Digital color restitution aims to digitally restore the original colors of a painting. Existing image editing applications can be used for this purpose, but they require a select-and-edit workflow and thus they do not scale well to large collections of paintings or different regions of the same painting. To address this issue, we propose an automated workflow that requires only a few representative source colors and associated target colors as input from art historians. The system then creates a control grid to model a deformation of the CIELAB color space. Such deformation can be applied to arbitrary images of the same painting. The proposed approach is suitable for restituting the color of images from a large photographic campaign, as well as for the textures of 3D reconstructions of a monument. We demonstrate the benefits of our method on a collection of mural paintings from a medieval monument.

Weathering effects are ubiquitous phenomena in cities. Buildings age and deteriorate over time as they interact with the environment. Pollution accumulating on facades is a particularly visible consequence of this. Even though relevant work has been done to produce impressive images of virtual urban environments including weathering effects, so far, no technique using a global approach has been proposed to deal with weathering effects. Here, we propose a technique based on a fast physically-inspired approach, that focuses on modeling the changes in appearance due to pollution soiling on an urban scale. We consider pollution effects to depend on three main factors: wind, rain and sun exposure, and we take into account three intervening steps: deposition, reaction and washing. Using a low-cost pre-computation, we evaluate the pollution distribution throughout the city. Based on this and the use of screen-space operators, our method results in an efficient approach able to generate realistic images of urban scenes by combining the intervening factors at interactive rates. In addition, the pre-computation demands a reduced amount of memory to store the resulting pollution map and, as it is independent from scene complexity, it can suit large and complex models by adapting the map resolution.

Abstract In urban design, estimating solar exposure on complex city models is crucial but existing solutions typically focus on simplified building models and are too demanding in terms of memory and computational time. In this paper, we propose an interactive technique that estimates solar exposure on detailed urban scenes. Given a directional exposure map computed over a given time period, we estimate the sky visibility factor that serves to evaluate the final exposure at each visible point. This is done using a screen‐space method based on a two‐scale approach, which is geometry independent and has low storage costs. Our method performs at interactive rates and is designer‐oriented. The proposed technique is relevant in architecture and sustainable building design as it provides tools to estimate the energy performance of buildings as well as weathering effects in urban environments.

Abstract This paper presents a method for the accurate rendering of path‐based surface details such as grooves, scratches and similar features. The method is based on a continuous representation of the features in texture space, and the rendering is performed by means of two approaches: one for isolated or non‐intersecting grooves and another for special situations like intersections or ends. The proposed solutions perform correct antialiasing and take into account visibility and inter‐reflections with little computational effort and memory requirements. Compared to anisotropic BRDFs and scratch models, we have no limitations on the distribution of grooves over the surface or their geometry, thus allowing more general patterns. Compared to displacement mapping techniques, we can efficiently simulate features of all sizes without requiring additional geometry or multiple representations.

Abstract Inverse surface design problems from light transport behavior specification usually represent extremely complex and costly processes, but their importance is well known. In particular, they are very interesting for lighting and luminaire design, in which it is usually difficult to test design decisions on a physical model in order to avoid costly mistakes. In this survey, we present the main ideas behind these kinds of problems, characterize them, and summarize existing work in the area, revealing problems that remain open and possible areas of further research.

Abstract Individually visible scratches, also called isolated scratches, are very common in real world surfaces. Although their microgeometry is not visible, they are individually perceptible by the human eye, lying into a representation scale between BRDF and texture. In order to simulate this kind of scratches in synthetic images we need to know their position over the surface (texture scale), so we can determine where to use the specific scratch BRDF instead of the ordinary surface BRDF. Computing the BRDF of a scratch is difficult because it depends on the scratch's invisible microgeometry. In this paper, we propose a new physically based model to derive this microgeometry by simulating the formation process of scratches. We allow specifying intuitively the parameters involved in the process such as the scratching tool, the penetration forces, and the material properties of the object. From these parameters, we derive the microgeometries of the scratches by taking into account the real behaviour of the process. This behaviour has been determined by analysing existing models in the field of materials engineering and some ``scratch tests'' that we performed on metals. Our method has the advantages of easily simulating scratches with a wide range of microgeometries and taking into account the variability of their microgeometry along the scratch path. Another contribution is related to the location of the scratches over the surface. Instead of using an image of the paths as in previous work, we present a new representation based on curves defining the paths. This offers an independence on the image resolution or the distance from the observer and accurately provides the scratch direction in order to compute scratch BRDFs. Categories and Subject Descriptors (according to ACM CCS): I.3.7 [Computer Graphics]: Color, shading, shadowing, and texture.

