining a high degree of accuracy achieving a quality similar to existing stationary BTF gantries that can be only used in a laboratory. The instrument design provides a good tradeoff between the accuracy of measurements and the practical applicability for measurement of locally flat samples. The instrument provides approximately 1000 HDR photographs in a minute that are necessary to capture spatially varying surface reflectance.

Compensating for color artifacts in the design of technical kaleidoscopes

Abstract. In the applications of computer graphics, bidirectional texture function (BTF) is used for realistic and predictive rendering. The goal of current research is to get a surface representation indistinguishable from the real world. We developed, built, and tested a portable instrument for BTF acquisition based on kaleidoscopic imaging. We discuss the color issues we experienced after the initial tests. We show that the same color balance cannot be applied to the whole image as the spectral response of the instrument varies depending on the position within the image. All optical elements were inspected for their contributions to the spectral behavior of the instrument. A software simulator of a mathematical model of the device was implemented. We found a way to implement all these contributions into the image processing pipeline. In this way, the correct white balance for each individual pixel in the image is found and applied, allowing for a more faithful color representation. Also proposed is an optimized dielectric protective layer for the kaleidoscope’s mirrors causing the least possible color aberration.