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A Survey of Pen-and-Ink Illustration in Non-photorealistic Rendering

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					Cs361: A Survey of Pen-and-Ink Illustration in Non-photorealistic Rendering   Jia Huang




   A Survey of Pen-and-Ink Illustration in Non-photorealistic

                                           Rendering


                                                CS361

                                     Computer Science Department

                                                 GWU

                                               Jia Huang




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Cs361: A Survey of Pen-and-Ink Illustration in Non-photorealistic Rendering                                                Jia Huang


                                                   Table of Contents

A Survey of Pen-and-Ink Illustration in Non-photorealistic Rendering ............................. 1
Introduction ......................................................................................................................... 3
Background ......................................................................................................................... 4
   Terminologies in hand-drawn pen illustration ................................................................ 4
   Principles......................................................................................................................... 5
   Classification................................................................................................................... 6
Object-based Approaches.................................................................................................... 8
   Algorithm1 ...................................................................................................................... 8
   Algorithm2 ...................................................................................................................... 8
   Algorithm3 .................................................................................................................... 10
   Algorithm4 .................................................................................................................... 11
   Algorithm5 .................................................................................................................... 13
   Algorithm6 .................................................................................................................... 14
Image-based Approaches .................................................................................................. 16
   Algorithm7 .................................................................................................................... 16
   Algorithm8 .................................................................................................................... 17
   Algorithm9 .................................................................................................................... 19
Application ........................................................................................................................ 19
Future Research................................................................................................................. 22
References ...................................................................................................................... 23




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                                              Abstract

This paper presents a survey of the work done in the pen-and-ink illustration field of non-

photorealistic rendering. Non-photorealistic rendering(NPR), as a relatively new area is

getting more and more attention from the computer graphics community. Pen-and-Ink

illustration, as one of the styles of NPR, received much research in the recent fifteen

years. In this paper, important terminologies and principles are first introduced. Then the

general pen-and-ink illustration techniques are categorized and examined with certain

focus. Afterwards one important application field is discussed. The paper also suggests

important areas for future research.

Introduction

Computer graphics research has focused on photorealistic rendering, which attempts to

create images of physical scenes with ever-increasing realism, that looks “just like the

real world.” However it is not as effective and efficient as non-photorealistic rendering in

a lot of situations. Non-photorealistic rendering (NPR) is any technique that produces

images of simulated 3d world in a style other than realism[1]. There are many styles of

NPR, for example, water color[2], impressionistic[3,4,32], pen-and-ink illustration,

engraving[5], etching[1], etc. Because of the broad spectrum of the area, the focus of the

paper is put on the pen-and-ink illustration style.

        Pen-and-ink is an extremely limited medium, allowing only individual

monochromatic strokes of the pen. However beautiful pen-and-ink illustrations

incorporating a wealth of textures, tones, and styles[18] can be created. Indeed, because

of their simplicity and economy, there are a lot of applications and advantages of this

illustration technique. With only a simple form, image creators are able to use expressive



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Cs361: A Survey of Pen-and-Ink Illustration in Non-photorealistic Rendering        Jia Huang


abstraction to emphasize the area of interest and focus the viewer’s attention without

being forced to depicting every detail. Therefore it is very effective and efficient for

purpose of illustrating and expressing(i.e. “effectively conveying the meaning or

feeling[28]), especially if the underlying models are too complex to render realistically.

Second, in architectural design and industrial design, stylized illustrations are often more

appropriate, especially in the initial design phase, than photorealistic pictures and CAD

because they give the impression of approximation and incompleteness and therefore are

more stimulating[30,27,25]. Third, because the files created using this method usually

consume less storage, they are reproduced more easily and transmitted more quickly and

especially convenient for laser printers. Forth, the illustrations made in this style are

blended well with texts, using the same ink as texts. Because of these characteristics and

advantages, pen-and-ink illustrations are widely used in textbooks, repair manuals,

advertising and many other forms of media.

        First we will discuss some fundamental terminologies, essential principles in pen-

and-ink illustration and a general classification of the general algorithms described here

will also be presented.

Background

Terminologies in hand-drawn pen illustration

For further discussion and instruction, interested reader should consult Guptil[6], a

classic comprehensible text on pen and ink illustration.