Abstract Inverse rendering problems usually represent extremely complex and costly processes, but their importance in many research areas is well known. In particular, they are of extreme importance in lighting engineering, where potentially costly mistakes usually make it unfeasible to test design decisions on a model. In this survey we present the main ideas behind these kinds of problems, characterize them, and summarize work developed in the area, revealing problems that remain unsolved and possible areas of further research. ACM CSS: I.3.6 Computer Graphics Methodology and Techniques I.3.7 Computer Graphics— Three‐Dimensional Graphics and Realism I.4.1 Image Processing and Computer Vision Digitization and Image Capture I.4.7 Image Processing and Computer Vision Feature Measurement I.4.8 Image Processing and Computer Vision Scene Analysis

This paper proposes an efficient method for the production of high quality radiosity solutions which uses an a priori knowledge of the dynamic properties of the scene to exploit temporal coherence. The method is based on a two-pass strategy that provides user-control on the final frame quality. In the first pass, it computes a coarse global solution of the radiosities along a time interval and then, in the second pass, it performs a frame-to-frame incremental gathering step using hardware graphic accelerators. Computing cost is thus reduced because the method takes advantage of frame-to-frame coherence by identifying the changes produced by dynamic objects and by decoupling them from computations that remain unchanged. The input data is a dynamic model of the environment through a period of time corresponding to the same camera recording. The method proceeds by incrementally updating two data structures: a space-time hierarchical radiosity solution for a given interval of time and a hierarchical tree of textures representing the space-time final illumination of the visible surfaces. These data structures are computed for a given viewpoint, either static or dynamic. The main contribution of this work is the efficient construction of the texture tree by identifying the changes produced by dynamic objects and by only recomputing these changes.

Realistic rendering animation is known to be an expensive processing task when physically-based global illumination methods are used in order to improve illumination details. This paper presents an acceleration technique to compute animations in radiosity environments. The technique is based on an interpolated approach that exploits temporal coherence in radiosity. A fast global Monte Carlo pre-processing step is introduced to the whole computation of the animated sequence to select important frames. These are fully computed and used as a base for the interpolation of all the sequence. The approach is completely view-independent. Once the illumination is computed, it can be visualized by any animated camera. Results present significant high speed-ups showing that the technique could be an interesting alternative to deterministic methods for computing non-interactive radiosity animations for moderately complex scenarios.

Abstract Realistic rendering animation is known to be an expensive processing task when physically based global illumination methods are used in order to improve illumination details. This paper presents the Multi‐Frame Lighting Method, an efficient algorithm to compute animations in radiosity environments. The method, based on global Monte Carlo techniques, performs the lighting simulation of groups of consecutive frames in a single process. All frames computed have the same accuracy as if they were computed independently while a significant high speed‐up is achieved. Results show that the method it is an interesting alternative for computing non‐interactive radiosity animations for moderately complex scenarios. Copyright © 2001 John Wiley &Sons, Ltd.

The paper describes a two pass algorithm capable of computing solutions to the global illumination in general environments (diffuse or glossy surfaces, anisotropically scattering participating media) faster than previous methods, by combining the strengths of finite element and Monte Carlo methods. A quick coarse solution is first computed with a clustered directional hierarchical method. This intermediate solution is used by a Monte Carlo method to accelerate computation of a final accurate solution by importance sampling, by means of link probabilities and adaptive probability density functions. Results from a first implementation of the algorithm for diffuse surfaces are presented.

Finite elements methods for radiosity are aimed at computing global illumination solutions efficiently. However these methods are not suitable for obtaining high quality images due to the lack of error control. Two‐pass methods allow to achieve that level of quality computing illumination at each pixel and thus introducing a high computing overhead. We present a two‐pass method for radiosity that allows to produce high quality images avoiding most of the per‐pixel computations. The method computes a coarse hierarchical radiosity solution and then performs a second pass using current graphics hardware accelerators to generate illumination as high definition textures.

In this paper we present a radiosity algorithm for dynamic scenes with a high level of frame-to-frame coherence. The algorithm is restricted to dynamic environments, where the object's movement is known a priori and the viewpoint is static. Each image is computed as the sum of two images: the base-level image, computed in a pre-process, and the frame-level image computed incrementally at each frame. The computation of the images is based on an importance-driven heuristic approach. The results of the implementation are analysed and discussed. © 1997 John Wiley & Sons, Ltd.

Abstract The performance of radiosity algorithms has been improved in recent years by means of using coherence when evaluating the visibility function for form factor computation. We present here a brief characterization of the uses of coherence for this goal. The characterization includes both previously proposed techniques and unexplored ones. Afterwards we propose new uses of coherence for visibility computation in form factor determination and we discuss the results of its implementation.

Abstract A survey on computer animation is presented with three specific goals: To clarify what people understand by computer animation. To separate the different aspects of computer animation and underline its main problems. To present the different solutions given to these problems. Several classifications for computer animation systems are first made attending to different criteria. The report is subsequently divided into two basic sections according to the historical classification of computer animation systems: computer‐assisted animation, where traditional animation is just helped by the use of computers, and modelled animation where the use of computers is essential in different aspects of the animation process. Special emphasis is placed on motion specification and generation. Finally, special attention is paid to human body animation because of the great interest of this specific field of computer animation and the peculiarity of its problems. Aspects of hardware aspects and postprocessing has just been taken into account in the classification section but have not been studied.

A scan line algorithm is proposed to render CSG modelled solids. Scan planes intersections are obtained by means of a proposed generalisation of Porter’s algorithm. 2-D scan plane primitives are decomposed in a way that provides a more accurate disjointness information then 3-D inclosing boxes and a simple way to class spans. Furtheremore, this scan plane organisation allows the use of most of scan line hidden surface strategies that make use of different types of coherence to speed up the visibility process.