InkBrush: A Sketching Tool for 3D Ink Painting

InkBrush is a new sketch-based 3D drawing tool for creating 3D ink paintings using free-form 3D ink strokes. It offers a digital calligraphy brush and various editing tools to generate realistic ink-like brush strokes with attributes like hairy edges, ink drips, and scattered dots. Users can adjust parameters such as moisture, color, darkness, dryness, and stroke style to customize the appearance of the brush strokes. The development of InkBrush was guided by a design study involving artists and designers. It was developed as a plugin for Blender, a popular 3D modeling tool, and its effectiveness and usability were evaluated through a user study involving 75 participants. Preliminary feedback from the participants was overwhelmingly positive, indicating that InkBrush was intuitive and easy to use. Following this, we also sought in-depth assessments from experts in ink painting and 3D design. Their evaluations further demonstrated the effectiveness of InkBrush.

calligraphy brush and various editing tools to generate realistic inklike brush strokes with attributes like hairy edges, ink drips, and scattered dots.Users can adjust parameters such as moisture, color, darkness, dryness, and stroke style to customize the appearance of the brush strokes.The development of InkBrush was guided by a design study involving artists and designers.It was developed as a plugin for Blender, a popular 3D modeling tool, and its efectiveness and usability were evaluated through a user study involving 75 participants.Preliminary feedback from the participants was overwhelmingly positive, indicating that InkBrush was intuitive and easy to use.Following this, we also sought in-depth assessments from experts in ink painting and 3D design.Their evaluations further demonstrated the efectiveness of InkBrush.

INTRODUCTION
Ink painting, an art form with a rich history in various Asian countries, can be traced back to its origins in the Han Dynasty of China.Over time, the practice of ink painting has undergone signifcant evolution, particularly with the advancements in digital technology.One notable development in this regard is the rise of digital ink painting, which has gained substantial popularity due to its ease of dissemination and reproduction [39].In recent years, there has been another notable shift driven by the increasing demand for 3D ink painting, primarily within the gaming, animation, and virtual reality (VR) industry [14,44].Distinct from their 2D counterparts, ink strokes drawn in a 3D space must be visually appealing from various angles while maintaining the intended aesthetic appeal of the ink strokes that form the drawn object.For instance, Figure 1 illustrates a cat painted with 3D ink strokes (using the proposed tool, InkBrush) that possess its visual charm when observed from diferent viewpoints.
However, the creation of visually appealing 3D ink paintings presents a signifcant challenge due to the lack of tools that enable artists to freely draw realistic 3D ink strokes that can capture the spontaneity and carefree aesthetic essential in ink painting [5,28].Current 3D modeling tools such as 3ds Max and Maya only ofer the option of simulating 3D ink painting by applying ink strokes created on a 2D canvas as a texture to a 3D model.Another approach involves 3D sketching tools like Google Tilt Brush, which can simulate ink strokes and their unique hairy edges to a certain extent by integrating hundreds of individual pen strokes manually drawn by the artist.However, these methods require the artist to focus on laborious and repetitive tasks, such as reproducing ink texture patterns, rather than the creative process of painting.To the best of our knowledge, no existing tool supports the freehand drawing of 3D ink paintings using realistic 3D ink strokes.
In this paper, we present InkBrush, a sketch-based 3D drawing tool specifcally designed for the creation of 3D ink painting on a desktop/laptop computer.InkBrush ofers users a digital calligraphy brush and a range of editing tools, allowing artists to sketch ink strokes freely on a 3D canvas using a mouse.InkBrush is uniquely capable of generating 3D brush strokes with distinct characteristics specifc to ink strokes, such as the characteristic hairy edge at the end of a stroke, ink drips, and scattered ink dots (Figure 1).Furthermore, InkBrush provides users with a set of tools that enable them to manipulate the strokes based on various parameters, such as the amount of moisture in ink, the color and darkness of the ink, the dryness of the brush, and the style of the trailing of the stroke.By ofering such a comprehensive range of editing options, our system allows users to achieve their desired artistic outcomes efectively.
The development of InkBrush was guided by a design study that brought together ink painting artists and experienced 3D designers to identify the crucial elements and parameters that infuence the appearance and behavior of ink strokes and generate guidance for an efective painting process within a 3D environment.We implemented InkBrush as a plugin for Blender, a popular 3D modeling and animation software widely used in the creative community.This allowed us to leverage the existing user base of the Blender community to evaluate the efectiveness and usability of InkBrush.Through a preliminary evaluation with 75 participants, we demonstrated that InkBrush was an efective and easy-to-use tool.Participants were able to easily navigate the interface and utilize the various features to create compelling ink paintings in a 3D environment.Building upon the positive outcomes of the preliminary evaluation, we sought to gather more detailed and expert feedback on InkBrush.To achieve this, we conducted a more focused evaluation involving eight participants who had expertise in either ink painting or 3D design, or both.Through the use of InkBrush by these experts, we were able to collect detailed feedback and insights regarding the features, functionalities, and overall usability of InkBrush.
The key contributions of our work are two-fold: (1) A novel tool that allows users to create 3D ink paintings using free-hand 3D ink strokes, and (2) The result of a user study to evaluate the efectiveness and usability of our tool.