        There are two different kinds of marks or strokes in pen-and-ink illustration:

outlines, hatching. Outlines are the external boundaries and internal edges, used to define

shapes. Outlines are exceptional in that they may be long and individually significant.



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Cs361: A Survey of Pen-and-Ink Illustration in Non-photorealistic Rendering          Jia Huang


The choice of whether or not to use outlines is largely an aesthetic one. Hatching is used

to indicate shading but should also follow the surface curvature.

        Pen-and-Ink limitation in being monochromatic is overcome to some extent

through the use of tone, style and texture. Tone is the darkness of a section of a drawing.

The perceived gray level or tone in an illustration depends largely on how dense the

strokes are in a region. Style is the brokenness of a line, for example solid line, dashed

line, dotted line. Texture is a tactile impression of a surface as rough, sandy, smooth.[7]

Texture is the collective result of many pen strokes, each individual stroke is not critical

and need not be drawn precisely. Indeed a certain amount of irregularities in each stroke

is desirable to keep the resulting texture from appearing mechanical. The most commonly

used textures include: hatching formed by roughly parallel lines; cross-hatching formed

by overlapped hatching in several directions; and stippling formed by small dots or very

short lines.

Principles

These are some fundamental principles of illustrating in pen and ink, useful for purely

computer generated illustrations. Interested readers are referred to [6] , [8] and [15].

        Strokes must look natural, not mechanical, the thickness of a line should vary

along its length, wavy lines are a good way to indicate that a drawing is schematic and

not yet completely resolved. It is not necessary to depict each individual tone accurately,

however presenting the correct arrangement of tones among adjacent regions is essential,

to disambiguate objects it is sometimes important to “ force tone” by enhancing contrast

or inventing shadows. And the character of strokes and outlines is important for

conveying texture, as well as geometry as lighting, e.g. straight lines are good for “glass”.



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S. Strassmann[9] “Hairy brush” proposed the “path-and-stroke” metaphor to emulate

brush using four different objects: brush, stroke, dip and paper to get a variety of

strokes[25]. S. Hsu and I. Lee[12] extended the metaphor by using general objects like

textures, images and recursively defined fractals that are drawn along a given path.

Haeberili[4] also focused on manipulating individual stroke by inspecting a collection of

attributes of the stroke, including location, color, size, direction and shape, and a path is

defined and physically simulated brush is used to generate the stroke.

Classification

There are three distinct types of input for stylized depiction processes[1]:

    (1) 3D scenes (described in terms of geometry, color, lighting, etc.) for rendering

    (2) images for processing

    (3) brush strokes from a user ( like the input to a paint system)

        Depending on the scene input(1,2), all the techniques presented here can be

classified into two classes, object-based and image-based[10]. Image-based systems

produce their illustrations directly from grayscale images. In object-based category, there

are two different kinds of image rendering algorithms, the image-centered and the scene-

centered algorithms[11]. Scene-centered algorithms project un-occluded objects onto the

image. Such techniques are qualified for polygonal scenes, while for other object kinds

special treatment is necessary, especially for free form surfaces. Image-centered

algorithms typically have a post-processing phase. Depending on (3), these techniques

can be categorized along the axis from interactive to fully automatic[1]. Below is a

classification of the general techniques that we will examine afterwards, represented by

tags.



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                           Object-based                                       Image-based

                           Object-centered           Image-centered

automatic                  Algorithm4,               Algorithm2,              Algorithm9

                           Algorithm5,               Algorithm3

                           Algorithm6

interactive                Algorithm1                                         Algorithm7,

                                                                              Algorithm8



        The advantage of geometry-based(object-based) systems is because of the

availability of the 3D geometry and viewing information they can produce illustrations

whose strokes not only convey the tone and texture of the surfaces in the scene, but also

convey the 3D forms of the surfaces by placing strokes along the natural contours of

surfaces. Existing image-based systems, on the other hand, until now have been able to

convey 3D information only by having a user draw individual strokes or specify

directions for orienting particular strokes. Whereas the ability to generate illustrations

with an image-based system offers several advantages. First, using an image-based

system greatly reduces the tasks of geometric modeling and of specifying surface

reflectance properties. Second, an image-based system provides the flexibility of using

any type of physical photograph, computer-generated image, or arbitrary scalar, vector or

tensor field as input, allowing visualization of data that is not necessarily physical in

nature. Finally, image-based systems offer more direct user control by providing the

ability to modify tone, texture, or stroke orientation[10]. General techniques of pen-and-




