468 research outputs found
Pushing the Limits of 3D Color Printing: Error Diffusion with Translucent Materials
Accurate color reproduction is important in many applications of 3D printing,
from design prototypes to 3D color copies or portraits. Although full color is
available via other technologies, multi-jet printers have greater potential for
graphical 3D printing, in terms of reproducing complex appearance properties.
However, to date these printers cannot produce full color, and doing so poses
substantial technical challenges, from the shear amount of data to the
translucency of the available color materials. In this paper, we propose an
error diffusion halftoning approach to achieve full color with multi-jet
printers, which operates on multiple isosurfaces or layers within the object.
We propose a novel traversal algorithm for voxel surfaces, which allows the
transfer of existing error diffusion algorithms from 2D printing. The resulting
prints faithfully reproduce colors, color gradients and fine-scale details.Comment: 15 pages, 14 figures; includes supplemental figure
High Dynamic Range (HDR) Display Perception
Displays have undergone a huge development in the last several decades. From cathode-ray tube (CRT), liquid crystal display (LCD), to organic light-emitting diode (OLED), even Q-OLED, the new configurations of the display bring more and more functions into industry and daily life. In the recent several years, high dynamic range (HDR) displays become popular. HDR displays usually refer to that the black level of the display is darker and the peak being brighter compared with the standard dynamic range (SDR) display. Traditionally, the peak luminance level can be used as the white in characterization and calibration. However, for HDR displays, the peak luminance is higher than the traditional diffuse white level. Exploration of the perceptual diffuse white in HDR image when presented in displays is proposed, which can be beneficial to the characterizing and the optimizing the usage of the HDR display. Moreover, in addition to the ``diffuse white , 3D color gamut volume can be calculated in some specific color appearance models. Calculation and modeling of the 3D color gamut volume can be very useful for display design and better characterizing display color reproduction capability. Furthermore, the perceptional color gamut volume can be measured through psychophysical experiments. Comparison between the perceptional color gamut volume and the theoretical 3D gamut volume calculations will reveal some insights for optimizing the usage of HDR displays. Another advantage of the HDR display is its darker black compared with the SDR display. Compared with the real black object, what level of black is `perfect\u27 enough in displays? Experiments were proposed and conducted to evaluate that if the HDR display is capable of showing ``perfect black for different types of background images/patterns. A glare-based model was proposed to predict the visual ``perfect black. Additionally, the dynamic range of human vision system is very large. However, the simultaneous dynamic range of human vision system is much smaller and is important for the fine tuning usage of HDR displays. The simultaneous dynamic range was measured directly for different stimulus sizes. Also, it was found that the simultaneous dynamic range was peak luminance level dependent. A mathematical model was proposed based on the experimental data to predict the simultaneous dynamic range. Also the spatial frequency effect of the target pattern on the simultaneous dynamic range was measured and modeled. The four different assessments about HDR displays perception would provide experimental data and models for a better understanding of HDR perception and tuning of the HDR display
Color image quality measures and retrieval
The focus of this dissertation is mainly on color image, especially on the images with lossy compression. Issues related to color quantization, color correction, color image retrieval and color image quality evaluation are addressed. A no-reference color image quality index is proposed. A novel color correction method applied to low bit-rate JPEG image is developed. A novel method for content-based image retrieval based upon combined feature vectors of shape, texture, and color similarities has been suggested. In addition, an image specific color reduction method has been introduced, which allows a 24-bit JPEG image to be shown in the 8-bit color monitor with 256-color display. The reduction in download and decode time mainly comes from the smart encoder incorporating with the proposed color reduction method after color space conversion stage. To summarize, the methods that have been developed can be divided into two categories: one is visual representation, and the other is image quality measure.
Three algorithms are designed for visual representation:
(1) An image-based visual representation for color correction on low bit-rate JPEG images. Previous studies on color correction are mainly on color image calibration among devices. Little attention was paid to the compressed image whose color distortion is evident in low bit-rate JPEG images. In this dissertation, a lookup table algorithm is designed based on the loss of PSNR in different compression ratio.
(2) A feature-based representation for content-based image retrieval. It is a concatenated vector of color, shape, and texture features from region of interest (ROI).
(3) An image-specific 256 colors (8 bits) reproduction for color reduction from 16 millions colors (24 bits). By inserting the proposed color reduction method into a JPEG encoder, the image size could be further reduced and the transmission time is also reduced. This smart encoder enables its decoder using less time in decoding.
Three algorithms are designed for image quality measure (IQM):
(1) A referenced IQM based upon image representation in very low-dimension. Previous studies on IQMs are based on high-dimensional domain including spatial and frequency domains. In this dissertation, a low-dimensional domain IQM based on random projection is designed, with preservation of the IQM accuracy in high-dimensional domain.
