1,720,991 research outputs found
Intensity histogram equalisation, a colour-to-grey conversion strategy improving photogrammetric reconstruction of urban architectural heritage
No full textThe paper presents a novel technique for the conversion of the colour signal into a grey-level signal tailored to the 3D reconstruction of urban scenes. The presented approach, named IHE from Intensity Histogram Equalization, derives from previous methods.
The proposed approach takes as input a set of images showing the same urban object possibly captured under different conditions and by different cameras. Then it processes the input images by maximizing the peaks of their chromatic channel distributions, in order to preserve the chromatic in-formation as much as possible.
IHE has been evaluated by comparing its performance with that of other state-‐of-‐the-‐art algorithms in terms of 3D reconstruction. The experiments, carried out on two datasets, show that IHE generally outperforms the other approaches
BID (Bruteforce Isoluminants Decrease) a RGB-to-gray conversion technique for automatic photogrammetry,
No full textIn the automatic reconstruction of three-dimensional objects and environments from two or more photographic sets, few solutions are able to take advantage of color information. Many of these reconstruction methods are conceptually designed to work on grayscale images in the sense that, sooner or later in the processing, for a given spatial location, the algorithm will only consider a single intensity value instead of the RGB triple.
Color to grayscale conversion is factually a dimensionality reduction problem. This process should not be underestimated, since there are many different properties that need to be preserved. Isoluminant color changes are usually not preserved with commonly used color to gray conversions. In any case, we can state that the 3D to 1D dimension reduction leads to information loss and that the appearance of this loss is related to the method.
Many conversion methods have been proposed in recent years; these methods mainly focus on perceptual accuracy in terms of the fidelity of the converted image when reproduced from color to grayscale tones. These kinds of approaches are not designed to fulfill the needs of visual stereo matching and image matching algorithms, where local contrast preservation is crucial in the process of matching by local operators. This is one the main reason why in Lowe’s “Scale-invariant feature transform” (SIFT) operator, the candidate keypoints with low contrast are rejected in order to decrease the number of ambiguous points in the matching process [1].
To face this issue, we evaluated some State-of-the-art algorithms designed to perform the so-called RGB-to-gray conversion. The results of this analysis phase led to develop a new procedure in which the most promising algorithm, instead of evaluating every single image separately, has been adapted to evaluate and find the best possible converting solution among the entire set of images.
In this paper, we present the BID (Bruteforce Isoluminants Decrease) a RGB-to-gray conversion technique that combines the idea of Multi-Image Decolorize (MID) [2] with our specifically developed framework. More specifically, the MID evaluates a whole set of images instead of a single one in order to preserve tonal coherence during the matching phase. On the other hand, our frameworks specifies the statistical properties of the input data with the help of a representative collection of image patches provided by the same images of which we realize the conversion. Differently from MID, that is an adaptation of the Grundland & Dogdson algorithm [3], our conversion is a generalization of the MATLAB RGB2Gray algorithm, and simultaneously takes as input the whole set of images to be matched.
BID bears some similarity to the previously introduced by Song[4]. However, significant critical difference lies in the measurement criterion used to evaluate the decolorization quality. In brief, Song et al. employs the bilateral filtering with high computational complexity; on the contrary, BID is based on the newly defined dominant color hypothesis and aims to maximize the tonal representation on the image set
Highlight and specular reflection removal in photogrammetric techniques applied to architectural heritage 3D modeling
Full text linkIn this paper, we present a new technique to remove specular effects from the photo-grammetric results in a automatic photogrammetric workflow for Architectural Heritage (AH) 3D model construction. Our solution provides a new reconstruction pipeline completely integrated in the automatic photogrammetric pipeline re-using existing data to arrange new results. The process of acquisition of the images to get the finished 3D model is therefore unique and the process for acquiring and visualizing the correct perceived color is fully integrated with the process of shape capture. Overall, the method does not require specific technical knowledge, being therefore relatively easy to use, and it can be used over many different urban settings and contexts. The proposed methodology is a high-level image-processing algorithm. As such, it uses several lower-level methods for its building blocks. We consider these methods as black boxes, and we explain below their input, output and purpose.
We demonstrated the efficiency of our method using case study of our work in many cases of the ca 43 km of historical porticoes system in Bologna, Italy, a superset of the family of AH objects that it belongs to
Cultural and architectural heritage conservation and restoration: which colour?
