Vis Comput (2012) 28:657–668
DOI 10.1007/s00371-012-0689-9

O R I G I N A L A RT I C L E

Automatic generation of accentuated pencil drawing with saliency
map and LIC
Michitaka Hata · Masahiro Toyoura · Xiaoyang Mao

Published online: 24 April 2012
© Springer-Verlag 2012

Abstract An artist usually does not draw all the areas in a
picture homogeneously but tries to make the work more expressive by emphasizing what is important while eliminating irrelevant details. Creating expressive painterly images
with such accentuation effect remains to be a challenge because of the subjectivity of information selection. This paper
presents a novel technique for automatically converting an
input image into a pencil drawing with such emphasis andelimination effect. The proposed technique utilizes saliency
map, a computational model for visual attention, to predict
the focus of attention in the input image. A new level ofdetail controlling algorithm using multi-resolution pyramid
is also developed for locally adapting the rendering parameters, such as the density, orientation and width of pencil
strokes, to the degree of attention defined by saliency map.
Experimental results show that the images generated withthe proposed method present the visual effect similar to thatof the real pencil drawing and can successfully direct theviewer’s attention toward the focus.
Keywords Pencil drawing · Saliency map · LIC · Focus ofattention · Multi-resolution pyramid

1 Introduction
Compared with photographs, paintings and drawings canoften convey information more effectively through emphasizing what is relevant while omitting extraneous details.
M. Hata · M. Toyoura · X. Mao ()Interdisciplinary Graduate School of Medicine and Engineering,
University of Yamanashi, 4-3-11, Takeda, Kofu, Yamanashi,
400-8511, Japane-mail: mao@yamanashi.ac.jp

A skilled artist can successfully direct the viewer’s attention
to the focus by accentuating the focus through changing therendering parameters in the picture. This paper presents a
novel technique for automatically converting an input photograph into a pencil drawing image with such kind ofaccentuation effect. Pencil drawings are excellent both as
the preparatory sketches and as finished renderings. The
monochrome tone and the specific mood of “unfinishedness” can often stir up one’s curiosity and imagination, andhence they are also widely used as an expressive medium
for apparel or architecture designs and scientific or technical illustrations. Being monochrome, the effect of accentuation is actually particularly important in achieving the charm
appearance, as well as the expressiveness of pencil drawings [1]. Figure 1(d) shows an example of pencil drawing by
an artist. To direct the viewer’s attention toward the squirrel,
irrelevant details are eliminated on outer area and the rendering parameters, such as the texture of strokes, are alsochanged accordingly to accentuate the squirrel. Such accentuation effect is not achieved in Fig. 1(b) which is generated with the existing automatic pencil drawing generation
method [2]. Figure 1(c) is the result generated with the proposed method. We can see the squirrel is expressively rendered in a similar way as that of Fig. 1(d).
The advance of non-photo-realistic rendering technology
in the past decade has made it possible to simulate almost
all kinds of traditional artistic media including the pencil
drawing. However, automatically creating true expressive
painterly images with such accentuation effect remains to
be a challenge. The difficulty mainly lies in the subjectivity of information selection. Given the same scene, different artists may create very different works by accentuating different areas or objects, depending on how they perceive or understand the scene as well as what kind of subjects they want to express. A known perspective to analyze

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Fig. 1 Accentuated pencil drawing generation using saliency map and LIC (Line Integral Convolution). (a) Input image. (b) Result of existing
method [2]. (c) Result of the proposed method. (d) Work drawn by an artist– A novel framework combining saliency map and LIC(Line Integral Convolution) for automatically generating
viewer-centered pencil drawings with the accentuation effect.
– A new multi-resolution scheme for achieving accentuation effect through locally adapting various rendering parameters to the degree of attention.
– New techniques for improving the image quality of existing LIC-based pencil drawing generation method.
– An evaluation experiment for validating the effect of accentuation in inducing the viewer’s visual attention.