INKBRUSH WALK THROUGH
This section provides an overview of the key features of InkBrush by illustrating the process of creating a morning glory painting, a subject commonly found in ink painting.
The user begins by selecting the ink brush tool from the tools panel located on the left side of the workspace.They then proceed to sketch a branch (Figure 2a).The frst stroke is automatically drawn on a plane that intersects the world origin, aligning parallel to the view plane.Each ink stroke is represented in 3D using two fat bands placed perpendicular to each other, giving the stroke a unique appearance from diferent viewing angles (Figure 2b).In this particular case, the user notices that the stroke is thicker than desired due to the default sets of brush parameters.To rectify this, they enter the editing mode, which reveals all the anchor points of the stroke's trajectory defned by a Bezier curve (Figure 2c).By clicking on each anchor point and adjusting the width widget, the user is able to modify the width of the stroke until achieving the desired thickness for each segment of the branch.These adjustments are recorded in the system to streamline the process of drawing subsequent branches.
Moving on, the user wishes to draw another branch at a diferent angle within the virtual space.To accomplish this, they employ the positioning tool to click on a desired location within the frst stroke where they want to start the second branch.This action specifes the correct depth for the starting point of the subsequent stroke, which aligns with the selected location inside the frst stroke.Next, the user rotates the camera view approximately 30 degrees around the vertical axis and proceeds to draw the second stroke.This results in a second stroke on a 2D plane that is 30 degrees apart from the plane where the frst stroke resides (Figure 2d).The user repeats this process to draw additional branches.
The following step involves drawing the petals.Similar to drawing the branches, the user frst designates the 3D location of the starting point for the next stroke by selecting a desired location on a branch.Since the petals require a thicker brush, the user adjusts the brush settings in the parameter panel located on the right side of the workspace (Figure 2e).The petals can be drawn using a curved trajectory.By default, the ink stroke consists of two bands arranged perpendicular to each other.However, since the user intends for the surface of the petal to be fat, they set the width of the vertical band of the stroke to zero (Figure 2f).Although they have the option to adjust the width at each anchor point using the width widget, the user chooses to modify all the widths simultaneously using the corresponding slider in the parameter panel.Additionally, they adjust the color of the stroke to purple and the moisture of the ink to achieve the desired appearance (Figure 2g).Once satisfed with the petal's look, the user rotates it to their desired orientation and fips it outward (Figure 2h).They repeat this process to draw the remaining petals of the morning glory and then proceed to draw the leaves (Figure 2i).
Lastly, the user rotates the camera view to examine their work from diferent angles.However, upon inspection, they notice that the branches appear fat when viewed from the side of the plane on which they were drawn.To address this, the user enters the edit mode and adjusts the location of one of the branch's anchor points slightly in the 3D space, giving the impression that it wraps around the other branch (Figure 2j).

RELATED WORK
This section provides a concise overview of previous research in the feld and highlights the relevance of our research in relation to three specifc areas: 3D ink painting, 3D sketching, and sketch-based 3D modeling.

3D Ink Painting
The current approaches used in generating 3D ink paintings or ink efects can be classifed into three primary categories: layering, texture drawing, and model rendering.It should be noted that these techniques do not function as freehand drawing tools, as they do not enable users to freely draw ink strokes within a 3D space.
Layering.Layering is a technique that uses multiple layers of 2d paint to create the illusion of a 3D efect.By separating various elements of a 2D scene onto designated layers within a 3D space, artists are able to produce a semblance of depth.This method has been used in animations, with one of the most renowned examples being the work titled Qiu Shi [51].However, it is important to note that artworks created through this technique can only be viewed from a fxed angle, as they do not possess true three-dimensionality. Additionally, the process of crafting each 2d layer is known to be tedious and time-intensive.
Texturing.Texturing is a prevalent approach in the creation of 3D ink paintings.This technique incorporates 2D ink paintings as textures onto a 3D model [5] and is currently the industry standard for producing such artworks.One notable example of a texturingbased ink painting animation is "Ode to Summer" [30].However, it is important to note that the major drawback associated with this method is that the texture applied to the model tightly adheres to its surface, like wrapping paper, resulting in a lack of brushwork efects along the edges.Consequently, the true essence and aesthetic of ink painting are greatly compromised.On the other hand, this approach requires complex 3d knowledge and the user needs to understand the correspondence between 2d textures and 3d models, which is not very novice friendly.
Model Rendering.Rendering techniques in computer graphics, such as non-photorealistic rendering (NPR) [41], have been employed to generate ink paintings based on the 3D model of an object.Notable works in this feld include those by Yuan et al. [50] and Deng et al. [10].Nevertheless, these methods necessitate the creation of 3D models prior to their conversion into ink paintings.Therefore, they cannot be used for freehand drawing.Furthermore, a common limitation of these approaches is their inability to produce ink painting brush strokes that possess naturalness and aesthetic appearance.To address this issue, Sun et al. [42] have proposed a novel multi-layer rendering algorithm that aims to simulate the efect of ink painting brushwork, efectively generating strokes like "Gou" and "Dun".Despite this advancement, one drawback of their approach is the absence of a 3D stereoscopic efect in the resulting ink paintings.To better simulate brush strokes that exhibit naturalness, recent research eforts have concentrated on optimizing the outcomes along the contour lines of the generated objects [13,25,31,43].Furthermore, specifc algorithms have been developed to optimize ink painting renderings for particular objects, such as mountains [46], trees [45], or animals [26,49].

3D Sketching
Much research has been conducted on the generation of 3D strokes from a user's 2D input, a lineage that can be traced back to Maneesh et al.'s pioneering method on non-parametric 3D polygon mesh surface painting that was proposed in 1994 [2].Presently, a prevalent method in this domain involves the creation of 3D strokes by projecting 2D strokes onto the surface of a 3D model [17,21,35].For example, Johannes et al. developed a system enabling users to draw 3D strokes directly on the surface of a 3D model [35].Similarly, Skippy, projects various segments of a stroke drawn on a 2D plane onto the curved surface of a 3D object [21].Another common approach to achieving this functionality is to anchor the user's drawing trajectory on the 2D screen plane.By adjusting the position of the camera, users can draw at diferent locations in 3D space.Many existing systems, such as Feather 3D [19], Mental Canvas [11], and the work by Lee et al. [24], have adopted this approach.In our implementation of InkBrush, we have also adopted this approach.
The use of 3D sketching for creative purposes has also gained signifcant attention in VR applications, where users move controllers to draw within a virtual 3D space.Software like Multibrush [33], Quill [40], CanvoX [20] and Tilt Brush [12] are commonly used for this purpose, with the built-in brushes typically consisting of singlecolored or simple noise-mixed strokes.Note that the absence of haptic feedback in the virtual 3D space can make freehand drawing, even on the surface of a virtual object, challenging.Consequently, eforts have been made to enhance the precision of drawing mid-air strokes.[3].Despite extensive research and commercial tools in this space, existing methods do not ofer a means to produce ink strokes.Consequently, artists have tried to mimic the shape and color of ink strokes by manually integrating hundreds of individual pen strokes (Figure 3).This method demands substantial user efort and often results in unsatisfactory results due to the limitations of this approach in capturing the subtle characteristics of ink painting, such as dryness and hazy efects [9,15,32].Consequently, artists' attention is often diverted from the creative process to the laborious and repetitive task of replicating ink texture patterns, detracting from their ability to fully concentrate on the act of creation.
Learning from these limitations, our tool aims to empower artists by granting them complete control over their strokes, enabling modifcations at any stage of the creative process.Additionally, each stroke can be customized in various adjustable dimensions, providing artists with the necessary control and ability to fne-tune their artwork according to their preferences.