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ink illustration are discussed in the following sections with an emphasis on the

advantages and disadvantages of the system, important steps and key features of the

specific system, the generation and placement of strokes( including rendering of outlines,

strokes, textures and tones) and applications of the algorithm.

Object-based Approaches

Algorithm1

Dooley and Cohen[13] proposed a system to enhance a traditional shading images with

illustration techniques. Their system is able to handle the difficulties arising from

triangulating complex objects when there are many surfaces with common borders

present.    Objects in the scene are attached to a set of semantic attributes that are

interactively determined. The rendering algorithm works with illustration rules that are

applied to the projected lines according to their attributes. They attempt a taxonomy of

line styles and semantics. They showed how line and surface qualities could be

customized by the user to create more effective images. For example, lines can vary

thickness, transparency and style and line attributes are described by a matrix on the

values of “importance”, “line type” and “hiddenness”.

Algorithm2

To produce outlined and contoured drawings instead of shaded image, T. Saito et al. [14]

propose an enhancement technique for 3D shapes that conceptualizes geometric

properties. This is an automatic image-centered approach, because all operations are

realized with 2D image processing operations and with no user interaction. The problems

with this method contain: aliasing and reflected or transparent images can not be

enhanced because each pixel can represent only one surface; inefficiency in both


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execution time and memory space because all images are preserved as floating point data

in order to avoid digitization errors.

        The technique can be divided into 3 separate processes: 1.geometric process based

on geometric factors such as object shapes and camera parameters(projection and hidden

surface removal); 2.physical process based on physical factor such as reflectance, colors,

textures(shading, texture mapping); 3.artificial process, such as outline enhancement. The

authors introduce a special “G-buffer” for geometric information used repetitively by the

algorithms. The contents of the buffer include: object/patch identifier, parametric patch

coordinates, Z-buffer, world coordinates and normal vector for the visible patch. G-buffer

is formed during the geometric process and used by the physical and artificial processes.

The basic enhancement operations, i.e. the drawing of discontinuity lines, contour lines

and curved hatching, are done using G-buffer contents during post-processing. It is

separated from geometric and physical processes.

Edge, Contour and Hatching Drawing

Edge contains profile and internal edge. Profile is the border line of an object on the

screen; internal edge is a line where two faces meet. Profile and internal edge are the 0th

and first order discontinuity of the depth image respectively. Discontinuity can be

extracted with a first order differential operator and discontinuity of the first order

differential of an image can be extracted with a second order differential operator. The

artifact resulted from these operations, such as confusing real discontinuities from rapid

depth change can be corrected using the minimum and maximum of neighboring

differential values. So both profile and internal edges can be drawn.




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        Using image processing technique, contour lines are generated as raster data and

consists of homogeneous calculations on each pixel and its neighbors. Curved hatching

rendering of a surface is performed using the contour method above in combination with

a technique that thins out and disappears the contours when they become too dense and

adds lines when the lines become too thin; the purpose is to have an overall periodic set

of lines in pixel space.

Application

This algorithm is commonly used in hand drawn illustrations in industrial design, line

illustration, topographical maps, medical imaging and surface analysis. The method is

also useful for photorealistic rendering.

        Piranesi[19] system proposed by Landsown and Schofield also uses NPR to create

illustrations from 3D models. Piranesi uses a standard graphics pipeline to create a 2D

reference image akin to a G-buffer. The user is then allowed to select specific regions of

the image and apply textures that emulate natural media interactively or automatically.

Algorithm3

Leiser[11] presents a technique to emulate cooper-plate rendering, an engraving

technique used for old system of printing. The goal is to render a copper plate, drawing

that consists of lines of varying thickness and of single points for 3d scene. A ray-tracing

approach is used to render curves on free-form objects. An advantage of this approach is

that it easily handles reflections and shadowing because ray tracing is used.