No full textColours play a crucial role in the field of architectural heritage. Colour analysis and colour rendition are involved in several critical facets of heritage preservation, conservation and restoration. These aspects are related to accurate documentation and an accurate representation of heritage artefacts and architectural works. The aim of this paper is to describe critical issues and open problems of the processes involved in this field. Documentation is performed in multiple ways, acquiring heterogeneous data ranging from archival images, photographs, drawings using various consumer or professional instruments (eg, digital cameras and spectrophotometers). The reliability of colour acquisition might be influenced by instrumental reasons (the technology used to acquire colour information), by environmental changes (architectural heritage surveys are often performed outdoors), by morphology (complex architectural objects are characterised by concavities and convexities which complicate the reflection evaluation), or by materials (showing different reflection, porosity and transparency indexes). Identification of materials, such as colorants, pigments and dyes, is also a vital process in the heritage field. Colour information could be used as an approach to the identification of materials, but these methods are still under development, and many issues need to be solved to achieve reliable results. Visualisation techniques of a heritage artefact also present the problem of the correctness of the colour representation. Several problems need to be faced in this context: the reliability of the acquisition, colour management of the rendering software, model complexity, and fragmentation of the devices upon which the model is visualised
Complex process of image color correction : a test of a target-based framework
Full text linkThis paper aims at presenting the complexity of process of image target-based color correction (CC). We present issues encountered from acquisition to rendering using colorimetric traditional tools. Target-based CC can be seen as an optimization problem. We have tested SHAFT (SAT & HUE Adaptive Fine Tuning) an automated framework for target- based CC. A key element of SHAFT is an iterative CIEDE2000 variation comparison between a reference and target image. In this work we replace the standard CIEDE2000 with the Euclidean color-difference formula for small–medium color differences in log-compressed OSA-UCS (Optical Society of America's committee on Uniform Color Scales) space. Results are presented using both formulae. A discussion on the complexity of scene color departures and correction performances concludes the paper. It is shown the effect of real scene complexity and how colors are subject to disordered shifts in the color space. Because of this complexity, it emerges the role of the CC method as a different color error minimizer
A technique to ensure color fidelity in automatic photogrammetry
No full textIn this conference we presented in 2013 a new process and some solutions for color acquisition, management, rendering and assessment in Architectural Heritage (AH) 3D models construction from reality-based data based on photographs, with the aim to illustrate easy, low-cost and rapid procedures that produce high visual accuracy of the image/model while being accessible to non-specialized users and unskilled operators, typically Heritage architects.
The 2013 solution was successfully tested on the digitization as 3D models of 10 kilometers of buildings with porticoes but its use demonstrated some bottleneck resulting mainly by the lack of automation in many steps.
In the last years the image-based pipeline for 3D reconstruction purposes has received large interest leading to fully automated methodologies able to process large image datasets and deliver 3D products with a level of detail and precision variable according to the applications. Most part of developed solutions allows to improve our pipeline, but none of them provides the solution to the problem of accurate color reproduction using automated processes.
In this paper we present an automated solution to increase the processing performances of the two central steps of the photogrammetric pipeline (camera orientation & bundle adjustment) and simultaneously radiometrically improve the quality of an image datasets.
Our solution aim to achieve a robust automatic color balance and exposure equalization to ensure (i) faithful color appearance of a digitized artifact and (ii) consistency of the color-to-gray conversion. A correct color balance allows to minimize the typical problem of incorrectly detected areas (e.g. different luminance value for the same color and/or isoluminant colors) that strongly appear in case of surfaces of the same color or colors with the same luminance value. Therefore, the color balance procedure aims to produce radiometrically-calibrated images ensuring the consistency of surface colors in all the images (i.e. as much as possible similar RGB values for homologous pixels). Starting from captured RAW images our workflow includes: exposure compensation, adaptive histogram equalization and color balance. Between the two general approaches (color characterization vs spectral sensitivities based on color targets) we adopted this last technique that uses a set of differently colored samples measured with a spectrophotometer.
A software exploiting a series of Xrite ColorChecker Classic was developed and a series of trials were put in place to demonstrate the efficiency of our automatic process in different case of use.
Color accuracy (‘color characterization’) was evaluated and expressed
Securing Color Fidelity in 3D Architectural Heritage Scenarios
Full text linkEnsuring color fidelity in image-based 3D modeling of heritage scenarios is nowadays still an open research matter. Image colors are important during the data processing as they affect algorithm outcomes, therefore their correct treatment, reduction and enhancement is fundamental. In this contribution, we present an automated solution developed to improve the radiometric quality of an image datasets and the performances of two main steps of the photogrammetric pipeline (camera orientation and dense image matching). The suggested solution aims to achieve a robust automatic color balance and exposure equalization, stability of the RGB-to-gray image conversion and faithful color appearance of a digitized artifact. The innovative aspects of the article are: complete automation, better color target detection, a MATLAB implementation of the ACR scripts created by Fraser and the use of a specific weighted polynomial regression. A series of tests are presented to demonstrate the efficiency of the developed methodology and to evaluate color accuracy (‘color characterization’)
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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