Abstract:
There are provided a unified framework based on extensible styles for 3D non-photorealistic rendering and a method of configuring the framework. The unified framework includes: 3D model data processing means for generating a scene graph by converting a 3D model input into 3D data and organizing the scene graph using vertexes, faces, and edges; face painting means for selecting a brusher to paint faces (interiors) of the 3D model using the scene graph; line drawing means for extracting line information from the 3D model using the scene graph and managing the extracted line information; style expressing means for generating a rendering style for the 3D model and storing the rendering style as a stroke, the rendering style being equally applied to a face-painting method and a line-drawing method; and rendering means for combining the stroke and the selected brusher to render the 3D model using both the face-painting method and the line-drawing method. The framework can be used to develop tools and new rendering styles for non-photorealistic rendering and animation.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a framework for 3D non-photorealistic rendering and a method of configuring the framework, and more particularly, to a framework that renders a 3D model in a general and intuitive manner like the way an artist selects a style and paints a model with the selected style and has a flexible style expandability for efficient non-photorealistic rendering by using a unified concept of expressing various rendering styles using just two rendering methods (face painting and line drawing), and a method of configuring the framework.  
         [0003]     2. Description of the Related Art  
         [0004]     With the increasing interest in non-photorealistic rendering and animation, a number of researches have been conducted to generate images in an artistic style that look like a painting or drawing. Early researches are focused on developing non-photorealistic rendering and animation systems that are individually dedicated to expressing a particular rendering style. Such systems use a non-photorealistic rendering pipeline that is obtained by slightly modifying a traditional rendering pipeline.  
         [0005]     Recently, unified frameworks have been researched to establish such an individual non-photorealistic rendering pipeline and provide a non-photorealistic rendering that is different from the existing photorealistic rendering. In a framework for non-photorealistic rendering based on a particle (1996), a particle is used to determine the position of a stroke (denoting a brush touch). The particle-based framework is the earliest framework that is still in use. Further, examples of recently proposed frameworks include a framework that expresses a 3D model using an adaptively sampled distance field (ADF) (an article titled “A New Framework for Non-photorealistic rendering”, 2001) and a framework based on a graphic processing unit (GPU). However, in the above-described frameworks, main concerns of non-photorealistic rendering systems (i.e., processing techniques for expressing styles) are not mentioned.  
         [0006]     A framework somewhat different from the above-described frameworks is disclosed in “OpenNPAR (2003, an article titled ‘OpenNPAR: A system for developing, programming, and designing non-photorealistic animation and rendering’).” This OpenNPAR system is an OpenInventor-based framework, in which each rendering style for expressing a non-photorealistic effect is matched with a combination of components and a desired style is generated by recombining the components. Although the OpenNPAR system modularizes detail non-photorealistic rendering styles (detail functions) and expresses a number of styles by recombining the modularized styles, the OpenNPAR system has a complicated basic concept in intuitive and non-photorealistic expressing. Further, the OpenNPAR system has a complicated basic pipeline that supports a developer (develops modules), a programmer (produces modifiers using the modules), a designer (generates styles using the modifiers), and a user.  
       SUMMARY OF THE INVENTION  
       [0007]     Accordingly, the present invention is directed to a unified framework based on extensible styles for 3D non-photorealistic rendering and a method of configuring the framework, which substantially obviates one or more problems due to limitations and disadvantages of the related art.  
         [0008]     It is an object of the present invention to provide a unified framework based on extensible styles for 3D non-photorealistic rendering and a method of configuring the framework. The framework has an intuitive structure such that rendering styles can be conveniently expanded for both face-painting and line-drawing methods.  
         [0009]     It is another object of the present invention to provide a rendering system employing a unified framework based on extensible styles for 3D non-photorealistic rendering, the unified framework unifying all rendering methods using face-painting and line-drawing methods such that system developers can easily provide styles satisfying designers&#39; demand.  
         [0010]     Therefore, the rendering styles generated by the unified framework of the present invention can be intuitively used for rendering a 3D model without distinguishing the interior, outside, and lines of the 3D model.  
         [0011]     Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.  
         [0012]     To achieve these objects and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, there is provided a unified framework based on extensible styles for 3D non-photorealistic rendering, the unified framework including: 3D model data processing means for generating a scene graph by converting a 3D model input into 3D data and organizing the scene graph using vertexes, faces, and edges; face painting means for selecting a brusher to paint faces (interiors) of the 3D model using the scene graph; line drawing means for extracting line information from the 3D model using the scene graph and managing the extracted line information; style expressing means for generating a rendering style for the 3D model and storing the rendering style as a stroke, the rendering style being equally applied to a face-painting method and a line-drawing method; and rendering means for combining the stroke and the selected brusher to render the 3D model using both the face-painting method and the line-drawing method.  