The method uses a kind of volume texturing in connection with image processing

algorithms and is suitable for implementation in a ray tracing algorithm. Shading is done

by regular hatchings in several thickness and distances. Copper plates can be generated



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from 2D pixel images with filter algorithms and image processing techniques. In the post-

processing step an edge detection algorithm is applied. Edges are sudden changes in the

image domain, that are difficult to determine with an image-centered method. This

technique is able to deal with not only polygonal scene but also freeform surfaces.

Experience shows that this method is especially interesting for illustration in books and

for generating icons on user interface.

Algorithm4

Winkenbach et al.[15] propose an automatic object-centered rendering system. Compared

to [Algorithm2 and Algorithm6] the use of strokes are more expressive. And this method

is resolution-independent while a lot of other computer drawing programs do not scale

well. The limitation is that it can only illustrate polyhedral models and can not be used

for curved surfaces. Also flat-shaded surfaces are assumed.

Main Steps

Firstly, the system computes the visible surfaces and the shadow polygons. Secondly, it

projects the polygons to NDC space to generate certain data structure. Thirdly, each

visible surface is then rendered. The procedural texture attached to each surface is

invoked to generate the strokes conveying the correct texture and tone for the surface.

Then all the strokes are clipped to the visible portion of the surface. Finally the outlines

are drawn by extracting form the structure generated in step2.

Key Features

Their rendering system is a basic graphics pipeline with a few notable changes. Some of

changes include rendering of texture and tone, clipping and outline. Because polygons are

no long scan converted, both texture and tone must be conveyed with hatching. And



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because the stroke is now distorted or displaced in some way, it is important to allow

strokes to sometimes stray slightly outside of the clipped region. To achieve this, we clip

the straight-line path of our strokes prior to adding in the function for waviness. And

boundary and interior outlines must be drawn in a way that takes into account both the

textures of the surrounded regions and the adjacency information.

Rendering Texture and tone

To render the texture and tone, we use prioritized stroke textures. A prioritized stroke

texture is a set of strokes each with an associated priority. When rendering a prioritized

stroke texture, all of the strokes of highest priority are drawn first; if the rendered tone is

still too light, the next highest priority strokes are added until the proper tone is achieved.

For example for a brick texture, the outlines of the individual brick elements have highest

priority, the strokes for shading individual bricks have medium priority, and the hatching

strokes that go over the entire surface have lowest priority. In the cross-hatching texture,

vertical strokes have priority over horizontal strokes, which have priority over the various

diagonal stroke direction. We express texture with outline. Each stroke texture has

associated with it a boundary outline.

Outline drawing

The interior outlines are used within polygons to suggest shadow directions or to give

view-dependent accents to the stroke texture. And for the sake of principles of pen-and-

ink illustration, we minimize outline by drawing it only if the tones of two neighboring

faces are not sufficiently different for disambiguation. Accented outlines, i.e., thickening

edges is a technique for providing subtle but important cues about the 3d aspects of an

illustrated scene. For example, the interior outlines of each brick in the “brick” stroke




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texture are drawn according to their relationship with the directin of the light source:

brick edges that cast shadows are rendered with thickened edges while illuminated brick

edges are not drawn at all. In addition to the light source direction, the viewing direction

is another important parameter that should be taken into account when drawing outline

strokes.

Algorithm5

Winkenbach et al.[16] extend the previous method[Algorithm4], and propose one

algorithm that can handle curved surfaces formulated parametrically, such as B-spline

surfaces, NURBS and surfaces of revolution. [Algorithm2] mainly uses image processing

technique in the post-processing stage for outline and hatching generation. While this

method integrates aspects of 2D and 3D rendering. In addition, traditional texture

mapping techniques can be used to extend the range of effects that can be achieved with

pen-and-ink rendering. The biggest limitation is that it deals only with surfaces

possessing a global parameterization. One possible solution is to parameterize such

surfaces.