Fig. 2 A viewer-centered painting by J.M.W. Turner “Rain Steam andSpeed the Great Western Railway”

the difference among pictures is whether the representation
is viewer-centered or object-centered [3]. Object-centeredpictures, as typified by Picasso’s works, are the interpretation of what artists know, namely the brain image, whilethe viewer-centered pictures are the interpretation of what
artists see, namely, the retinal image. While information
selection in an object-centered picture can be purely subjective, a salient feature or location on a retinal image isvery likely to be chosen as the subject to be emphasized
in a viewer-centered picture. Figure 2 is a typical viewercentered work by the famous English artist J.M.W. Turner.
We can see that he has made the front of train the focusby blurring out the details at other areas. The front of the
train is likely to be what has drawn his attention at his firstglance of the scene. Inspired by Turner’s work, we cameto the idea of employing saliency map [4], a computationalmodel of visual selective attention, for automatically predicting the focus of attention in a viewer-centered picture.
To accentuate the focus, we also developed a technique forcontrolling the level of details as well as the appearance of
pencil strokes according to the degree of attention given bysaliency map. Our major contributions can be summarized
as following:

Our experimental results show that the images generated
with the proposed method present the similar accentuation
effect as in the real pencil drawing and can successfully direct the viewer’s attention toward the focus.
The remainder of the paper consists of the following sections. Section 2 reviews the related works. Section 3 briefly
introduces the existing LIC-based pencil drawing generation technique and saliency map. Section 4 presents the details of the proposed technique by showing the basic idea,
the overall procedure and the details of newly proposed
techniques. Section 5 describes the evaluation experiment
after showing some results. Section 6 concludes the paper.

2 Previous work
In this section, we review the related work from two aspects:
pencil drawing generation and the application of visual attention models in image abstraction.
2.1 Pencil drawing generation
As one of the most popular artistic media, pencil drawing
was also well explored for non-photorealist rendering in thepast decade. There are mainly two approaches to addressing
the pencil drawing computationally. The first approach is toprovide physical simulation to the materials and skills, andAutomatic generation of accentuated pencil drawing with saliency map and LIC

has been mainly combined with interactive painting or 3D
non-photorealistic rendering for generating realistic pencil
drawing images. The second approach is the painterly filtering, which involves taking an image and applying somekind of image processing or filtering techniques to convert
it into an image visually resembling a pencil drawing. Our
technique takes the second approach. It takes a 2D image
as the input and converts it into a pencil drawing like image
automatically.
As simulation-based approach, Sousa and Buchanan developed an observation model of pencil drawings [5, 6]
by investigating real pencil drawing with scanning electron microscope. Takagi et al. proposed to model the paper micro-structure and color pigment distribution as 3Dvolume data and use volume ray-tracing for renderingcolor pencil drawings [7]. Semet et al. used an interactive artificial ant approach to generate a pencil drawing from a reference image [8]. Melikhov et al. used adisk B-spline curves for converting an image into a pencil drawing strokes with physically simulated interior [9].
Lee et al. developed a GPU-based real-time techniquefor rendering 3D meshes using in the pencil drawing
style [10].
More recently, AlMeraj et al. [11] designed an algorithm
for creating lines that perceptually resembles human drawn
pencil strokes, based on a physical model of human arm
movements as well as an observational analysis of humandrawn lines.
Among large number of pencil drawing generation techniques based on image processing or filtering [12–18], a major scheme is to use LIC (Line Integral Convolution). LIC isa texture-based flow visualization technique which visualizes a flow field by low-pass filtering a white noise along
the local streamlines of the vector field. Inspired by the visual similarity between the pencil strokes and the traces of
streamlines in LIC image, Mao et al. proposed a techniquefor automatically converting a 2D input image into a pencil
drawing by convolving the dithered version of the input image with a vector field defining the directions of strokes [2].
The method has been extended for colored pencil drawing[12] and video [13, 14]. Recently, several more works havebeen done to add further improvements to the original LICbased method [15–17].
To the best of our knowledge, however, all those previous
works on pencil drawing put their efforts mainly on how to
generate realistic strokes or hatching, and the issue of how toautomatically generate the accentuated pencil drawing has
not been explored yet. The proposed method extends the existing LIC scheme for automatically generating accentuatedpencil drawings. We employ a new multi-resolution pyramid based approach to locally adapt the parameters of LIC
to the degree of relevance so as to accentuate the focus ofattention.

