Non-photorealistic rendering

We present the concept of volumetric depth-peeling. The proposed method is conceived to render interior and ex-terior iso-surfaces for a fixed iso-value and to blend them without the need to render the volume multiple times. The main advantage of our method over pre-integrated ...

This paper describes a novel extension of the photon mapping algorithm, capable of handling both volume multiple inelastic scattering and curved light paths simultaneously. The extension is based on the Full Radiative Transfer Equation (FRTE) and Fermat's law, and yields physically accurate, high-dynamic data than can be used for image generation or for other simulation purposes, such as driving simulators, underwater vision or lighting studies in architecture. Photons are traced into the participating medium with a varying index of refraction, and their curved trajectories followed (curved paths are the cause of certain atmospheric effects such as mirages or rippling desert images). Every time a photon is absorbed, a Russian roulette algorithm based on the quantum efficiency of the medium determines whether the inelastic scattering event takes place (causing volume fluorescence). The simulation of both underwater and atmospheric effects is shown, providing a global illumination solution without the restrictions of previous approaches.

We combine a machine vision system that recognises emotions and a non-photorealistic rendering (NPR) system to automatically produce portraits which heighten the emotion of the sitter. To do this, the vision system analyses a short video clip of a person expressing an emotion, then tracks the movement of facial features and uses this tracking data to analyse which emotion was expressed and what the temporal dynamics of the expression were. The image where the emotion is expressed strongest, the location of the facial features in that image and a keyword describing the emotion detected are passed to the NPR software. This keyword is used to choose appropriate (simulated) art materials, colour palettes, abstraction methods and painting styles, so that the rendered image may heighten the emotion being expressed. We describe the vision and rendering systems and their combination, and provide examples of portraits produced in this emotionally aware fashion.

A rendering is an abstraction that favors, preserves, or even emphasizes some qualities while sacrificing, suppressing, or omitting other characteristics that are not the focus of attention. Most computer graphics rendering activities have been concerned with photorealism, i.e., trying to emulate an image that looks like a highquality photograph. This laudable goal is useful and appropriate in many applications, but not in technical illustration where elucidation of structure and technical information is the preeminent motivation. This calls for a different kind of abstraction in which technical communication is central, but art and appearance are still essential instruments toward this end. Work that has been done on computer generated technical illustrations has focused on static images, and has not included all of the techniques used to hand draw technical illustrations. A paradigm for the display of technical illustrations in a dynamic environment is presented. This display environment includes all of the benefits of computer generated technical illustrations, such as a clearer picture of shape, structure, and material composition than traditional computer graphics methods. It also includes the three-dimensional interactive strength of modern display systems. This is accomplished by using new algorithms for real time drawing of silhouette curves, algorithms which solve a number of the problems inherent in previous methods. We incorporate current non-photorealistic lighting methods, and augment them with new shadowing algorithms based on accepted techniques used by artists and studies carried out in human perception. This paper, all of the images, and a mpeg video clip are available at

Shading for cel animation based on images is a recent research topic in computer-assisted animation. This paper proposes an image-based shading pipeline to give a 3D appearance to a 2D character by inspecting the hand-drawn image directly. The proposed method estimates normal vectors on the character's outline and interpolates them over the remaining image. The method does not limit the animator's creative process and requires minimal user intervention. The resulting shading pipeline can be easily applied to photorealistic and non-photorealistic 2D cel animation. In the proposed method, the animator can easily simulate environment reflections on the surface of 2D reflecting objects. As far as the authors are concerned, the proposed technique is the only one in the literature that is genuinely an image-based method for 2D animation.

We present a novel drawing system for composing and rendering perspective scenes. Our approach uses a projective 2D representation for primitives rather than a conventional 3D description. This allows drawings to be composed with the same ease as traditional illustrations, while providing many of the advantages of a 3D model. We describe a range of user-interface tools and interaction techniques that give our system its 3D-like capabilities. We provide vanishing point guides and perspective grids to aid in drawing freehand strokes and composing perspective scenes. Our system also has tools for intuitive navigation of a virtual camera, as well as methods for manipulating drawn primitives so that they appear to undergo 3D translations and rotations. We also support automatic shading of primitives using either realistic or non-photorealistic styles. Our system supports drawing and shading of extrusion surfaces with automatic hidden surface removal and highlighted silhouettes. Casting shadows from an infinite light source is also possible with minimal user intervention.