Generation of Stroke Textures

The progress compared to their last implementation is in the generation of the stroke

texture. Firstly they use a grid of lines, which consists of parallel lines, running in one or

more user-specified directions in the parameter domain, to orientate hatching strokes

along a surface. Secondly, they introduce a technique called “controlled-density

hatching”. This technique allows strokes to gradually disappear in light areas of a surface

or in areas where too many strokes converge together and allows new strokes to gradually




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come into existence in dark areas or areas in which the existing strokes begin to diverge

too much. Specifically a recursive algorithm is used.




                                               Figure 1. Illustration with Texture Map

Controlled-density hatching allows “fine grain” control over the tone of a pen-and-ink

illustration. With this new capability, we can use traditional texture mapping to vary the

tone on the surface of an object.

Algorithm6

Markosian et al.[17] build a system to deliberately trade accuracy for speed. In contrast

Winkenbach’s pen-and-ink rendering system produces decidedly finer images, but takes

several minutes to do so. This system is very quick and can be easily extended to other

styles. But there is no shadow created.

Key Features

This real-time NPR technique is based on economy of line – the idea that a great deal of

information can be effectively conveyed by very few strokes. This algorithm only renders

silhouettes, certain user-chosen key features and some minimal shading of surface

regions. One obstacle to achieving real-time NPR is the problem of determining

visibility, since a straightforward use of z-buffer may give incorrect results.

        So the key idea is rapid identification of silhouette edges using interface

coherence of silhouette edges and fast visibility determination using improvements and

simplification in Appel’s hidden-line algorithm.


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Main Steps

The overall structure of the algorithm is: 1.determine the silhouette curves in the model

2.determine the visibility of silhouette and other feature edges by a modified Appel’s

algorithm 3.render the silhouette and feature edges.

Generating Strokes

World-space polylines to be rendered are first projected into the film plane. Artistic or

expressive strokes are then generated by modifying the resulting 2D polylines. There are

three techniques for generating expressive strokes: drawing the polylines directly with

slight enhancements such as variations in line width or color; adding offsets to the

polylines to define high-resolution “artistically” perturbed strokes; and texture-mapped

strokes which follow the shape of the polyline. In the second case the polylines are

parameterize by arc length. A new parametric curve Q(t) is based on the original

parametric curve p(t) by adding a vector offset v(t) defined in the tangent-normal basis,

i.e. Q(t) =p(t) + vx(t)p’(t) + vy(t)n(t). The third method builds a texture-mapped mesh

using the polyline as a reference spine. Each texture map represents a single brush stroke.

The textures are repeated along the reference spine, approximately preserving its original

aspect ratio.

Placement of Strokes

Shading strokes (particles) are put in world space (not the surface) rather than define

them in screen space. This is the approach used by Meier in her “painterly rendering”[3].

The advantage of this approach is it maintains frame-to-frame coherence. Stroke

directions are defined by the cross product of local surface normal and the ray from the




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camera to the stroke location and the ray from the camera to the stroke location, so that

stroke line up with silhouette lines.

Applications

Haro[24] implements a non-photorealistic renderer for producing pictures that look like

sketches using this technique. Interested reader may refer to [24] for details.

Image-based Approaches

Algorithm7

Salisbury et al.[18] present an algorithm for rendering subdivision surface models of

complex scenes using an interactive editable particle system, with 2D grayscale images

as a starting point.

Interaction

Strassmann [9] presents interactive simulation of traditional artist tool. Compared to his

approach, rather than focus on individual strokes, this new system tries to directly support

the higher-level cumulative effect that the strokes can achieve: texture, tone and shape.

The interaction between the user and system is high-level in that the user “paints” using

textures and tones, and the computer draws the individual strokes. Exceptions are outlines

that have individual significance; in addition an artist might occasionally need to touch

up fine details of textured work.

Texture generation

Different from Winkenbach al. ‘s approach, a combination of non-procedural and

procedural stroke textures are used. Non-procedural texture is that the textures tiled the

plane and the stroke selected for drawing at a point was the one that happened to pass

through that point. A library of user-defined stored stroke textures is supported, as well as



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several built-in procedural stroke textures. A stored texture is simply a collection of

strokes. Drawing a texture at a given darkness is a matter of choosing from the collection

a subset that has enough strokes to reach the desired tone. For some textures such as

scribbles, the choice of strokes to draw for a given tonality is not critical. In these cases

the system simply selects strokes from the texture in a random sequence, generating

candidate strokes and testing the tonal effect of candidate strokes. For other textures, the

system supports a predefined priority for each stroke. In addition to modifying individual

strokes, the user can edit collections of strokes, like the “lighting” operation which

incrementally removes strokes.