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2.2 Application of visual attention models in image
abstraction
Visual attention models have been explored by many researches for achieving the abstraction effect in painterly rendering. DeCarlo and Santella proposed a method for controlling the level of details in stylized images according touser’s gaze information [19]. This method has the advantage of being able to reflect the real intention of a user, but
has the shortage of requiring the eye-tracking which limits
its usefulness in real applications. Our method uses saliency
map to predict user’s attention and hence it is fully automatic. Saliency map is a spatial map showing the strength(degree of saliency) of the visual attention. Since there is alimit in the throughput of the brain, the information of broad
perspective cannot be processed in an instant. So human aggregates multiple features and then turns attention to where
it unconsciously acquired as a more important place.
Saliency map has been used for generating other kinds ofpainterly images. Collomosse and Hall used saliency map
to generate the images in oil-painting style [20]. The visual attention of an edge is detected and only those edges
with high attention are drawn later. Winnemöller et al. proposed an automatic real-time video abstraction framework[21], in which areas with lower frequency are further filteredout so that the most informative area (the area consistingof higher frequency) can be further emphasized. Kyprianidis and Kang have published a series of excellent papers
for high quality real-time image and video abstraction using different filtering kernels and edge-enhancement techniques [22–26]. Zhao et al. used saliency map to control the
degree of abstraction [27]. Although our technique can beviewed as a kind of image abstraction technique and shares
the same idea of using saliency map to define the degree ofabstraction, we have developed a set of new techniques for
applying the saliency map to the specific application of pencil drawing generation.
3 Preliminaries
As mentioned in previous sections, saliency map and LICbased pencil drawing generation scheme are two importantelemental techniques upon which we build the proposed
techniques. As the preliminaries required for understanding
the algorithm of the proposed technique, we briefly describe
in this section the procedures for generating pencil drawing
with LIC and for computing saliency map.
3.1 Pencil drawing generation using LIC
As depicted in Fig. 3, given a photograph, Mao et al.’s technique generates a pencil drawing image in the following 7
steps [2]:660

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Fig. 3 Pencil drawing generation using LIC

1. Generate a gray scale image from the input image.
2. Generate an edge image from the grayscale image.
3. Generate the noise image by applying random ditheringto the grayscale image generated at Step 1.
4. Generate the vector field either by assigning a random
direction to each segmented region or using the gradientof the input image.
5. Applying LIC to the white noise and the vector field.
6. Generate stroke image by adding the edge image to the
LIC image.
7. Obtain the output image (pencil drawing) by compositing
the stroke image with the paper sample.
By using the dithered image as the noise, the resulting
pencil drawing has a tone matching that of the input image.
Changing the granularity of the noise or the length of the
1D convolution kernel in LIC can result in pencil strokesof different widths or lengths. We will describe in the next
section how we adapt those parameters to the local saliency
value to achieve the accentuation effect.
3.2 Saliency map
We use Itti et al.’s model [4] to compute the saliency map.
As depicted in Fig. 4, firstly, intensity, color (red, green,
blue and yellow) and orientation (degrees of 0°, 45°, 90°,
and 135°) are extracted in multi-resolution from the Gaussian pyramid and Gabor pyramid. Then a Feature Map isgenerated for each of them by computing the difference between the layers of the pyramids, which imitates the centersurround type receptive field. The multi-resolution layers ofeach Feature Map are combined into a Conspicuity Map
where similar features are suppressed and distinct features
are further emphasized nonlinearly. Finally, the intensity,
color and orientation of Conspicuity Map are unified to obtain the Saliency Map which topographically encodes forconspicuity (or “saliency”) at every location in the input image.

Fig. 4 Computational model of saliency map

4 Proposed method
4.1 Basic idea
In the real pencil drawings, artists achieve the effect of accentuation by controlling not only the level of details but
also the appearance of strokes. Figure 5 is an example ofpencil drawing by an artist. The focusing region (area enclosed by the dotted line) is drawn in fine details with clear
and sharp strokes. The periphery area between the dottedline and dashed line is drawn with rough and flat strokes andthe outer region is simply omitted. To achieve the similar effect, the proposed method uses a Gaussian filtered versionof saliency map, called Draw Map, to define the relevanceof each area. To locally adapt the density and appearance ofstrokes to the relevance, we first build a Gaussian pyramidfrom the input image, and then select appropriate resolution from the pyramid according to the saliency value in the
Draw Map for computing the noise, edge and vector field.