Recently, in the area of non-photorealistic rendering, there has been a significant effort in digital painting research to simulate traditional painting styles and painting pigments. In particular, the simulation of painting tools is very important because this enables an intuitive painting experience for a user and generates rich painting effects. Many painting systems have been proposed using brushes, but less for other painting tools. As a result, there is still a limitation in the variety of painting effects that can be generated. In this paper, we propose a method to simulate painting using a painting knife, which is an important tool in oil painting. We model a painting knife and model the pigments such that the system is suitable for realizing the impasto style. We also present a technique for simulating the interaction between a painting knife and pigments on a canvas in real-time.

We present a method that takes a raster image as input and produces a painting-like image composed of strokes rather than pixels. Our method works by first segmenting the image into features, finding the approximate medial axes of these features, and using the medial axes to guide brush stroke creation. System parameters may be interactively manipulated by a user to effect image segmentation, brush stroke characteristics, stroke size, and stroke frequency. This process creates images reminiscent of those contemporary representational painters whose work has an abstract or sketchy quality. Our software is available at

The purpose of this project was to create several methods for achieving non-photorealistic appearance of the objects or, in other words, the goal was to create several different implementations of the cartoonish shader, which can sometimes be referred to as a cel-shader or just toon-shader.

We present a paint model for use in interactive painting systems that captures a wide range of styles similar to oils or acrylics. The model includes both a numerical simulation to recreate the physical flow of paint and an optical model to mimic the paint appearance.Our physical model for paint is based on a conservative advection scheme that simulates the

In this publication we will look at the different methods presented over the past few decades which attempt to recreate digital paintings. While previous surveys concentrate on the broader subject of non-photorealistic rendering, the focus of this paper is firmly placed on painterly rendering techniques. We compare different methods used to produce different output painting styles such as abstract, colour pencil, watercolour, oriental, oil and pastel. Whereas some methods demand a high level of interaction using a skilled artist, others require simple parameters provided by a user with little or no artistic experience. Many methods attempt to provide more automation with the use of varying forms of reference data. This reference data can range from still photographs, video, 3D polygonal meshes or even 3D point clouds. The techniques presented here endeavour to provide tools and styles that are not traditionally available to an artist.

Drawing surfaces using hatching strokes simultaneously conveys material, tone, and form. We present a real-time system for nonphotorealistic rendering of hatching strokes over arbitrary surfaces. During an automatic preprocess, we construct a sequence of mipmapped hatch images corresponding to different tones, collectively called a tonal art map. Strokes within the hatch images are scaled to attain appropriate stroke size and density at all resolutions, and are organized to maintain coherence across scales and tones. At runtime, hardware multitexturing blends the hatch images over the rendered faces to locally vary tone while maintaining both spatial and temporal coherence. To render strokes over arbitrary surfaces, we build a lapped texture parametrization where the overlapping patches align to a curvature-based direction field. We demonstrate hatching strokes over complex surfaces in a variety of styles.

In this paper, we present a novel robotic system that produces watercolour paintings by means of a 6-degree-of-freedom collaborative robot. After an analysis of traditional watercolour, different non-photorealistic rendering techniques are applied in order to elaborate digital images into artworks. Several algorithms, aimed at processing both the backgrounds and the details, are implemented. Then, the resulting rendering is converted into a sequence of trajectories that the robot reproduces on paper. During the process, the artist controlling the system can change both the algorithm parameters and the hardware variables (e.g. brush type, colour dilution, etc.) in order to obtain a different artistic rendering. The challenge is indeed not to faithfully reproduce an image but to introduce a personal and original contribution to the artwork. The robotic painting system described in this paper was named "Busker Robot" and it is the first automatic system that uses the watercolour technique for artistic rendering. It was installed at the "Algorithmic Arts and Robotics" exhibition during the international event "Trieste Next" (Trieste, Italy, September 2017) and won an Honorable Mention at the 2018 International Robotic Art Competition (RobotArt).

This paper describes the various artistic effects of watercolor and shows how they can be simulated automatically. Our watercolor model is based on an ordered set of translucent glazes, which are created independently using a shallow-water fluid simulation. We use a Kubelka-Munk compositing model for simulating the optical effect of the superimposed glazes. We demonstrate how computergenerated watercolor can be used in three different applications: as part of an interactive watercolor paint system, as a method for automatic image "watercolorization," and as a mechanism for nonphotorealistic rendering of three-dimensional scenes.