Sketch-Based Interactive Modeling
Researchers have also conducted work on sketch-based 3D modeling techniques to enhance users' work efciency by assisting in the semi-automated creation of 3D models [18,22,23,29].For instance, Takeo et al. developed a sketch-based modeling interface that can automatically generate a 3D model based on the 2D sketches drawn by a user on the screen [16].Cherlin et al. further improved this approach by enabling the creation of higher quality 3D models with fewer user strokes [6].Xing et al. proposed an immersive data-driven approach for hair modeling, utilizing the strip method, which enables users to quickly create high-quality 3D hair using a hair brush [48].The gaming and animation industry's rapid development has led to the emergence of specialized sketchbased modeling tools designed for complex objects such as hair, smoke, and water [1,36,47].These tools alleviate users' burden of focusing on intricate and repetitive details while ensuring the production of high-quality outcomes.Drawing inspiration from these existing works, our system aims to generate 3D models of ink brush strokes based on the trajectory of a user's input trace, encompassing small details like ink drips and unique characteristics specifc to ink painting.

CORE INK PAINTING PATTERNS
This section aims to elucidate the signifcant characteristic features of ink strokes that were considered to shape the functionality of our tool.It is worth mentioning that although these features pertain primarily to a 2D space, their exploration in the existing literature for interactive tools has been limited.Therefore, conducting a thorough examination of these features is crucial to efectively inform the design of our system.A more detailed overview and background introduction of this traditional art can be found in books such as [37].
Visual foundation.In ink painting, the application of various techniques to form ink strokes is closely referred to as the concept of "line".This notion extends beyond mere delineation of outlines or contours, encompassing the ability to communicate a sense of form, movement, and emotion.Figure 4 provides an illustration of the distinctive appeal of ink painting in comparison to other painting arts, such as oil painting and watercolor painting.From a visual perspective, the strokes in ink paintings exhibit a multitude of changes.They showcase an initial movement, followed by a dispersal at their ends, and undulations in the middle (Figure 5a).These dynamic transformations imbue the artwork with a sense of liveliness and energy, capturing the essence of the subject matter.Our tool employs a dynamic generation of ink textures to faithfully replicate the distinct characteristics of ink strokes.It utilizes mesh bands to represent the strokes in a 3D space, allowing for diferent viewing angles while preserving their natural and unique appearance.
The proportion of water to ink is equally important in ink painting, as it contributes to the creation of depth efects and color variations, typically within a grey scale.Artists often manipulate various factors such as the moisture content of the brush, its dryness, the darkness of the ink, the length of the trailing , and the color of the ink itself in order to achieve a diverse range of ink efects (Table 1) .Combining these techniques even gives rise to unique brushwork and ink wash efects, which are defning characteristics of the visual appearance of ink painting (Figure 5).Our tool has incorporated all of these parameters, allowing users to adjust their values according to their individual preferences.Ink dots and drips.The incorporation of ink dots and drips serves as an additional visual element that enhances the vocabulary of brushstrokes and facilitates the artist's expression of emotions.These dots and drips ofer a unique avenue for conveying a sense of spontaneity and unrestricted creativity.The haphazard arrangement of ink dots results from the initial contact of the brush with the paper's surface, efectively conveying the artist's inclination towards arbitrary and uninhibited creation.In line with this artistic tradition, our tool allows users to incorporate ink dots and drips into their paintings.Moreover, the tool allows for adjusting the dots' and drips' placement and density, enabling users to exercise precise control over these elements.Like the 3D strokes, ink dots and drips can also be viewed from diferent angles, adding depth and dimension to the overall composition.

FORMATIVE STUDY
The primary goal of our research was to develop an efective and easy-to-use freehand drawing tool for 3D ink painting.We began our research by conducting a formative study to gather insights from ink painting artists regarding our proposed tool's potential demands and applications.To achieve this, we devised a semi-structured interview protocol and recruited four ink painting artists, two of whom had extensive experience in 3D design (P1 and P2).We sought their perspectives on suitable use scenarios and explored the artistic techniques that could be employed to achieve commonly desired brushwork efects.Moreover, we sought the artists' opinions on the challenges and opportunities associated with integrating 3D elements into a traditional 2D art form like ink painting.This allowed us to understand their thoughts on the potential implications and impacts of incorporating technology into their artistic practice.
Procedure.The artists were briefed on the study's objective prior to responding to our questions (see the list of questions Appendix A).To provide them with a comprehensive understanding of the current state of technology in relation to 3D art creation, we shared information about various techniques and tools that could potentially assist in the creation of 3D ink paintings, such as Multibrush [33] and Tilt Brush [12].Our intention was to stimulate their creativity and provide them with a broader perspective on the possibilities that arise from the integration of technology into their artistic process.
Result.All the artists were enthusiastic about a tool that could facilitate the creation of 3D ink paintings.They believe that such The amount of water in the brush signifcantly afects the edge blurring of a stroke.
The level of dryness of a brush determines the degree of brokenness in brushstrokes The intensity of the ink determines how black a stroke is The length that becomes transparent at the tail end due to lifting the brush Colored ink, such as purple for morning glory Depicting bright spots and sunlight is ideal for rendering clouds, gradations, and ethereal atmospheres.
Commonly used in landscape painting for the textures of mountain rocks Commonly used in drawing large areas of shadow, dark regions, and outlining shaded parts of withered branches, and houses Drawing fake-like or hair-like structures, such as tails, feathers, bamboo leaves, etc While black and grays are dominant, artists sometimes add subtle color hints for objects.
Table 1: Detailed information on the fve parameters a tool has the potential to promote ink painting as a new and versatile medium in various felds, including animation.However, concerns were raised by an artist (P2) with experience in 3D design regarding the time-consuming and tedious nature of the current process, which hinders their creativity during production.Furthermore, these artists (P1, P2, P4) commented that previous works heavily relied on 3D modeling and machine rendering, resulting in minimal involvement from the artists themselves and making it challenging to convey the expressive feeling inherent in their artwork.In relation to the proposed tool, another artist (P4) expressed concerns about the potential learning curve and the high entry barrier, fearing that they may struggle to adapt quickly to the new process.They explicitly stated that the workfow of the tool should align with their existing painting habits.Additionally, it is crucial to note that all the artists emphasized the importance of accurately capturing the charm of ink painting from all angles, as opposed to being limited to specifc viewpoints like existing tools tend to do.These insights highlight the key user needs related to rendering efects, the creative process, and efciency.By incorporating these fndings, we distilled several design objectives.
• 3D Aesthetics It is crucial for 3D strokes to maintain consistency in their aesthetic appearance regardless of the viewer's angle of observation.• Ease of use The painting workfow should incorporate the established practices employed by artists in order to facilitate a seamless shift from traditional physical media to digital media.• Real-time rendering Real-time rendering is a crucial requirement that must be supported to provide users with a seamless and interactive experience.