Automatic generation of accentuated pencil drawing with saliency map and LIC

4.2 Overall procedure
Given an arbitrary input image, the proposed method automatically converts it into an accentuated pencil drawing in
the following 9 steps (Fig. 6):
1. Generate a saliency map from the input image.Smooththe saliency map to obtain the Draw Map.

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2. Generate a grayscale image from the input image. Builda multi-contrast Gaussian pyramid with descending contrast along with the decreasing of resolution.
3. Generate a Multi-Resolution Image by selecting pixels
from different layers of the multi-contrast Gaussian pyramid according to the pixel’s saliency value in the Draw
Map.
4. Generate an edge image from the Multi-Resolution Image.
5. Generate a vector field with the Gabor pyramids whichhave been used for extracting orientation features when
computing saliency map.
6. Generate a noise pyramid by applying random dithering
to each layer of the Gaussian pyramid.
7. Apply LIC to the noise pyramid and vector field to generate a LIC pyramid. Then generate a multi-resolution LIC
image by selecting the pixels at appropriate layers of the
LIC pyramid based on the Draw Map.
8. Generate stroke image by adding multi-resolution edge
image to the multi-resolution LIC image.
9. Obtain the Output Image (pencil drawing) by compositeof the stroke image and the paper sample.
4.3 TechniquesFig. 5 A focus and its rendering. An accent is attached by thecontrast of strokes. The inner side of the dotted line is distinctly drawn compared with the area between the dotted line andthe dashed line. And the outside of the dashed line is omitted.
From The Walters Art Museum (http://art.thewalters.org/detail/15810/
little-girl-dressing-her-little-brother/) (CC BY-NC-SA 3.0)

Fig. 6 Overall procedure of the proposed method

4.3.1 Draw Map
In saliency map, the saliency value is nonlinearly emphasized or suppressed to simulate the lateral inhibition mech-

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anism (Fig. 11(b)) [4]. For this reason, if we simply use theoriginal saliency map for controlling the local rendering parameters, almost no strokes would be drawn for most areasof the image except for a very small region around the position with highest saliency value (Fig. 11(d)). However, as
shown in Fig. 5, the periphery of the focus is usually also
drawn to provide the context for the focusing area in real
pencil drawings. It is psychologically known that periphery
vision is very important in scene recognition [28]. Here thecontext in periphery, rendered with strokes of different appearance can provide a further cue to emphasize the focusing region.
To solve this problem, we first suppress the nonlinear effect of saliency map by taking a square root of the saliency
value and then further smooth it with a Gaussian filter. Since
it is known that the best periphery size is about the half of thescene [28], we set the standard deviation σ of the Gaussian
filter to be 1/12 of the image size, so that the filter supports
half of the image size. We call the resulting smoothed map
Draw Map and use it instead of the original saliency map for
locally controlling the parameters of strokes. Figure 11(e) isa pencil drawing image generated by using the Draw Map ofFig. 11(c), where we can see an accentuation effect similar
to Fig. 5 has been achieved.
4.3.2 Multi-Contrast Gaussian Pyramid
To make the strokes look sharper and clearer in the focusing
area, we use an image of enhanced contrast for computing
the parameters of strokes for the focusing area and use images of low contrast for other areas. To get the local control
over the contrast, we process the Gaussian pyramid to havea descending contrast along with the decreasing of resolution to get a Multi-Contrast Gaussian Pyramid. Thus, by using the appropriate layers from the Multi-Contrast Gaussian
Pyramid for generating noise as well as for detecting edges,
we can locally adapt the sharpness of strokes to the saliency
value.
4.3.3 Multi-Resolution Image (integration of pyramid
layers based on Draw Map)
For keeping a smooth transition between the areas of different level of detail, instead of computing edges from MultiContrast Gaussian Pyramid directly, we first generate a single Multi-Resolution Image by choosing pixels from thesuitable layer of the pyramid according to the saliency value
in the Draw Map. Denoting the depth of the layer with thehighest resolution as N , the layer r from which the pixel(x, y) in the Multi-Resolution Image should be chosen is
calculated by the following equation:r = DM(x, y) · N(1)Fig. 7 Edges in real pencil drawing. From The Walters Art Museum(http://art.thewalters.org/detail/11007/mountain-and-river-scene/) (CC
BY-NC-SA 3.0)