(2) A no-reference image blurring metric. Based on the edge gradient, the degree of image blur can be measured.
(3) A no-reference color IQM based upon colorfulness, contrast and sharpness
HDR-ChipQA: No-Reference Quality Assessment on High Dynamic Range Videos
We present a no-reference video quality model and algorithm that delivers
standout performance for High Dynamic Range (HDR) videos, which we call
HDR-ChipQA. HDR videos represent wider ranges of luminances, details, and
colors than Standard Dynamic Range (SDR) videos. The growing adoption of HDR in
massively scaled video networks has driven the need for video quality
assessment (VQA) algorithms that better account for distortions on HDR content.
In particular, standard VQA models may fail to capture conspicuous distortions
at the extreme ends of the dynamic range, because the features that drive them
may be dominated by distortions {that pervade the mid-ranges of the signal}. We
introduce a new approach whereby a local expansive nonlinearity emphasizes
distortions occurring at the higher and lower ends of the {local} luma range,
allowing for the definition of additional quality-aware features that are
computed along a separate path. These features are not HDR-specific, and also
improve VQA on SDR video contents, albeit to a reduced degree. We show that
this preprocessing step significantly boosts the power of distortion-sensitive
natural video statistics (NVS) features when used to predict the quality of HDR
content. In similar manner, we separately compute novel wide-gamut color
features using the same nonlinear processing steps. We have found that our
model significantly outperforms SDR VQA algorithms on the only publicly
available, comprehensive HDR database, while also attaining state-of-the-art
performance on SDR content
Modeling Perceptual Trade-offs for Designing HDR Displays
Display technology has evolved in pursuit of perceptual pleasure by providing realism and visual impact. The endeavor of the evolution has brought HDR displays to the market. HDR displays, which have become the mainstream display technology recently, are considered not only the present but also the future of displays because of their daunting technical goals: A peak luminance of 10,000 cd/m^2 and near-monochromatic primaries. However, both positive and negative prospects in terms of perceptual aspects for future HDR displays coexist. On the positive side, it is expected that HDR displays will provide better image quality and more vivid color. On the negative side, apart from technical barriers such as production cost and power consumption, HDR displays will induce side effects, for example, observer metamerism, which refers to the phenomenon that color matches for one observer result in color mismatches for other observers. This particular side effect could be a severe issue in HDR displays as their narrow-band primaries likely worsen the color mismatches. Hence, critical to the success of future HDR displays is dealing properly with the perceptual trade-offs. In other words, future HDR display designers need to select physical specifications that maximize perceptual benefits while minimizing adverse effects. This dissertation aims at exploring both potentially positive and negative aspects of future HDR displays, using various perceptual assessments. In particular, the dissertation focuses on two physical factors of a display device: peak luminance and chromaticity color gamut, and the effects of the two factors on related human perception: image quality, observer metamerism, and colorfulness. The ultimate goal of this dissertation is to address the related human perception aroused by the physical factors and propose models to help design future HDR displays. In order to achieve the goal, the dissertation first addresses the image quality trade-off relationship between peak luminance and chromaticity color gamut. A psychophysical experiment was used to develop models to predict equivalent image quality under the trade-off between peak luminance and chromaticity gamut as a function of the perceptual attributes lightness and chroma. Second, a novel approach based on a computational evaluation to investigate potential observer metamerism in HDR displays was explored. This research shows how observer metamerism in HDR displays varies with varying peak luminance and chromaticity color gamut. This research aims at developing a straightforward model to predict observer metamerism in HDR displays based on the computational evaluation. Third, a psychophysical experiment to derive a colorfulness scale for very saturated colors is carried out. This experiment focuses on understanding how the sensitivity of the human visual system responds to highly-saturated colors that extend beyond the stimuli studied in previous research. The colorfulness scale would help both advanced lighting system and display system designers. Fourth, the dissertation suggests an evaluation tool devised based on the observer metamerism and colorfulness scale works that can be utilized to determine the physical specification of HDR displays, maximizing perceptually positive effects while minimizing perceptually negative effects at the same time
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AN APPRAISAL OF THE DEVELOPMENTS IN THE REPRODUCTION OF COLOUR IN COMPUTER PUBLISHING SYSTEMS
The plethora of coloured images that are reproduced in any printed media is facilitated through a variety of related processes that collectively constitute traditional printing techniques. The aim of this research is to appraise recent developments that have occurred within the colour prepress process. The colour prepress process involves the preparation of colour separated halftone films that are used in the production of offset lithographic printing plates. Over recent years the application of desktop publishing technology to perform many of the functions associated with the colour prepress process has raised a number of significant issues and debates.