Outline Drawing

The system allows scanned, rendered or painted images to be used as a reference for tone

and shape. The reference image is used for several purposes: as a visual reference for the

artist, as a tone reference for painting, as a source image from which the outline is

extracted, as a reference for determining stroke and texture orientation.

Algorithm8

P. Salisbury et al. [10] are the first to use orientable textures for image-based pen-and-ink

illustration. This is also an improvement of the previous method[Algorithm7]. This

system is able to render strokes and stroke textures according to a vector field in such a

way that they also produce the proper texture and tone; and estimate tones as new

oriented strokes are progressively applied.

Interaction

Editor allows the user to specify the three components of a layer (tone, direction, and

texture), the system does the tedious work of placing all the strokes.



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                          Figure 2. The User Input(from left to right) tone, direction and a stroke example

        In their previous work, the textures tiled the plane and the stroke selected for

drawing at a point was the one that happened to pass through that point. By contrast, in

this new system the placement of strokes on the final illustration is independent of their

relative position in the texture. Spacing between strokes is instead maintained indirectly

by the rendering system.

Placement of Strokes

Dynamic placement of stroke is import to maintain the density. Dynamic is implemented

by drawing in order of importance, the fraction of its intended darkness that has not yet

been accumulated at that point. Rendering consists of looking for the location with

greatest importance, placing a stroke, update an image that records the importance and

repeat until the importance everywhere is below a termination threshold. The whole

process of matching the illustration to the tone image is recorded in a difference image,

updated after each stroke is drawn, whose value at each pixel is the difference between

the tone image and the blurred version of the illustration. The importance image is

derived from the difference image, its value at each point is the current difference derived

by the initial value of the difference.             This algorithm is called Difference Image

Algorithm.



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        After knowing where to put the stroke, orient and bend the stroke. Map the stroke

into the direction field so that at every point along its length, the stroke’s new angle

relative to the direction field is the same as the prototype stroke’s angle with respect to

the vertical direction.

Algorithm9

Curtis[26] implements a “loose and sketchy” filter to automatically draw the visible

silhouette edges using image processing and a stochastic, physically-based particle

system. The only input is a depth map of the model and it will be converted into two

images: template image and force field. Template image represents the amount of ink

needed for each pixel. And the force field pushes particles along the silhouette edges.

The particles are generated randomly initialized by the template image. Acceleration of

each particle is based on the force field. Particle generates ink and remove ink during its

traveling, a technique used in [10, 31]

Application

In the following text, one application area of pen-and-ink illustration, the rendering of

trees or complex natural objects, is discussed .The advantage of art-based illustration is

evident in the kind of application where the underlying model is so complex that it is very

time-consuming to model and render while the whole thing can be rendered with a few

strokes which evoke the impression of complexity using pen-and-ink illustration. Trees

are one of those complex objects.

        Kowalski et al. [20] suggest an algorithm to render fur, grass, trees etc. by pen-

and-ink illustration. This approach is image-centered and interactive. Their approach is to

generate abstract sketches of trees by using geometric primitives like spheres for defining



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rough approximations of a tree’s foliage. These primitives were rendered conventionally

to achieve gray-scale images. In the second step, the images were used to procedurally

place graftals – small objects representing leaves or hair – on the surfaces by applying the

“difference image algorithm” proposed earlier by Salisbury[Algorithm8].                               Their

algorithm controls the density of hatching strokes in order to match the gray tones of the

target image. And this method enables different textures assigned to different regions,

while in the previous version, [Algorithm6] all the surfaces are assigned with textures

uniformly. It is accomplished by dividing models into one or more regions (patches).

        The particle placing is a hybrid of screen and object. To meet the requirement

that graftals appear to stick to surfaces in the scene, graftals are placed in the scene after

converting the 2D screen position to a 3D position on some surface[3].