In this paper, we present non-photorealistic rendering techniques that are applied together with a painting robot to realize artworks with original styles. Our robotic painting system is called Busker Robot and it has been considered of interest in recent art fairs and international exhibitions. It consists of a six degree-of-freedom collaborative robot and a series of image processing and path planning algorithms. In particular, here, two different rendering techniques are presented and a description of the experimental setup is carried out. Finally, the experimental results are discussed by analyzing the elements that can account for the aesthetic appreciation of the artworks.

This paper discusses the possibilities of video games as visual-oriented training. By focusing on the game Ōkami and its worldwide praised visual presentation, I will criticize the Western notion of dichotomy of word and image and provide examples from Japanese cultures to problematize this Western oriented dichotomy. After demarcating the layer of 'typical Japanese' of Ōkami, comparison between Ōkami's visual style and traditional forms of Japanese art during the game will be made and subsequently put into perspective to illustrate the issues of the dichotomization of word and image. I will argue that video games would not be an inappropriate medium to combat visual illiteracy, considering the level of artistic freedom and popularity they enjoy.

This paper reports preliminary results of ongoing interdisciplinary research in digital humanities and animation. This research explores visualisation techniques (e.g., motion graphics, 3D animation, Non-Photorealistic Rendering) to empower the study of depicted objects in Renaissance treatises on architecture and engineering. The aim is to pioneer a method that can be adopted by both scholars in the humanities and practitioners in animation to 1) take advantage of available editions and scholarship via interactive online systems (e.g., Engineering Historical Memory); 2) map and decode visual information and knowledge embodied in manuscripts; 3) create philologically correct 3D models and storytelling to unfold narratives embedded in drawings. As a showcase, this paper used Francesco di Giorgio Martini's drawings of the Corinthian capital in the manuscripts Ashburnham 361 (Biblioteca Medicea Laurenziana, Florence) and Saluzzo 148 (Musei Reali, Turin).

We present a system that lets a designer directly annotate a 3D model with strokes, imparting a personal aesthetic to the non-photorealistic rendering of the object. The artist chooses a "brush" style, then draws strokes over the model from one or more viewpoints. When the system renders the scene from any new viewpoint, it adapts the number and placement of the strokes appropriately to maintain the original look.

Abstract: L’obiettivo di questo studio è la ricostruzione
e l’analisi di un progetto non realizzato: il Gran Cimitero
di Giuseppe Damiani Almeyda (1834-1911),
a partire dai suoi disegni originali, disegnati a matita,
inchiostro e acquerello su cartoncino. Il progetto ha
vinto la medaglia d’oro sia alla Prima Mostra Nazionale
di Belle Arti di Firenze, nel 1861, sia alla Mostra
di Sicilia di Belle Arti, nel 1863. La ricostruzione del
modello tridimensionale è servita per dedurre elementi
di interpretazione e viste prospettiche, ombreggiate
o renderizzate, elaborate cercando rendere visibili gli
spazi immaginati nei disegni originari in proiezione
ortogonale.

Manga is a type of stylised bitonal drawings pleasing in terms of screen variety and tidy lines. In this paper, we present a method to draw architecture, spatial compositions and their details, including colours, textures, and tones, using manga drawing techniques. Given a colour image, such as photography, or drawing, our method finds the optimal solution for screening during the bi-tonal transformation, as well as a set of ideally abstracted line drawings. This offers a novel rendering appearance for the communication of architectural design. In our results we demonstrate successfully that our method generates manga-like drawings from an architectural image that preserve architectural key-elements, such as surface or material properties.

This paper proposes an image-based approach to animate painting procedure of Chinese ink painting at the brush stroke level. Given a painting image, first we obtain its brush strokes by a stroke decomposition method. Then based on our brush stroke model built by vectorization and painting trajectory extraction, animation of its painting procedure can be displayed by our real-time rendering model. As for the applications, our approach could be useful for analysis of Chinese ink painting, as well as for art education.