         [0013]     In another aspect of the present invention, there is provided a method of configuring a unified framework based on extensible styles for 3D non-photorealistic rendering. The method configures the framework to render a 3D model in a general and intuitive manner like an artist selects a style and paints a model with the selected style, and to apply various rendering styles to face-painting and line-drawing such that all 3D models can be rendered only by face-painting and line-drawing regardless of rendering styles for the 3D models.  
         [0014]     In a further another aspect of the present invention, there is provided a method of configuring a unified framework based on extensible styles for 3D non-photorealistic rendering, the method including the step of configuring the unified framework to generate a rendering style for a 3D model and simultaneously apply the style by combining the style with a face-painting brusher and a line-drawing brusher, wherein the step of configuring the unified framework includes: parsing 3D model data of the 3D model according to a predetermined format to generate a scene graph; organizing the 3D model data based on vertexes, faces, and edges using the scene graph; selecting a brusher for painting an interior of the 3D model using the scene graph; extracting line characteristic information including silhouette, crease, and boundary line information by using the scene graph; generating interior lines (hatch lines) using the scene graph for the interior of the 3D model; and generating a rendering style for being used to render the 3D model finally.  
         [0015]     It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     The accompanying drawings, which are included to provide a further understanding of the invention, are incorporated in and constitute a part of this application, illustrate embodiments of the invention and together with the description serve to explain the principle of the invention. In the drawings:  
         [0017]      FIG. 1  is a schematic view illustrating a framework for non-photorealistic rendering according to the present invention;  
         [0018]      FIG. 2  is a block diagram illustrating a framework for non-photorealistic rendering according to an embodiment of the present invention;  
         [0019]      FIG. 3  is a schematic view illustrating a set of a style and a brusher used in a non-photorealistic rendering framework as a basic component for style expression according to the present invention;  
         [0020]      FIG. 4  is a view for explaining intermediate outputs of operation steps of a non-photorealistic rendering framework according to the present invention; and  
         [0021]      FIG. 5  shows examples of rendering styles developed based on a non-photorealistic rendering framework according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]     A unified framework based on extensible styles for 3D non-photorealistic rendering will now be described in detail with reference to the accompanying drawings according to preferred embodiments of the present invention.  
         [0023]      FIG. 1  is a schematic view illustrating a framework for generating a non-photorealistically rendered image according to the present invention. Referring to  FIG. 1 , the framework of the present invention is configured based on the way a person draws a picture.  
         [0024]     In the framework of the present invention, non-photorealistic images are rendered by imitating the way an artist paints a picture and based on the concept that all non-photorealistic images can be rendered through interior area (face) painting and line drawing.  
         [0025]     The framework can be easily expanded and modified for non-photorealistically rendering a 3D model using various styles. For this, the framework is configured such that rendering is performed using two methods (face painting and line drawing) for a 3D model selected for “what to paint?” and a style selected for “how to paint” is defined as a pair of a stroke and a brusher (rendering tool).  
         [0026]      FIG. 2  is a block diagram illustrating a framework for non-photorealistic rendering according to an embodiment of the present invention. Each part of the non-photorealistic rendering framework will now be described with reference to  FIG. 2  according to the embodiment of the present invention.  
         [0027]     The non-photorealistic rendering framework includes a 3D model data manager  100 , a style manager  200 , a face painting manager  300 , a line drawing manager  400 , a characteristic line extracting manager  500 , an interior line extracting manager  600 , a motion line extracting manager  700 , a face painting brusher group  800 , a line drawing brusher group  900 , and a state manager  1000 .  
         [0028]     In detail, the 3D model data manager  100  is a processor that processes and manages all 3D data used in the non-photorealistic rendering framework. The 3D model data manager  100  includes a 3D model data parsing module  110  that reads 3D model data and generates a scene graph in a predetermined format, and a 3D model data organizing module  120  that expresses the generated scene graph using vertexes, faces, and edges.  
         [0029]     The style manager  200  is a processor that produces and manages non-photorealistic rendering styles used in the non-photorealistic rendering framework. The style manager  200  includes a style generating module  210  providing common functions for generating various styles, and a detail style generating module  220  for expressing detail features of each style generated by the style generating module  210 .  