where DM(x, y) ∈ [0, 1] is the saliency value in the Draw
Map. Since r is usually a floating number, we calculateMI(x, y), the value of the pixel (x, y) in Multi-Resolution
Image, by linearly interpolating the values of the pixels on
the two adjacent layers of the pyramid:MI(x, y) = (1 − a) · PI floor(r), x, y
+ a · PI floor(r) + 1, x, y ,a = r − floor(r),where PI(n, x, y) is the value of the pixel (x, y) at the nth ∈[0, N] layer of pyramid.
4.3.4 Edge
As shown in Fig. 7, edge is also an important visual factor contributing to the effect of accentuation. The density
and strength of the edge should decrease as getting further from the focus. This effect can be achieved naturally
by detecting the edges from the Multi-Resolution Imageon which the frequency and contrast decrease with the decrease of saliency value. DoG filter is commonly used forthe edge detection in non-photorealistic rendering applications [22, 23, 27] because it is relatively insensitive to noisecompared with differential filters. But we choose adaptive
thresholding method [29] for achieving an easy control over
the width of edges. The edge image ATh(x, y) is calculated
in the following way:
1, MI(x, y) > Ave(x, y) − E
ATh(x, y) =0, otherwise
where Ave(x, y) is the average of the 5 ∗ 5 neighborhood of
pixel (x, y). E is an offset to control the density and widthAutomatic generation of accentuated pencil drawing with saliency map and LIC

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Fig. 9 Accentuation of edges. (a) Uniform edges. (b) Accentuatededges with their density, width and strength adapted to local saliency
valueedge the appearance of pencil stroke matching the strokes inother interior regions. Figure 8(c) shows the result of applying LIC along the edge.
By applying the above edge detection as well as the edge
enhancement technique to the Multi-Resolution Image generated according to Draw Map, we can obtain the edge
images with the density and appearance of edges that are
adapted to local saliency value. Figure 9(b) is an example
of such a result. Compared with the edge image generated
without using Draw Map (Fig. 9(a)), we can see the result of
proposed technique (Fig. 9(b)) which resembles the accentuation effect of the work drawn by artist (Fig. 7).
4.3.5 Vector field

Fig. 8 Noise removal of edge refinement. (a) Result of the adaptive
threshold. (b) Noise removal. (c) LIC enhancementof the edges. By increasing E, fewer and narrower edges aredetected.
The edge image obtained by the adaptive thresholding
may contain short edges and noise as shown in Fig. 8(a).
We remove those short edges and noise by eliminating
the edge pixel whose neighborhood has low average edge
strength. Figure 8(b) is the result obtained by eliminating
the edge pixels whose 5 ∗ 5 neighborhood has small edge
strength. Finally Line Integral Convolution is performed on
the edge image to produce coherent edges with the vector field described in the next paragraph. Since LIC blurs
the image along local streamlines of the vector field, applying LIC to the edge image results in the edge enhancement in a way similar to flow-based edge enhancement approaches [22, 23, 27]. Furthermore, LIC filtering gives the

Existing LIC-based methods used gradient and Fourier analysis for defining the vector field representing local texture
directions. We chose to use the result of Gabor filtering forthree reasons: (1) Gradient method is very sensitive to noise;(2) Fourier analysis method usually cannot generate a correct vector field except for the case when the local texture
has a very uniform direction; (3) Gabor filter is known to bebiologically correct texture descriptor and we already havethe multi-resolution Gabor pyramid for 4 orientations (0°,45°, 90°, 135°) available during the process for generating
Saliency Map.
Denoting by λ the wavelength, σ the standard variation,
θ the orientation, ψ the phase offset, and γ the spatial aspectratio, the Gabor filtering of image I can be represented as thefollowing convolution:
2

GF λ,σ,θ,ϕ (x, y) = e

2 2− x́ +γ2 ýx́ = x cos θ + y sin θ ,

2σ



x́
cos 2π + ϕλý = −x sin θ + y cos θ664

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low-resolution layer when generating the noise. This is realized by adjusting the intensity of Multi-Contrast Gaussian Pyramid according to the depth of the layer before performing dithering. Thus the value at pixel (x, y) on the nthlayer of noise pyramid NP is calculated in the followingway:
NP(n, x, y) =Fig. 10 Calculating local orientation by compositing the 4 vectors detected by Gabor filtering