Portrait artists using oils, acrylics or pastels use a specific but open human vision methodology to create a painterly portrait of a live sitter. When they must use a photograph as source, artists augment their process, since photographs have: different focusing -everything is in focus or focused in vertical planes; value clumping -the camera darkens the shadows and lightens the bright areas; as well as color and perspective distortion. In general, artistic methodology attempts the following: from the photograph, the painting must 'simplify, compose and leave out what's irrelevant, emphasizing what's important'. While seemingly a qualitative goal, artists use known techniques such as relying on source tone over color to indirect into a semantic color temperature model, use brush and tonal "sharpness" to create a center of interest, lost and found edges to move the viewers gaze through the image towards the center of interest as well as other techniques to filter and emphasize. Our work attempts to create a knowledge domain of the portrait painter process and incorporate this knowledge into a multispace parameterized system that can create an array of NPR painterly rendering output by analyzing the photographic-based input which informs the semantic knowledge rules.

We present a simple technique to improve the perception of an object's shape. Bump mapping is well known in the computer graphics community for providing the impression of small-scale geometrical features, which are not explicitly modelled. Here, we propose a similar approach (variation of normals) for the purpose of enhancing the perception of a given geometry. Our approach is based on a simple modification of the surface normals in order to enhance the geometric features of the object during the rendering. The enhanced normals produced by this approach can be used in any rendering technique. The technique presented is particularly well suited to improve the rendering of mechanical parts where common straightforward shading techniques can often generate shading ambiguities. r

Cubism, pioneered by Pablo Picasso and Georges Braque, was a breakthrough in art, influencing artists to abandon existing traditions. In this paper, we present a novel approach for cubist rendering of 3D synthetic environments. Rather than merely imitating cubist paintings, we apply the main principles of analytical cubism to 3D graphics rendering. In this respect, we develop a new cubist camera providing an extended view, and a perceptually based spatial imprecision technique that keeps the important regions of the scene within a certain area of the output. Additionally, several methods to provide a painterly style are applied. We demonstrate the effectiveness of our extending view method by comparing the visible face counts in the images rendered by the cubist camera model and the traditional perspective camera. Besides, we give an overall discussion of final results and apply user tests in which users compare our results very well with analytical cubist paintings but not synthetic cubist paintings.

Silhouette is a drawing feature that is popular in illustrations and line-drawing artworks and is an important focus of research in the field of Non-Photorealistic Rendering (NPR). Current work on silhouette extraction and rendering tends to assume a polygonal mesh input, where connectivity information is available and the formation of silhouettes involves tracking of edges. On the other hand, the availability of 3D scanning devices nowadays leads to the proliferation of dense point data set for which no connectivity information is initially available. We present in this paper a hybrid image/object-space method to directly detect and render stylized silhouettes for such data set. Our method is simple to implement and runs at an interactive frame rate. Its basic form involves rendering silhouette points, each with a unique color value, to a color buffer. We show how lines can then be quickly fitted to the points projected. We further show how stylized silhouette can be used effectively with two other NPR techniques that happen to work very naturally with pointsampled geometry: stippling, and direct drawing onto surfaces.

Figure 1: (a) A photo of a car engine (b) Stylized rendering highlighting boundaries between geometric shapes. Notice the four spark plugs and the dip-stick which are now clearly visible (c) Photo of a flower plant (d) Texture de-emphasized rendering.

Phong-shaded 3D imagery does not provide geometric information of the same richness as human-drawn technical illustrations. A non-photorealistic lighting model is presented that attempts to narrow this gap. The model is based on practice in traditional technical illustration, where the lighting model uses both luminance and changes in hue to indicate surface orientation, reserving extreme lights and darks for edge lines and highlights. The lighting model allows shading to occur only in mid-tones so that edge lines and highlights remain visually prominent. In addition, we show how this lighting model is modified when portraying models of metal objects. These illustration methods give a clearer picture of shape, structure, and material composition than traditional computer graphics methods.

We present a system that lets a designer directly annotate a 3D model with strokes, imparting a personal aesthetic to the non-photorealistic rendering of the object. The artist chooses a “brush” style, then draws strokes over the model from one or more viewpoints. When the ...

At the core of our analysis and design implementations is the observation that N-way rotational symmetries can be described by symmetric N-th order tensors, which allows an efficient vector-based representation that not only supports coherent definitions of arithmetic operations on rotational symmetries but also enables many analysis and design operations for vector fields to be adapted to rotational symmetry fields.