INKBRUSH
Based on the above design goals, we developed InkBrush1 , a software tool that allows users to create 3D ink paintings using freeform 3D ink strokes.InkBrush provides a range of functionalities to facilitate users in sketching and refning their 3D strokes.These functionalities include an ink brush tool, as well as tools for modifying stroke shape, adjusting ink texture parameters, incorporating ink dots and drips, and replaying a painting process (Figure 6).InkBrush was implemented as a plugin for Blender, a popular 3D modeling and animation software widely used in the creative community.This integration allows for seamless utilization of InkBrush within the existing workfow of Blender, enabling users to harness the full potential of 3D ink painting.
Figure 6: The UI of our tool includes a tool panel, a parameter panel, and a 3D canvas.

Ink Brush
The key component of InkBrush is the ink brush tool, which allows users to create 3D ink strokes by freely drawing with their mouse.Users can also use the middle mouse button for zooming, rotating, and panning the view within 3D canvas.InkBrush ofers two different modes of drawing, each based on the location of the brush tip: (1) 2D panel mode and (2) the surface mode.2D panel mode.Similar to the existing 3D sketching tools, the 2D panel allows users to draw on a 2D panel that is parallel to the screen.In situations where the user wishes to start a stroke along an existing one, such as drawing a secondary branch from the main branch of morning glory, they can utilize the positioning tool to click on the desired location.This action translates the 2D panel to intersect with that specifc point, allowing users to accurately specify the depth within the 3D space.
Surface mode.Contrasting the frst mode, the surface mode forces the brush tip to adhere to the surface of a 3D object present in the scene.Consequently, users can directly draw ink strokes onto the object's surface, creating an efect akin to grafti on the object.Once the drawing is complete, the 3D object can be removed.This approach allows users to use the 3D object as a shape guide, facilitating the drawing of complex stroke paths.

Shape Adjustment Tools
The shape adjustment tools allow users to modify the geometry of a completed brush stroke, enabling them to create strokes that align with their artistic preferences.While some of these tools are commonly found in 3D editing software, such as Blender, their inclusion in InkBrush ofers users the means to generate unique artistic expressions while also providing foundational support for their 3D editing tasks.
Move tool.The move tool allows users to manipulate the 3D position of a selected anchor point of the stroke trajectory.
Length tool.The length tool allows users to modify the length of a stroke by manipulating the anchor point located at the end of the stroke.Radius tool.The radius tool allows users to adjust the thickness of a specifc section of a stroke by manipulating a selected anchor point.By simply dragging its widget, the width of the bands that form the stroke model can be increased or decreased as a whole.This tool is particularly useful when editing the tail end of a stroke, as it allows for the creation of unique ink patterns resulting from lifting or pressing the brush tip (Figure 7 a b).
Tilt tool.The tilt tool allows users to manipulate strokes by twisting them at specifc anchor points.By using this tool, users can generate shapes that resemble DNA helixes, adding a unique visual efect to 3D ink strokes that cannot be achieved in their 2D counterparts (Figure 7 c d).Like the other tools listed here, although the tilt tool is not a new addition to 3D editing tools, its integration into 3D ink painting ofers fresh avenues for artistic expression that were previously unattainable with traditional 2D media.

Texture Attributes
Based on our discussions with ink painting artists, the appearance of an ink stroke is infuenced by various factors, among which the most important ones include the moisture level, dryness, darkness, trailing, and color of the ink (Figure 8).InkBrush allows users to adjust each of these factors to achieve their desired efects.
Moisture level.The level of moisture is used to simulate the amount of water absorbed by the brush.A smaller value indicates less amount of water, resulting in a rougher edge to the brushstroke.In contrast, a higher value produces a block boundary, thereby creating the efect of water difusing on paper (Figure 8 a).
Dryness.The level of dryness determines the degree of hollowness created at the center of a stroke.It is a widely used technique in ink painting, typically achieved through the use of rapid brush movement.Higher level of dryness results in a more pronounced hollowness efect.In contrast, a lower level of dryness leads to a more flled appearance in the stroke (Figure 8 b).
Darkness.The level of darkness represents the concentration of ink in a stroke.A higher darkness value results in a stroke appearing more black and less susceptible to color infuence.In contrast, a lower darkness value gives a stroke a greyer appearance, allowing the addition of color to stand out more prominently (Figure 8 c).In ink painting, darkness is frequently used in conjunction with color.
Trailing.The level of trailing determines the positioning of the most ink-heavy part of a stroke caused by a heavy press of the brush.A higher value for trailing results in the ink-heavy part being located near the beginning of the stroke, which gives the appearance of a longer tail.In contrast, decreasing the value of trailing gradually shifts the ink-heavy part towards the middle of the stroke, resulting in a shorter tail (Figure 8 d).In our implementation, the smallest value of trailing produces a pattern that mimics a brush being lightly lifted right after the start of the stroke, followed by gradually increasing pressure during the movement, with the strongest pressure applied at the center of the stroke.
Color.Our implementation allows for strokes to have up to four diferent colors along their path.Users can modify the color of each individual segment, as well as manipulate the distribution of color gradients (Figure 8 e).

Ink Dots and Drips
A crucial feature of ink strokes lies in the presence of scattered ink dots and drips around the head and body of the stroke.To convey these details, we have placed ink dots around the head of the stroke (Figure 9 a).Furthermore, ink drips are positioned along the bottom of the stroke (Figure 9 a).To provide users with fexibility and customization options, we have included options in the parameter panel that allow them to adjust the number of ink points and ink drips according to their specifc needs.Additionally, users can modify the size and relative position of the ink dots and ink drips along the stroke's body (Figure 9 b c).Furthermore, the degree of dispersion of the ink dots can also be adjusted to meet users' preferences (Figure 9 d).It is important to note that the default values of these parameters have been carefully determined by experienced ink painting artists.These default settings ensure a natural and aesthetically pleasing result, thereby signifcantly reducing the time and efort required for users to achieve desirable outcomes.
Figure 9: An illustration of diferent adjustable efects of dots and drips

Recording and Replaying Painting Process
InkBrush automatically captures and records the complete process of ink painting.This feature allows users to review the artwork stroke by stroke.By using replay, users gain access to the details of each brushstroke, allowing for a closer examination of the artist's technique and style.This way, users can gain a deeper understanding of the artist's creative process, witnessing their skill and creativity unfold in real-time.

IMPLEMENTATION
In this section, we present a detailed description of the implementation of InkBrush, focusing on how ink strokes are represented in a 3D space and how ink meshes and textures are generated.