We can get the strength of the various orientation components by changing θ of Gabor filter.
Gabor energy is obtained by calculating the root-sumsquare between a Gabor and a 90°-phase-shifted version:Gθ (x, y) =


2 
2
GI θ,0 (x, y) + GI θ,− π2 (x, y)GI θ,ϕ (x, y) = I (x, y) ∗ GF θ,ϕ (x, y)=I (k, l)GF θ,ϕ (x − k, y − l)kl

As shown in Fig. 10, we detect the direction vector at eachpixel as the composition of vectors obtained by scaling thefour unit vectors in the directions of 0°, 45°, 90°, and 134°.
By subtracting the Gabor energy in each of the four directions with the minimum energy among the four, we can
avoid false detection of a vector in case the pixel does nothave an explicit direction.
As shown in Fig. 10, when the direction of the texture is
between 135° and 180°, instead of 0°, unit vector of 180°
should be used for the composition.
⎧−→
θ=180◦ ,45◦ , (Gθ − m) e θ ,⎪⎪90◦ ,135◦⎪−→ ⎨ G ◦ > max(G ◦ , G ◦ )
135
45
90
V =
⎪
⎪
⎪
−
→
⎩
◦ ◦ (G − m) e ,otherwise
θ=0 ,45 ,90◦ ,135◦θθm = min(G0◦ , G45◦ , G90◦ , G135◦ )0,

1 + Nn (PI(n, x, y) − 1) < T

1,otherwise

where T is the threshold of random dithering.
4.3.7 LIC image
We apply LIC to the vector field and noise pyramid to getan LIC pyramid, and then generate the multi-resolution LIC
from the LIC pyramid by referring to the Draw Map. Since
the process of generating the multi-resolution LIC requires
to up-sample the low resolution layer to have the same sizeas the highest resolution, it results in rough and wide strokes
naturally at the area with low saliency value.

5 Results
5.1 Implementation
We implemented the proposed technique using C# andEmgu CV (wrapper to the OpenCV). We used iLab Neuromorphic Vision C++ Toolkit VirtualBox (http://ilab.
usc.edu/toolkit/) for computing the saliency map. Depthof the pyramid is 4 layers (N = 3). The length of the 1D
filter kernel used in LIC is 10 for the layer of highest resolution. Part of images are from Berkeley Segmentation Data
Set (http://www.eecs.berkeley.edu/Research/Projects/CS/
vision/bsds/) and PublicDomainPictures.net (http://www.
publicdomainpictures.net/).

G180◦ = G0◦
Again, using the Gabor pyramid we can adapt the level ofdetails of the orientations to that of the saliency value. As
we can see in a rabbit image in Fig. 10, with our Gabor pyramid based orientation detection method we can generate thestrokes well depicting the orientation of local texture.
4.3.6 Noise
In order to achieve the omission of a stroke in the irrelevant areas, we reduce the density of black pixels at the

5.2 Results
Figure 11 demonstrates the effect of adapting the rendering parameters to the saliency value. Figure 11(b) is generated by only adapting the level of details to the saliency
value while Fig. 11(a) also changes the directions, widthand length of strokes, the density and strength of edges. It
is obvious that by changing the rendering parameters, thefocusing area can be further accentuated making the image
more expressive and charming.

Automatic generation of accentuated pencil drawing with saliency map and LIC

Fig. 11 Draw Map and rendering parameter control. (a) Input image.
(b) Saliency Map. (c) Draw Map. (d) A result generated with the proposed scheme but using the original saliency map. (e) A result gen-665

erated with the proposed scheme but only adjusting the intensity with
Draw Map. (f) A result generated with the proposed method which adjusting all the rendering parameters with Draw Map

Fig. 12 Comparison of results. (a) Input images. (b) Draw Map. (c) Results of proposed method. (d) Results of Mao et al.’s method [2]Figure 12 shows several more results generated with our
new techniques. For comparison, the images generated withthe existing LIC-based method [2] are also shown. We cansee from the images that compared with the result of existing
method, the focus area has been emphasized by eliminating
irrelevant details in the remaining area. The change of rendering parameters also makes the image more charming andpleasing as an art work.

