LIM 2023 Program

Abstract: Naturalness is a complex concept and a number of parameters might impact naturalness perception. In this work, we addressed how the combination of different elevation levels and size of prints impacts naturalness perception of 2.5D prints. The results of a subjective ranking experiment showed that observers perceived 2.5D prints as more natural at higher elevation with larger size of print. Moreover, we observed that elevation seems to be a more dominant parameter than size of the print for observers when evaluating naturalness.

Abstract: We introduce a quantitative metric to analyze the visually perceived differences between colors that are visualized on different displays, or that are visualized on the same display but using different visualization methods. This metric is validated by analyzing perceived visual differences as scored by observers using an iPad Air 2 display under different ambient light conditions. Our results show that the metric calculations are well aligned with the visual data from this experiment.
      We use the new metric to investigate the reproducibility of spectroradiometer data from three different displays (iPad Air 2, and iPad models from 2017 and 2018). Our results show that color visualizations based on these datasets are virtually identical for the iPad Air 2 and iPad 2017. For the iPad 2018, in circa 10% of the colors a visually noticeable difference occurs between visualizations based on the older and on the new dataset. We use the same metric to also compare color visualizations on these three displays. Our results show that color visualizations between iPad 2017 and iPad Air 2 are often visually different, with color differences larger than CIEDE2000 = 4.0 for 50% of the colors. But comparing iPad 2017 with iPad 2018 the color visualizations are often visually identical. For 90% of the colors, color differences CIEDE2000 < 2.4.

Abstract: In this paper, we introduce the analysis for gloss unevenness by using developed multiple directional incident lights optics. Gloss unevenness is strongly related to recognition of material texture. However, it looks different depending on the angles, so it has been difficult to measure quantitatively. The gloss unevenness caused by surface roughness is able to analyze by the distribution of normal on the surface. We have developed the optical system that simultaneously illuminates with light from three different directions, angles, and captures images of gloss unevenness in one shot. We confirmed that the normal distribution of the surface can be estimated by analyzing the image. This proposed method can not only measure the gloss unevenness image, but also estimate the shape of the surface. As an application of this gloss unevenness observation technology, it is also possible to detect of scratches and coating unevenness in industrial quality control.

Abstract: Accurate color reproduction is an essential parameter in many 3D printing applications.  Although current technologies in full-color 3D printing have enabled the reproduction of thousands of colors, reproducing the precise target color is still challenging and requires tuning. In this paper, we integrate halftoning with a multi-layer printing approach, where ink is deposited at variable depths, to improve the reproduction of tones and fine details in poly-jet 3D printing. The proposed approach is implemented using a manually controlled ink placement add-on for a commercial 3D printer and is compared to the default software of that printer. Results demonstrate that the proposed multi-layer halftoning performs more accurately in reproducing the tones and details of the target appearance.

Abstract: The primary objective of this paper is to demonstrate the utility of symplectic numerical techniques for ray tracing within gradient-index media. The relevant mathematics are explained in brief, deriving the optical Hamiltonian independently of the Lagrangian optical formalism before constructing a symplectic ray tracing algorithm. Numerical experiments with the Lüneburg and Maxwell fish-eye lenses compare the effectiveness of symplectic methods with standard numerical integration techniques, challenging the idea that the increased accuracy of higher-order numerical methods justifies their elevated computational cost. Further uses for symplectic ray tracing are also discussed..

Abstract: The Munsell dataset holds a prominent position in the field of color science. This dataset describes large color differences covering a wide color gamut, making it highly valuable for the development of color models. Currently, the widely used version is the Munsell Renotation, which is the second version of the dataset. In this paper, we analyze the third version, known as the Munsell Re-renotation, identify significant errors within it, and provide corrections for obvious typos. We propose a novel method for detecting nonuniformities, utilizing the L₁-STRESS measure and the proLab uniform color space (UCS). Our findings demonstrate that the revised version of the Munsell Re-renotation dataset achieves significantly better consistency with established UCSs compared to the original Munsell Re-renotation data. Additionally, we discuss modifications of the STRESS measure for data with unknown scales. Unlike previous modifications, the proposed measure, STRESS_group, is identical to the classic STRESS measure when the scales are the same.

Abstract: In material appearance we are interested in objectively measuring a physical aspect of a material, such as reflectance, and also in understanding how we see that material. In perceptual experiments we typically display simple stimuli to an observer, record their response, and then try to build a theory of why the observer responded in a given way. Often the latter model is implemented as a computer algorithm, and many of these are, for example, now implemented in camera pipelines for smartphones. However, the stimuli that are shown to observers are necessarily either very simple, such as rectangular patches of colour, or small in number. This raises the question as to whether the recorded responses to simple stimuli actually shed light on how we perceive scenes in the real world.
     In this paper, we look at a specific example of perceptual stimuli: Mondrian images, and investigate the extent to which, in the sense of their autocorrelation matrix, they represent the real world. We show that by modelling paths of pixels through an image using a statistical model that captures the statistics of real Mondrians, the autocorrelation matrix of Mondrians is Toeplitz, and moreover this Toeplitz structure is also found in real images. Although Mondrian images do not contain typical visual cues, our path model can be tuned to replicate the statistics of real images in the autocorrelation sense. The practical utility of this method is that paths through images and their autocorrelation statistics are a key tool for developing algorithms to predict the perceptual response to complex scenes. For example, this approach is at the foundation of retinex image processing. Experiments validate our method.