Stroke Mesh
Intuitively the 3D mesh representation of an ink stroke can be created by modeling the complex geometry of the stroke using a hollow mesh, incorporating small details such as ink dots by utilizing spheres or cylinders.While this traditional method accurately captures the intricacies of the stroke, it signifcantly increases the model's complexity, potentially leading to delays in generating and rendering the output, especially when dealing with a large number of strokes.To overcome these challenges, we have chosen a simplifed approach.Instead of employing a complex mesh, we represent the ink strokes using two fat band meshes, composed of multiple quads, oriented perpendicular to each other (Figure 10a).These bands follow the trajectory of the stroke in a 3D space, providing a fat surface to which the ink texture can be attached and viewed from various angles.This simplifed representation not only reduces the model's complexity but also enhances the efciency of generating and realizing real-time rendering.To create a realistic rendering of the hairy end of a stroke, we introduced several supplementary bands along the trajectory of the ink stroke.These supplementary bands are randomly added and are parallel to the main band meshes (Figure 10b).By attaching ink textures to these supplementary bands, a lifelike hairy pattern can be achieved for the end of the stroke (Figure 10c).Moreover, as these supplementary bands are visible from diferent viewing angles, the overall realism of the 3D representation of the ink stroke is further enhanced (Figure 10d).It is important to note that an ink stroke often begins with a region that contains a greater amount of ink than the subsequent parts of the stroke.This is due to the initial heavy landing of the brush on the paper to initiate the stroke.To accurately represent this efect, we create a head mesh, which is wider than the body of the stroke, and attach it to the starting point of the stroke.While a cube or sphere could be used to create the head mesh, for the purpose of simplifying the model, we employed a single quad.Similarly, for representing individual ink dots or ink drips, we use a single quad.However, it should be noted that using fat quads to represent the stroke head or dots poses a challenge.Once their orientation is fxed, they become almost invisible from the side due to their fat nature.To address this issue, we consistently orient them towards the camera.This efectively creates a convincing 3D illusion without the need for complex 3D meshes.

Mesh Generation
To generate the stroke mesh, the process begins with the resampling of the input trajectory that is provided by the user.Following resampling, the smoothed input trajectory is obtained by ftting it into a Bezier curve, which is defned by a limited number of anchor points.This Bezier curve then serves as the medial axis for generating the stroke mesh.The attributes of each anchor point, including its position, direction vector, radius vector, and normal vector, are adjustable by the user and determine the shape and geometry of the stroke mesh.
Next, the square head mesh was generated at the start of the trajectory facing the camera.We then proceeded to generate two band meshes positioned perpendicular to each other along the medial axis.The anchor points of the medial axis divide the two band meshes into diferent regions.These regions can be modifed by adjusting parameters such as radius vector.Users can manipulate the anchor point widget using the mouse cursor to make these adjustments.To ensure a continuous and smooth surface for the band meshes, we employed multiple Bezier curves within each region.The surface is defned by the following equation: where , () and , () are the Bernstein basis functions used to defne the interpolation of Bezier curves., represents the control points that defne the shape of the Bezier curves on the plane.The orders of the surface are denoted by and , respectively, and they determine the smoothness of the resulting surface.
The generation of supplementary bands follows a similar process to that of the stroke meshes.However, there are some key diferences.Firstly, the default width of the supplementary bands is determined by a random proportion of the stroke mesh width.Furthermore, the number of supplementary bands and their specifc locations on the stroke mesh are randomized.This means that each stroke mesh will have a unique arrangement of supplementary bands, elevating the visual aesthetics and overall complexity of strokes.

Ink Texture Generation
To achieve a realistic appearance of 3D ink strokes, we applied a 2D ink texture onto each band of the 3D stroke model.This ink texture can be generated dynamically or statically.While existing literature ofers various techniques for dynamically generating stroke patterns that capture the key characteristics of ink textures, such as fow of ink [8], the dynamic generation of ink textures may demand signifcant computational resources, such as GPU-powered machines [38], which may not be readily accessible to the broader audience of InkBrush and Blender.Therefore, in this early iteration of our system, static textures were chosen, as they were also found to be efective.
In our implementation, we generated the 2D ink texture by combining a Color Texture and an Alpha Texture.The Color Texture consists of four diferent colors arranged linearly, as determined by the color parameter described in Section 6.3.The Alpha Texture, on the other hand, defnes the shape and texture of the stroke.
To create the alpha texture for a band of the 3D stroke model, we frst generated an Ellipse Mask using the inscribed ellipse for that particular band.This Ellipse Mask incorporated a radial gradient of changes in transparency.(Figure 11c) shows the resulting Ellipse Mask, with black and white areas representing transparent and opaque regions, respectively.
Next, we aimed to simulate the unique contour shape of the ink stroke.To achieve this, we used Perlin noise to generate a Noise Texture, with each minimum noise cell being about 1/10 of the width.By applying this Noise Texture to the Ellipse Mask, with contrast amplifed, we were able to generate an efect similar to that of a stroke created with a highly saturated brush soaked in a signifcant amount of water (Figure 11d); referred to as high moisture in Section 6.3).
To simulate a stroke created with a very dry brush (low moisture), we employed a simple yet efective method.Prior to applying the Noise Texture, we stretched it by a factor of 3 in the x-direction relative to the y-direction in the stroke's UV space.The result is illustrated in Figure 11f.To achieve variations in moisture levels between high and low, we assigned diferent weights to the Ellipse Mask and Noise Texture.
Within the contour of the stroke, we also aimed to simulate the distinctive appearance of the body of the ink stroke.Note that when ink is applied to the surface of rice paper, its texture causes an uneven difusion of ink, resulting in a visually unique appearance.In order to recreate this pattern, we decomposed the Noise Texture into its HSV (Hue, Saturation, Value) components and focused solely on the H (Hue) channel.An example of this can be seen in Figure 11h.By multiplying this texture with the Ellipse Mask, which was Figure 11: Ink texture generation translated towards the negative x direction by 0.25, we were able to create an appearance similar to ink difusing from the left (start of the stroke) to the right (end of the stroke) of the image.Lastly, the Alpha Texture was generated by combining the contour shape of the stroke and its internal texture, as shown in Figure 11k.
In contrast, the texture for dots and drips is relatively consistent and does not require dynamic generation based on the shape and size of the stroke.Therefore, for these elements, we opted to use a static image as their texture.However, the colors of ink drips and dots are dynamic and vary based on the color of the adjacent part of the stroke.Using ink dots as an example, for each ink dot, we calculate the color of the closest vertex on the stroke mesh.This color is then blended with the ink dot to create a cohesive color scheme that matches the local area of the stroke.