The reproduction of coloured images in the printed medium demands that certain fundamental criteria are adhered to in order to maintain professional standards of colour fidelity. Such criteria include: successful digital halftone production, the elimination of moiré patterns, and maintaining colour fidelity between the coloured original and the coloured reproduction. This research thesis shall therefore establish the principles and techniques involved in the reproduction of colour in a printed medium. It will also asses whether desktop publishing systems are able to facilitate successful professional colour reproduction by examining current debates that challenge the viability of desktop publishing solutions. Current debates concerning desktop publishing solutions are primarily concerned with assessing the value of Adobe PostScript level 2 solutions, computer interchange spaces for colour matching purposes, and rational supercell techniques that attempt to eliminate moiré patterns. The research also attempts to establish the validity of current debate findings by comparing them with statistics derived hom a questionnaire (undertaken as part of the research program) that seeks the opinions of system users on the effectiveness of their individual systems at processing and delivering acceptable colour separations
N-colour separation methods for accurate reproduction of spot colours
In packaging, spot colours are used to print key information like brand logos and elements for which the colour accuracy is critical. The present study investigates methods to aid the accurate reproduction of these spot colours with the n-colour printing process. Typical n-colour printing systems consist of supplementary inks in addition to the usual CMYK inks. Adding these inks to the traditional CMYK set increases the attainable colour gamut, but the added complexity creates several challenges in generating suitable colour separations for rendering colour images.
In this project, the n-colour separation is achieved by the use of additional sectors for intermediate inks. Each sector contains four inks with the achromatic ink (black) common to all sectors. This allows the extension of the principles of the CMYK printing process to these additional sectors. The methods developed in this study can be generalised to any number of inks. The project explores various aspects of the n-colour printing process including the forward characterisation methods, gamut prediction of the n-colour process and the inverse characterisation to calculate the n-colour separation for target spot colours. The scope of the study covers different printing technologies including lithographic offset, flexographic, thermal sublimation and inkjet printing.
A new method is proposed to characterise the printing devices. This method, the spot colour overprint (SCOP) model, was evaluated for the n-colour printing process with different printing technologies. In addition, a set of real-world spot colours were converted to n-colour separations and printed with the 7-colour printing process to evaluate against the original spot colours. The results show that the proposed methods can be effectively used to replace the spot coloured inks with the n-colour printing process. This can save significant material, time and costs in the packaging industry
Modelling Colour Appearance: Applications in Skin Image Perception
Humans are trichromatic, and yet their perception of colours is rich and complex. The research presented in this thesis explores the process of colour appearance of uniform patches and natural polychromatic stimuli. This is done through the measurement and analysis of the achromatic locus (Chapter 2), modelling of chromatic adaptation in a large dataset of unique hues settings (Chapter 3), and measurement of thresholds for uniform and polychromatic stimuli derived from simulated skin images (Chapter 4). Chapter 2 proposes a novel navigation scheme based on unique hues for traversing colour space. The results show that when colour adjustments are made using this novel scheme, the variability of achromatic settings made by observers is reduced compared to the classical method of making colour adjustments along the cardinal axes of the CIELUV colour space. This result holds across the tested luminance levels (5,20,50 cd/m^2) in each of the three tested ambient illumination conditions – dark, simulated daylight and cool white fluorescent lighting. The analysis also shows that the direction of maximum variance of the achromatic settings lies along the daylight locus. Chapter 3 evaluates models of chromatic adaptation by using unique hues settings measured under different ambient illumination conditions. It is shown that a simple diagonal model in cone excitation space is the most efficient in terms of the trade-off between accuracy and degrees of freedom. It is also found that diagonal and linear models show similar performances, reiterating their theoretical equivalence. Performances of these diagonalisable models are found to be worse for UR and UG unique hue planes compared to UY and UB planes. Chapter 4 presents a set of three experiments reporting estimations of perceptual thresholds for polychromatic and uniform stimuli in a 3-D chromaticity-luminance colour space. The first experiment reports thresholds for simulated skin images and uniform stimuli of the corresponding mean CIELAB colour. The second and third experiments investigate the effect of ambient illumination and the location of the stimuli in colour space. The thresholds for the polychromatic stimuli are found to be consistently higher than those for the uniform patches, for both the chromatic, and the luminance projections. The area of the chromaticity ellipses shows a gradual increase with distance from the illuminant chromaticity. The orientations of these ellipses for simulated skin are found to align with the vector joining the mean patch chromaticity and the illuminant chromaticity
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