        Figure 4,5. scene rendered without graftal texture(left) and with graftal texture(right)

        Fraftals are based on a flat tapering shape by gradually reducing width about a

central spine, which is a planar polyline. After being placed with the DIA, each graftal

determines how to orient and draw itself.

        The drawbacks of the algorithm[20] are that each new graftal texture requires a

procedural implementation that included writing code. Also graftals are regenerated in

each frame in a way that leads to excessive introduction and elimination of graftals even

for small changes in camera parameters. Thirdly graftals choose from among a small




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number of discrete “levels of detail” at which to draw, making transition between levels

noticable.

         Markosian et al.[21] propose a new system which overcome these drawbacks by

a more expressive interface, static placement and continuous levels of detail.

        The new approach is to use a “static” placement scheme, where graftals are

distributed onto surfaces during the modeling phase. Certain graftals have a multi-

resolution structure, so that a single graftal, seen from larger distance, transforms into

several graftals, when viewed from nearby. Further, these transformations are carried out

in a continuous manner by smoothly varying the shape, size, position and orientation.

        Deussen et al.[22] take a very different approach which models detailed tree trunk

and leaves while the goal of the previous approaches is to avoid complex modeling. The

lines drawn are the result of visually combining many drawing primitives instead of

placing graftal objects on some large geometries. A drawback of this approach is that

they potentially have to deal with more input data. The solution to this problem is to

represent a tree at several levels of detail, for example if the current model has too many

leaves a much simpler model can be used instead. The advantage of this method is the

ability to draw both an abstract tree and a specific plant which will be not very different

from its realistic image and the ability to make use of existing tree libraries.

        To draw the trunk, the method raised by Markosian in [17][Algorithm6] to render

the outline of the trunk can be used. The skeleton can be crosshatched using “difference

image algorithm” by Salisbury[19][Algorithm7]. The direction of the strokes is similar to

what is used in [17] [Algorithm6] either at random or affected by the normal vector of the




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Cs361: A Survey of Pen-and-Ink Illustration in Non-photorealistic Rendering   Jia Huang


stem geometry. Leaves are drawn using abstract drawing primitives. For example, each

leaf can be represented as the outline of a disk.



Future Research

    1. Improve the procedural stroke textures and automate further methods for creating

        them

    2. Incorporate other illustration effects

    3. Add more interactive controls to help designing 3d illustrations

    4. Render other natural objects

    5. Create animation

    6. Explore other forms of illustration besides pen-and-ink, including traditional

        forms like water color and air brushing




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Cs361: A Survey of Pen-and-Ink Illustration in Non-photorealistic Rendering                     Jia Huang


References

[1] Craig Reynolds. Stylized Depiction in Computer Graphics. http://www.red3d.com/cwr/npr

[2] Cassidy J. Curtis, Sean E. Anderson, Kurt W. Fleischer, David H. Salesin. In SIGGRAPH ’97

Conference Proceedings.

[3] Barbara J. Mier. Painterly Rendering for Aniamtion. In SIGGRAPH ’96 Conference Proceedings.

[4] Paul Haeberli. Paint by Numbers: Abstract Image Representation. In SIGGRAPH ’90 Conference

Proceedings, Volume 24, Number 4, August 1990.

[5] Victor Ostromoukhov. Digital Facial Engraving. In SIGGRAPH ’99 Conference Proceedings.

[6] Arthur L. Guptil. Rendering in Pen and Ink. Watson-Guptil Publications, New York, 1976.

[7] Tom Jazen. Ten Papers on Non-Photorealistic Rendering. http://www.world.std.com/~tej/pni.html

[8] Frank Lohan. Pen and Ink Techniques. Contemporary Books, inc., Chicago, 1978.

[9] Steve Strassmann. Hairy Brush. In SIGGRAPH ’86 Conference Proceedings, Volume 20, Number 4,

August 1986.

[10] M. Salisbury, M. Wong, J.F. Hughes, and D. Salesin. Orientable textures for image-based pen-and-ink

illustration. In SIGGRAPH ’97 Conference Proceedings.