USER STUDY
We conducted a user study to assess the efectiveness and usability of InkBrush.Our goal was to gain initial insights into how well users were able to use our tool to create ink paintings and to gather feedback on their overall experience with it.Additionally, we aimed to identify the strengths and weaknesses of InkBrush and determine areas for improvement.Considering InkBrush as the pioneering freehand 3D ink painting tool, our primary focus has been directed towards gaining insights of user interaction with the tool, rather than incorporating a baseline for comparative analysis.

Procedure and Task
We released InkBrush on a popular social media platform in China several weeks before the study.Participants were invited to download and use InkBrush on their own computers to create ink paintings.The study was conducted in two phases, with the frst phase focusing on gathering preliminary feedback on the usability and learnability of InkBrush from a large and diverse group of users, despite their expertise in ink painting and 3D design.Before the study began, participants were provided with a 5-minute instructional video to familiarize themselves with InkBrush.The video outlined the general purposes and features of the brush, followed by a step-by-step tutorial on how to use it.Following the instructional video, participants were asked to engage in a 10-minute free-form painting using InkBrush.They were given complete freedom in terms of the content of their painting.After completing the task, each participant was given a questionnaire that included questions related to the InkBrush features they used to complete their task, as well as the usability of InkBrush, assessed by the System Usability Scale (SUS).
The second phase of the study aimed to gain a deeper understanding of the user experience of InkBrush through feedback from a smaller group of participants with expertise in ink painting and/or 3D design.During this phase, participants were asked to complete another ink painting task on a topic of their choice, but without any time constraints.This allowed them to freely explore and express their creativity using InkBrush.Upon completion of the task, we conducted one-on-one interviews with each participant.These interviews were designed to gather their feedback, opinions, and insights regarding the various features, functionalities, and aspects of InkBrush.The aim was to understand how these elements either facilitated or hindered their creative process.Additionally, participants were asked to complete a survey using the Creativity Support Index (CSI) [7] to gauge the degree to which they perceived creative support while using InkBrush.
In addition to gathering insights from the users' perspective, we were also interested in evaluating the aesthetic appeal of the artworks produced using InkBrush.To achieve this, a panel of four experts was recruited to act as a jury.This panel consisted of two experts specializing in ink painting and two specializing in 3D design, with the latter also possessing experience in ink painting.Their role was to assign scores to the artworks produced by the eight participants.The criteria for evaluation were based on the aesthetic appeal of the paintings, including elements such as Visual Impact, Creative Expression, Use of Color, Harmony, and Composition and Layout, as well as their fdelity to traditional principles, such as Brushwork, Ink Color, and Charm of Ink.

Participants
In the frst phase of our study, a total of 75 participants were recruited, consisting of 43 males and 32 females.Among these participants, 50 reported being familiar with 3D design, while 60 had previous experience in ink painting.For the second phase of our study, a subset of eight participants voluntarily agreed to continue their involvement.This subgroup included four males and four females, all exhibiting expertise in ink painting and/or 3D design, including modeling and texturing.Specifcally, three participants reported a strong profciency in 3D design, while four participants reported a basic skill level in ink painting.Moreover, one participant reported an advanced skill level in ink painting.

Results
The study results are reported based on data collected from all 75 participants in Phase 1 and 8 participants in Phase 2.