Abstract: Visual perception of material properties is useful for remote determination of the physical characteristics of objects. Here we ask what information in the proximal image is used by human observers use to make gloss judgements? There are two novel features of our study. The first is to use online psychophysical testing to collect a large dataset of human judgements (n_participants= 297) of many computer-generated images (n_images= 3888). The second is to use machine learning to uncover image features that consistently drive human judgements. Importantly, in this approach, the networks are trained to reproduce human judgements, rather than the physical ground-truth. The data-driven analysis revealed a simple, biologically plausible filter model that explains the majority of successes and failures in human gloss constancy, as well as reproducing known effects outside the training set. We discuss the value of large-scale online testing and deep learning in perception research.

Abstract: In colour science, colour matching functions (CMFs) are essential for measuring how sensitive the human eye is to various light wavelengths and determining the colour of stimuli in various viewing situations. It has traditionally taken a lot of time and effort to conduct colour-matching studies to describe an observer’s perception of colour. This article presents a simple and compact 3D-printed colorimeter designed to conduct colour-matching experiments. A pilot study was conducted using the colorimeter with four observers participating in a maximum saturation-type colour matching experiment., where they would match spectral lights in the 400-720 nm range to three narrow band LED primaries. The study aimed to assess the accuracy and performance of the system in measuring individual observer CMFs. Results showed that the CMFs of the four observers showed normal characteristics of a colour-normal observer. However, the limited number of measurements per observer may have contributed to the lack of smoothness in the CMFs. The CMFs of one of the observers were compared with Stiles and Burch 1955 RGB CMFs, after normalising to the same primaries. We noted that the red and green functions fell within the expected range, while the blue function showed some unusual characteristics. The limitations of the colorimeter and overall pilot study were also discussed. In conclusion, the colorimeter showed promising results in measuring CMFs, however the limitations need to be addressed to improve matching accuracy. Additionally, further measurements are required to better characterise intra-observer and inter-observer variabilities.

Abstract: Predicting the final appearance of a print is crucial in the graphic industry. The aim of this work is to build a mathematical model to predict the visual gloss of 2D printed samples. We conducted a psychophysical experiment where the observers judged the gloss of samples with different colours and different gloss values. For the psychophysical experiment, a new reference scale was built. Using the results from the psychophysical experiment, a mathematical prediction model for the visual assessment of gloss has been developed. By using the Principal Component Analysis to explain and predict the perceived gloss, the dimensions were reduced to three dimensions: specular gloss measured at 60°, Lightness (L*) and Distinctness of Image (DOI).

Abstract: The Ministry of Culture in France has its own center dedicated to the conservation of works of art from its 1200 national museums, called Center of research and restoration for French museums. It works on all types of artworks: jewelry, painting, furniture, bronze statues... The research department has several instruments, optical orchemical, dedicated to analyse the artwork’s materiality before restoration or for art history purpose. Except color that has been studied since the opening of the laboratory in 1931, other apparence attributes were not included in analysis workflow. Recent advances in color and gloss measurements and their application to cultural heritage analysis and restoration are presented in this talk.

Abstract: Translucency is an appearance attribute, which primarily results from subsurface scattering of light. The visual perception of translucency has gained attention in the past two decades. However, the studies mostly address thick and complex 3D objects that completely occlude the background. On the other hand, the perception of transparency of flat and thin see-through filters has been studied more extensively. Despite this, perception of translucency in see-through filters that do not completely occlude the background remains understudied. In this work, we manipulated the sharpness and contrast of black-and-white checkerboard patterns to simulate the impression of see-through filters. Afterward, we conducted paired-comparison psychophysical experiments to measure how the amount of background blur and contrast relates to perceived translucency. We found that while both blur and contrast affect translucency, the relationship is neither monotonic, nor straightforward.

Abstract: Recent advances in pigment production resulted in the possibility to print with RGBW primaries instead of CMYK and performing additive color mixing in printing. The RGBW pigments studied in this work have the properties of structural colors, as the primary colors are a result of interference in a thin film coating of mica pigments. In this work, we investigate the angle-dependent gamut of RGBW primaries. We have elucidated optimal angles of illumination and observation for each primary ink and found the optimal angle of observation under diffuse illumination. We investigated dot off dot halftoned screen printing with RGBW inks on black paper and in terms of angle-dependent dot gain. Based on our observations, optimal viewing condition for the given RGBW inks is in a direction of around 30∘ to the surface normal. Here, the appearance of the resulting halftoned prints can be estimated well by Neugebauer formula (weighted averaging of the individual reflected spectra). Despite the negative physical dot gain during the dot off dot printing, we observe angularly dependent positive optical dot gain for halftoned prints. Application of interference RGBW pigments in 2.5D and 3D printing is not fully explored due to the technological limitations. In this work, we provide colorimetric data for efficient application of the angle-dependent properties of such pigments in practical applications.

Abstract: Appearance, definition from the New Shorter English Dictionary: 1. The action of coming into view or becoming visible. 2. The action of appearing formally at any proceedings. 3. The action or state of seeming or appearing to be. 4. State or form as perceived. 5. Outward show or aspect. 6. A phenomenon, an apparition. 7. A gathering of people. 8. The action or an instance of coming before the world.
     From the definition, it can be understood that appearance is always a show with an audience. Without the viewer, appearance does not exist. What does it mean for ‘Material Appearance’? In this lecture I would like to explore how in portraits, as examples of an outward show coming before the world, the recording and the reproduction of appearance relies on shared knowledge. Is appearance in the eye of the beholder? What shortcuts can be taken? What misinterpretations will happen when the cultural background of the audience is different from the people who have recorded and recreated appearance. The case studies will come from different continents and different eras.