[11] Wolfgang Leister. Computer Generated Copper Plates. Computer Graphics Forum, Volume 13,

Number 1, March 1994, pp. 69-77, Blackwell, ISSN 0167-7055.

[12] S. Hsu and I. Lee. Drawing and animation using skeletal strokes. In SIGGRAPH ’94 Conference

Proceedings, pages 109-118.

[13] Debra Dooley, Michael F.Cohen. Automatic Illustration of 3D Geometric Models: Lines. In

SIGGRAPH ’90 Conference Proceedings, Volume 24, Number 2, pages 77-82, March 1990.

[14] Takafumi Saito, Tokiichiro Takahashi. Comprehensible Rendering of 3D Shapes. In SIGGRAPH ’90

Conference Proceedings, Volume 24, Number 4, pages 197-206, August 1990.

[15] G. Winkenbach and D. Salesin. Computer-generated pen-and-ink illustration. In SIGGRAPH ’94

Conference Proceedings, pages 91-100.

[16] G. Winkenbach and D. Salesin. Rendering parametric surfaces in pen and ink. In SIGGRAPH ’96

Conference Proceedings, pages 469-476.




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Cs361: A Survey of Pen-and-Ink Illustration in Non-photorealistic Rendering                    Jia Huang


[17] Lee Markosian, Michael A. Kowalski, Sam Trychin, Lubomir Bourdev, Daniel Goldstein, John F.

Hughes. Real-Time Non-Photorealistic Rendering. In SIGGRAPH ’97 Conference Proceedings.

[18] Michael P. Salisbury, Sean E. Anderson, Ronen Barzel, David H. Salesin. Interactive Pen-and-Ink

Illustration. In SIGGRAPH ’94 Conference Proceedings.

[19] Piranesi. http://www.arct.cam.ac.uk/research/cadlab/irender/index.html

[20] Michael A. Kowalski, Lee Markosian, J.D.Northrup, Loring S. Holden, John F. Hughes. Art-based

Rendering of Fur, Grass, and Tree. In SIGGRAPH ’99 Conference Proceedings.

[21] Lee Markosian, Barbara J. Meier, Michael A. Kowalski, Loring S. Holden, J.D. Norhthrip, John F.

Hughes. Art-based Rendering with Continuous Levels of Detail. In NPAR’00 Conference Proceedings.

[22] Oliver Deussen, Thomas Strothotte. Computer-generated Pen-and-ink Illustration of Trees. In

SIGGRAPH ’00 Conference Proceedings.

[23] Thomas Strothotte, Bernhard Preim, Andreas Raab, Jutta Schumann, David R. Forsey. How to Render

Frames and Influence People. In Computer Graphics Forum (13) 3, Proceedings of euroGraphics 1994,

pages 455-466,1994.

[24] Antonio Haro. A nonphotorealistic render. http://www.cc.gatech/edu/~hato/cs7490

[25] Stefan Schlechtweg. Lines and How to Draw Them. http://isgwww.cs.Uni-

Magdeburg.DE/~stefans/pubi/norsigd2.html

[26] Cassidy Curtis. Loose and Sketchy Animation.

http://www.cs.washington.edu/homes/cassidy/loose/sketch.html

[27] Paul Bourke. Computer Sketching. http://www.swin.edu.au/astronomy/pbourke/fractals/sketch/

[28] Matthew Kalplan, Bruce Gooch, Elaine Cohen. Interactive Artistic Rendering. In NPAR’00

Conference Proceedings.

[29] Maic Masuch, Stefan Schlechtweg, Bert Schonwalder. DaLi – Drawing Animated Lines!. In

Proceedings of Simulation and Animtation ’97, S.87-96, SCS Europe, 1997.

[30] Jutta Schumann, Thomas Strothotte, Stefan Laser. Assessing the Effect of Non-Phototealistic

Rendered Images in CAD. http://www.acm.org/sigs/sigchi/chi…dings/papers/Schumann/chi96fi.html

[31] Greg Turk and David Banks. Image-Guided Streamline Placement. In SIGGRAPH ’96 Conference

Proceedings.




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[32] Peter Litwinowicz. Processing Images and Video for an Impressionist Effect. In SIGGRAPH ’97

Conference Proceedings.




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