Phase 1 results.
Our study fndings revealed that InkBrush achieved an average SUS score of 79.3 (SD = 6.9).Specifcally, the usability aspect of InkBrush received an average score of 80.2 (SD = 7.9), while the learnability aspect received an average score of 75.7 (SD = 14.2).To interpret these scores, we referred to the Curved Grading Scale of SUS [34] and the established correlation between SUS scores and adjective scales [4].According to these guidelines, the overall grade for the usability of InkBrush is an A-, indicating a high level of usability.Additionally, the learnability of InkBrush receives a B grade, suggesting that it is relatively easy to learn.Participants with prior experience in using 3D design software can easily grasp the tutorial content and initiate ink painting within 2-3 minutes.On the other hand, even for novice users without any prior knowledge of 3D design, following the tutorial once is typically adequate for them to engage in the creation of ink paintings independently.
Aside from the SUS scores, we also sought to understand the common InkBrush features used by participants.We found that participants employed a wide range of tools provided by InkBrush to accomplish various objectives related to shaping stroke forms, ink textures, and drips.Within the three primary tool categories, namely Shape Adjustment, Texture Attributes, and Ink Dots and Drips, we found that the tools for adjusting ink texture were used most frequently, as reported by a majority of participants (53).These tools allowed participants to modify ink attributes such as darkness, trailing, or color.The Shape Adjustment tools were also well-received, cited by 45 participants in diferent instances to facilitate adjustments to the length, radius, or tilt of their ink stroke.Surprisingly, the Ink Dots and Drips tool was also widely used, with 30 participants indicating their usage for refning the fner details of their ink strokes, a need that is expected only when all the other main details of the work are satisfying.This diverse usage of tools is promising, indicating the versatility of InkBrush and its user-friendly interface for new users.8.3.2Phase 2 results.On average, our participants spent 70 minutes completing their task.Figure 12 shows the ink paintings created by the participants.From a stylistic perspective, the works produced by P2, P4, and P5 exhibit a loose painting style, presenting common subjects found in traditional ink paintings such as fruits, mountains, and orchids.Interestingly, P1, P3, P6, P7, and P8 have chosen more innovative and unconventional topics that are less frequently explored in traditional ink paintings, including cranes, calligraphy, dragons, and fowing hair.It is worth noting that even participants lacking prior experience in 3D design (P5, P6, P7) were able to generate high-quality 3D ink paintings quickly.Moreover, those with experience in both 3D design and ink painting (P1, P2, P3, P4, P8) found our tool to be a valuable asset, signifcantly enhancing their efciency in completing their artwork.
Nearly all participants expressed satisfaction with InkBrush as a drawing tool designed for 3D ink painting.The InkBrush was found to be efective in achieving a wide range of brushstroke efects commonly used in ink painting, including the creation of feathers, hair, and leaves.The participants emphasized the absence of any other existing tool capable of achieving the same level of artistic expression in 3D ink painting."The 3D ink dots produced by InkBrush exhibited a natural appearance from various viewing angles, resulting in a truly amazing ink efect in three dimensions."(P5).Another participant (P8), who had extensive experience in ink painting since childhood, commented on the transformative nature of InkBrush, stating that "prior to discovering this tool, I believed ink painting was limited to two-dimensional mediums.However, the seamless integration of my ink strokes into 3D artworks astonished me.", leading them to describe the experience as truly amazing.2: Expert scores on the aesthetic appeal and fdelity to traditional principles of the 3D ink paintings created by our participants.The scores were assigned on a scale of 0 to 5.
Interestingly, participants P6 and P7 found pleasure in utilizing the InkBrush to write calligraphy characters for graphic design purposes.This unexpected application of the InkBrush showcases its versatility and expands its potential use in various scenarios.
Aesthetic appeal.In terms of the aesthetic appeal of the 3D ink paintings, Table 2 presents the average scores assigned to each painting across various dimensions.Overall, the jury expressed high praise for the 3D ink paintings produced by the participants, with the majority receiving commendable scores.Notably, the works by P2 and P3 garnered the highest scores, with the jury highlighting the participants' adept handling of "Liu Bai" (the deliberate use of negative space, allowing the unpainted areas to hold as much signifcance as the painted ones).(P3) and the level of completeness (P2), both crucial aspects in the realm of ink painting.Additionally, P5's creation gained favor among several jury members due to the choice of subject matter and the good use of color.It is noteworthy that all the 3D ink paintings obtained relatively high scores in the brushstroke dimensions (the degree of similarity to the brushstrokes in traditional ink painting), highlighting the efectiveness of InkBrush in rendering realistic brushstrokes, thereby meeting the experts' expectations for ink painting.
Creativity support.Concerning creativity support, the average CSI score for InkBrush was 84.96 (SD = 12.86) out of 100.This score indicates that InkBrush provides efective support for participants to engage in their creative process of ink painting.The detailed scores for each item reported by every participant are presented in Table 3.Note that due to the study's design not allowing for collaboration, participants uniformly ranked Collaboration the lowest, thus receiving zero scores across the board.Our data shows that the participants scored higher in dimensions such as Exploration, Expressive, and Immersion, suggesting that InkBrush efectively encourages them to explore various output efects and facilitates the expression of their ideas.
InkBrush vs Texturing.Given several participants' prior experience in producing 3D ink painting using 3D modeling and texturing, we also sought to gather their initial perspectives regarding their experiences using InkBrush compared to the conventional method of texturing 2D painting onto 3D models.As discussed in the literature review, the latter is the method widely used in the industry for generating 3D ink paintings.Our fndings suggest that InkBrush ofers several advantages over existing tools.Unlike the existing 3D modeling tools, which necessitate manual modeling, premeditated planning, and the creation of texture maps in advance, the InkBrush functions as a genuine painting tool that facilitates creativity during painting (P1, P4, and P8)."The advantage of using InkBrush is the freedom it provides.It is a real drawing tool.In contrast, incorporating 2D paintings as textures onto 3D models requires too much efort because I need to adjust the texture carefully to accurately match the complicated geometry of the 3D models in order to achieve the right outcome." (P8).
InkBrush also ofers users a more efcient means of creating ink paintings.In comparison, existing methods require users to draw paintings using 2D graphic editing tools such as Photoshop.Subsequently, they have to import the resulting textures into 3D editing software to view the fnal result.This process often involves multiple iterations and can be time-consuming.However, such a cumbersome process becomes unnecessary when utilizing InkBrush.Moreover, InkBrush provides users with the tools to adjust the attributes of their strokes.This feature signifcantly reduces the need for repeated iterations, as users can easily modify and fne-tune their artwork in real-time (P2, P3, P4, and P7).
Improvements.Nonetheless, there are areas where InkBrush could be improved.One notable issue mentioned by participants is the difculty in controlling the angle and direction of the brush in a 3D space.This challenge is especially pronounced for beginners who may struggle to manipulate the brush efectively.They fnd it cumbersome to accurately draw thin objects like hair in a single stroke due to the limited control over the angle at the start and end of the brush stroke (P1, P3, P5).Furthermore, our current implementation does not fully facilitate the quick drawing of objects that are primarily composed of spherical or square shapes, such as a gourd.These objects can be more efciently constructed using the traditional method of 3D modeling rather than relying on multiple strokes.A potential solution to improve our system involves integrating a sketch modeling approach similar to Simpmodeling [27] into our system.Additionally, our current implementation falls short in meeting the needs of expert users.For example, some ink painting artists noted that when subjects require precise stroke movements and sudden changes, such as mountains and terrains, the tool still lacks the ability to accurately replicate the brushstroke dynamics inherent in traditional ink paintings.Moreover, several artists have provided feedback suggesting that certain aspects of the ink efect could greatly beneft from improvements.Specifcally, they have highlighted the need for better expression of changes in haloing, density, and dryness of the ink across a stroke (P3, P4, P5, P8).As   3: The CSI scores calculated based on the data of the eight participants involved in the second phase of the study.These scores were calculated on a scale of 0 to 100.The CSI score was calculated by dividing the total score of the six dimensions by 3 [7].
mentioned by P5, there is a noticeable ink transition at the end of the brush, but the absence of a halo efect on the side gives it an unnatural appearance.Furthermore, the ink transition itself is not perceived as being sufciently natural.
Our implementation of the stroke model consists of two fat bands positioned at a 90-degree angle to each other.This confguration simplifes the complexity of the model, but it results in a lack of natural appearance at the intersection of the two bands, particularly when the stroke is viewed at a zoomed-in level.This issue arises due to the misalignment of the ink texture at the intersection edges (potential solutions discussed in the Fugure work).Additionally, in real-world ink painting, artists use various practical techniques to alter the ink profle, such as adjusting the darkness and intensity of the ink color, as well as creating a halo efect on the edges of brushstrokes.These efects are achieved by adding additional ink on top of an already wet ink surface.However, in our current implementation of InkBrush, strokes are treated as independent entities, lacking the ability to interact with one another.Based on the feedback from our participants (P4, P5, P8), it is evident that future iterations of InkBrush should incorporate these features to facilitate the use of these painting techniques.Additionally, participants also expressed the need for more editable predefned attributes, such as the material and texture of the rice paper (P1, P4, P7).
Dissemination.The online release of InkBrush has gained significant attention and positive feedback from users and artists worldwide, including those within and outside of China.This favorable reception has been ongoing for several months since the initial release.In addition to the positive reception, users have been exceptionally generous in sharing their artwork created using InkBrush.A small random selection of them is showcased in Figure 13.