The time required of generating a 962 ∗ 642 image is 51
minutes on a Core2Duo 2.26-GHz PC. This does not include the time for computing saliency map. The most timeconsuming parts are the local orientation detection based on
Gaussian pyramid and LIC. Since fast LIC [30] and Gaborfiltering algorithm using GPU [27] are already available, we
can expect to improve the speed of the proposed technique
by utilizing those acceleration techniques.

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Fig. 13 Eye-tracking data (30 seconds). (a) Input. (b) Results of proposed method. (c) Results of Mao et al.’s method [2]5.3 Evaluation
We have conducted an experiment to validate whether the
pencil drawing images generated with the proposed technique have the effect of directing the viewer’s attention to
the focus area. The subjects are divided into 3 groups. Group
1 is presents the original images, Group 2 is presented withthe pencil drawing images generated with the existing LICbased method, and Group 3 is presented with the images
generated with the proposed method. Each group has 3 subjects and there is no overlapping of subjects among thesegroups. In other words, each subject watched only one version for each image. We recorded the eye positions of subjects for the first 30 seconds using EMR-AT VOXER eyetracker with a sampling rate of 60 Hz from NAC Image
Technology, Inc. Figure 13 visualizes the eye positions of
the 3 subjects (each in different color) over the images. We
can see, for all 3 different images that the eye positions on
the images generated with the proposed technique are moreconcentrated around the focusing area. Note that the eye positions on the images generated with the existing method
are even more scattered than those on the original images.
From the evaluation results we can conclude that the images generated with the proposed technique do have the effect of drawing the viewer’s attention toward the focusingarea.
5.4 Discussion
Figure 14 shows an example of failure. Only half of thehorse’s face was drawn. Saliency map is a bottom-up modelFig. 14 An example of failure

based on low-level visual cues and hence it may fail topredict human attention correctly since our attention is
also top-down and task-based. Some other factors may
influence the visual attention. For example, it is known
that humans tend to attend the pattern of a face. Combining the newest face detection technique may help us
improve the results for an image including human objects. Recently, several more sophisticated visual attention models have been developed. Judd et al. created asaliency model which uses machine learning technique tolearn the visual attention from the eye-tracking results [31].
Since this model could partially learn the top-down visual attention, we can expect to generate pencil drawings
more similar to human drawn works by employing theirmodel.

Automatic generation of accentuated pencil drawing with saliency map and LIC

6 Concluding remarks
We proposed a novel method for automatically converting
an input image into pencil drawing. By using saliency map
to predict the attention of a viewer and by adapting the local rendering parameters to the saliency value, we succeededin generating viewer-centered pencil drawing images simulating the effect of emphasis and eliminations found in human pencil drawings. We are now improving the method byemploying a more sophisticated visual attention model andsome newest computer vision technologies. Through suchan effort we expect to be able to partially simulate the effectfound in object-centered pencil drawings.
Acknowledgement The authors thank Mr. T. Sawada from University of Yamanashi for his help in conducting the evaluation experiment.
This work was supported by KAKENHI 21300033 Grant-in-Aid for
Scientific Research (B), Japan Society for the Promotion of Science(JSPS).

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Michitaka Hata received the B.Sc.
and M.Sc. degrees in Engineering from University of Yamanashi,
Japan. He is a Ph.D. student in Interdisciplinary Graduate School of
Medicine and Engineering, University of Yamanashi, Japan. His research interests include non-photorealistic rendering and visualization.

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M. Hata et al.
Masahiro Toyoura received the
B.Sc. degree in Engineering, M.Sc.
and Ph.D. degrees in Informatics
from Kyoto University. He is currently an Assistant Professor at Interdisciplinary Graduate School of
Medical and Engineering, University of Yamanashi, Japan. His research interests are augmented reality, computer and human vision.
He is a member of IEEE Computer
Society.

Xiaoyang Mao received her B.Sc.
in Computer Science from Fudan
University, China, M.Sc. and Ph.D.
in Computer Science from University of Tokyo. She is currently a Professor at Interdisciplinary Graduate
School of Medical and Engineering,
University of Yamanashi, Japan. Her
research interests include texture
synthesis, non-photo-realistic rendering and their application to scientific visualization. She is a member of ACM SIGRRAPH and IEEE
Computer Society.