While traditional graphics techniques provide for the realistic display of three-dimensional objects, these methods often lack the flexibility to emulate expressive effects found in the works of artists such as Michelangelo and Cezanne. We introduce a technique for capturing custom artistic shading models from sampled art work. Our goal is to allow users to easily generate shading models which give the impression of light, depth, and material properties as accomplished by artists. Our system provides real-time feedback to immediately illustrate aesthetic choices in shading model design, and to assist the user in the exploration of novel viewpoints. We describe rendering algorithms which are easily incorporated into existing shaders, making non-photorealistic rendering of materials such as skin, metal, or even painted objects fast and simple. The flexibility of these methods for generating shading models enables users to portray a large range of materials as well as to capture the look and feel of a work of art.

In this paper, we present a new system of non-photorealistic rendering which allows landscape photographs to be automatically converted to look like Oriental paintings. Using various computer vision and image processing techniques, we can generate images with the rules and features commonly found in Oriental paintings. With such a system, anyone can create realistic Oriental paintings easily, without the years of practice that are usually required by Oriental artists.

We propose a hybrid method for computing apparent ridges, expressive lines recently introduced by Judd et al. Unlike their original method, which works over the mesh entirely in object space, our method combines object-space and image-space computations and runs partially on the GPU, producing faster results in real time.

This paper describes the various artistic effects of watercolor and shows how they can be simulated automatically. Our watercolor model is based on an ordered set of translucent glazes, which are created independently using a shallow-water fluid simulation. We use a Kubelka-Munk compositing model for simulating the optical effect of the superimposed glazes. We demonstrate how computergenerated watercolor can be used in three different applications: as part of an interactive watercolor paint system, as a method for automatic image "watercolorization," and as a mechanism for nonphotorealistic rendering of three-dimensional scenes.

Creating 3D plant models is often a hard and laborious task. To make it easier and more natural, we propose a sketch-based interface for modeling single-compound plant structures with phyllotactic arrangements. Our approach is based on the traditional illustration technique of concept sketching. The user sketches the key construction lines for the main plant body and lateral organs. Our system then automatically constructs the 3D plant arrangement in phyllotactic patterns rendered as pen-and-ink line drawings. The user is then able to edit the model by oversketching the construction lines, adjusting density of lateral organs, and specifying different phyllotactic patterns. We demonstrate the capabilities of our system for a variety of plant models.

In this paper, we analyze the key factors underlying the implementation, evaluation, and optimization of image processing and computer vision algorithms on embedded GPU using OpenGL ES 2.0 shader model. First, we present the characteristics of the embedded GPU and its inherent advantage when compared to embedded CPU. Additionally, we propose techniques to achieve increased performance with optimized shader design. To show the effectiveness of the proposed techniques, we employ cartoon-style non-photorealistic rendering (NPR), speeded-up robust feature (SURF) detection, and stereo matching as our example algorithms. Performance is evaluated in terms of the execution time and speed-up achieved in comparison with the implementation on embedded CPU.

This paper presents an interactive system for designing and browsing volumetric illustrations. Volumetric illustrations are 3D models with internal textures that the user can browse by cutting the models at desired locations. To assign internal textures to a surface mesh, the designer cuts the mesh and provides simple guiding information to specify the correspondence between the cross-section and a reference 2D image. The guiding information is stored with the geometry and used during the synthesis of cross-sectional textures. The key idea is to synthesize a plausible cross-sectional image using a 2D texturesynthesis technique, instead of sampling from a complete 3D RGB volumetric representation directly. This simplifies the design interface and reduces the amount of data, making it possible for non-experts to rapidly design and use volumetric illustrations. We believe that our system can enrich human communications in various domains, such as medicine, biology, and geology.

We describe an approach to generating animations of drawings that start as a random collection of strokes and gradually resolve into a recognizable subject. The strokes are represented as tree based genetic programs. An animation is generated by rendering the best individual in a generation as a frame of a movie. The resulting animations have an engaging characteristic in which the target slowly emerges from a random set of strokes. We have generated two qualitatively different kinds of animations, ones that use grey level straight line strokes and ones that use binary Bezier curve stokes. Around 100,000 generations are needed to generate engaging animations. Population sizes of 2 and 4 give the best convergence behaviour. Convergence can be accelerated by using information from the target in drawing a stroke. Our approach provides a large range of creative opportunities for artists. Artists have control over choice of target and the various stroke parameters.