LIMITATION AND FUTURE WORK
In this section, we discuss the insights obtained from our research and acknowledge the limitations of our current approach.We will also propose potential directions for future research to further build upon our work and address the gaps identifed in our study.
Study.The current study serves as an initial exploration into the efectiveness and usability of InkBrush.This preliminary investigation lays the groundwork for further research to thoroughly investigate the capabilities of InkBrush.An immediate next step involves conducting a study similar to the second phase of the current experiment with a larger and more diverse pool of participants.This will not only serve to validate our current fndings but also promise to yield deeper insights into various aspects of InkBrush.With such a study, we also aim to uncover additional innovative usage scenarios.Specifcally, we intend to explore the potential of InkBrush to facilitate collaborative 3D ink painting processes.Furthermore, we plan to investigate the learning processes and outcomes of both novices and expert users of InkBrush, providing insight into the development of 3D ink painting skills across varying expertise levels.
Our qualitative evaluation of the visual appeal of the ink strokes generated by InkBrush was based on human judgment.In our next step, a more quantitative approach will be adopted to objectively gauge the adherence of the generated ink strokes to traditional ink painting principles and aesthetics, potentially using metrics such as stroke geometry and color.Additionally, we plan to expand our initial comparison of InkBrush and the standard texturing method by incorporating a more controlled environment with a diverse range of real-world scenarios.This comparative analysis will enable a better understanding of the advantages and limitations of our freehand drawing approach.Moreover, we plan to conduct longitudinal studies to validate our initial fndings and gain insights into evolving behavioral patterns, learning outcomes, and the development of workfows over time.Such information can guide future improvement to InkBrush.
Ink stroke efects.The current implementation of our ink brush presents several areas for potential improvement.One notable limitation is the lack of a natural appearance of ink strokes at the intersection of the brush's fat bands.As part of our ongoing research eforts, we are directing our focus towards exploring two potential solutions to address this issue.Firstly, we can aim to develop an improved mapping algorithm that facilitates automatic alignment of the ink texture at the intersection.Alternatively, another solution is to employ a blurring technique in the region of the texture near the intersection.By applying a controlled blur efect, we can mitigate the visibility of the misalignment and create a smoother transition between the two bands.This would help to alleviate the unnatural appearance observed when zoomed in on the stroke.
Furthermore, there is room for improvement in the holo efect of our ink strokes.Feedback from our study participants suggests that while our current implementation of the ink brush is efective for most usage scenarios, particularly those involving thick ink painting, it proves to be relatively challenging to achieve the desired efect of light ink spreading out on rice paper.In order to address this issue, we plan to improve the implementation of the holo efect in our tool.
Expert mode.Participants who possess extensive knowledge in both ink painting and 3D modeling have ofered us valuable perspectives on how to enhance the expert mode of our system.They suggested that there are certain situations where it would be benefcial for our implementation to permit the conversion of an existing 3D model into 3D strokes.Although our current surface painting mode partially addresses this requirement, the manual approach was inefective and lacks the desired level of precision.
Immersive 3D experience.Drawing in a 3D space poses a challenge for users as they often struggle to locate accurately where they are drawing via a 2D screen.This difculty arises primarily due to the inherent limitation of perceiving depth on a fat screen.While various approaches have been proposed to enhance the experience of drawing in a 3D space through a 2D interface, the problem remains unresolved.One potential solution to this issue is Virtual Reality VR.However, some participants who have experimented with VR painting have reported that drawing in VR is less efcient and lacks the same level of control fexibility as using a mouse.To overcome these challenges, our future research will explore ways to integrate InkBrush into the VR environment and provide a more immersive 3D ink painting experience to our users.

CONCLUSION
Our research addresses the signifcant challenges involved in the creation of visually appealing 3D ink paintings.The demand for such artwork in industries such as gaming, animation, and virtual reality is rapidly increasing, thus making InkBrush a valuable addition to the ink painting community.The unique contribution of InkBrush lies in its pioneering approach to 3D ink painting using freehand drawing.This sets it apart from current alternatives, which typically require substantial efort in 3D modeling and texturing or the conversion of 2D ink paintings into a 3D format.InkBrush aims to empower users by ofering a powerful digital calligraphy brush and editing tools that replicate the subtle characteristics of traditional ink strokes.Through a user study involving artists and 3D designers, we provide initial insights that demonstrate InkBrush's alignment with the needs and expectations of its target users.This validation serves to further afrm its potential as a useful tool for the ink painting community and beyond.By seamlessly integrating with Blender, InkBrush has the potential to foster a vibrant community of artists who will continue to push the boundaries of 3D ink painting.Looking ahead, we anticipate that InkBrush will enable artists to explore the intersection of traditional ink painting aesthetics and the emerging VR environment.This will open up exciting possibilities for traditional artistic expression in the new era of mixed reality.

Figure 2 :
Figure 2: A walkthrough of how a 3D ink painting can be created using our tool.(a) The branch of the morning glory, created using a single stroke; (b) The 3D model of the branch comprises two fat bands arranged perpendicular to each other (colored diferently for clarity) and several supplementary bands and square ink-dot meshes positioned around them; (c) Anchor points of the stroke; (d) The second branch is drawn on a plane 30 degrees from that of the frst branch; (e) The parameter panel contains adjustable settings for the ink stroke; (f) One of the two fat bands of the stroke model is removed; (g) The color and moisture level of the ink can be adjusted to achieve the desired appearance of a petal; (h) The petal is rotated to the desired orientation; (i) The completed fower of the morning glory; (j) The shape of a branch was adjusted in the 3D space to make it look like the branch wraps around the other branch.

Figure 3 :
Figure 3: The illustration of a horse's tail created using Tilt Brush and InkBrush.(a) The original target image served as the reference for the artistic rendition.(b) The outcome and 3D model produced using Tilt Brush, involving a multitude of brush strokes.Each stroke is manually drawn, requiring extensive labor to emulate the aesthetic of ink painting.(c) The outcome and 3D model created using InkBrush involved four brush strokes.

Figure 4 :
Figure 4: A rooster painted in three diferent painting techniques: (a) ink painting, (b) oil painting, and (c) watercolor painting.

Figure 5 :
Figure 5: An illustration of varying unique patterns used in ink painting.

Figure 7 :
Figure 7: An illustration of the efect of the Radius tool and Tilt tool.

Figure 8 :
Figure 8: An illustration of six diferent texture attributes that can be adjusted to create various efects.

Figure 10 :
Figure 10: The 3D model of an ink stroke.(a) The bare bone of the stroke model; (b) The stroke model with supplementary bands; (c) The stroke model with ink texture; (d) The same stroke viewed from a diferent angle.

Figure 13 :
Figure 13: A random selection of 3D ink paintings created by InkBrush users worldwide (viewed from diferent angles).