         [0030]     The face painting manager  300  is a processor for painting faces (interior areas) of a 3D model used in the non-photorealistic framework according to a rendering style generated by the style manager  200 . The face painting manager  300  includes a face painting control module  310  for receiving controls from a user.  
         [0031]     The line drawing manager  400  is a processor for drawing lines of a 3D model used in the non-photorealistic framework according to a rendering style generated by the style manager  200 . The line drawing manager  400  includes a line drawing control module  410  for receiving controls from a user.  
         [0032]     The characteristic line extracting manager  500  is a process that extracts characteristic lines from a 3D model used in the non-photorealistic rendering framework and manages the extracted characteristic lines. The characteristic line extracting manager  500  includes a characteristic line extracting group  510  that provides upper functions commonly used for extracting characteristic lines, a characteristic line extracting module  520  that provides a common interface based on each extracted characteristic line, and a detail characteristic line extracting module  530  that extracts characteristic lines according to detail categories.  
         [0033]     The interior line extracting manager  600  is a process that generates interior lines (hatch lines) of a 3D model used in the non-photorealistic rendering framework. The interior line extracting manager  600  includes an interior line extracting group  610  that provides upper functions commonly used for generating interior lines, an interior line extracting module  620  providing a common interface based on each generated interior line, a detail interior line extracting module  630  extracting interior lines according to detail categories, and a particle placer module  640  determining start positions when interior lines are generated.  
         [0034]     The motion line extracting manager  700  is a process that generates motion lines of a 3D model used in the non-photorealistic rendering framework. The motion line extracting manager  700  includes a motion line extracting group  710  that provides upper functions commonly used for generating motion lines, a motion line extracting module  720  providing a common interface based on each generated motion line, and a detail motion line extracting module  730  extracting motion lines according to detail categories.  
         [0035]     The characteristic line extracting manger  500 , the interior line extracting manager  600 , and the motion line extracting manager  700 , which are provided for generating three kinds of lines, have the same configuration. Thus, the framework can be easily extended or modified according to the size of a developed system.  
         [0036]     The face painting brusher group  800  is a processor for painting the inside of a 3D model used in the non-photorealistic rendering framework. The face painting brusher group  800  includes a brusher group  810  providing a face painting brusher as a painting tool and a brusher module  820  providing a detail face painting function.  
         [0037]     The line drawing brusher group  900  is a processor for drawing lines of a 3D model used in the non-photorealistic rendering framework. The line drawing brusher group  900  includes a line drawing brusher module  910  providing a line drawing brusher as a line drawing tool and a detail line drawing brusher module  920  providing a detail line drawing function. The state manager  1000  stores and manages states of all processors of the non-photorealistic framework and states of a system developed based on the non-photorealistic framework.  
         [0038]     A face painting render  20  and a line drawing render  30  perform rendering functions (face painting and line drawing) using data output from the above-described processors  100  to  900  based on states stored in the state manager  1000 . Processing operations of the processors  100  to  900  are performed for each frame of a rendering model by a non-photorealistic render  10 .  
         [0039]     The structures and functions of the processors  100  to  900  of the non-photorealistic rendering framework shown in  FIG. 2  will now be more fully described with reference to  FIG. 4  that shows intermediate outputs of the processors  100  to  900 .  
         [0040]     The 3D model data manager  100  performs various functions for processing 3D data using a scene graph generated by the 3D model data parsing module  110 . The 3D model data organizing module  120  performs the most importance function in the non-photorealistic rendering process using the scene graph. That is, the 3D model data organizing module  120  calculates boundary information for vertexes and faces. Therefore, in the non-photorealistic rendering framework of the present invention, all physical data are stored based on vertexes, and in other structures, pointer information of the vertexes are used. Thus, costs for storing and processing data can be minimized.  
         [0041]     That is, in the framework of the present invention, an Xparser is provided as a basic module for parsing directX data, and optimization is performed by searching and removing overlapped vertex information to improve the efficiency of vertex-based calculation since the directX data are stored in the form of a connected structure of overlapped vertexes. Here, a current vertex is compared with a vertex located after the current vertex in the data structure (one-on-one comparison) to determine whether it is overlapped. Therefore, the time for this optimization depends on the number of vertexes of an object.  
         [0042]     The style manager  200  manages color and test information and allows the face painting manager  300  and the line drawing manager  400  to access a stroke array  230  in which various styles are stored for expressing a 3D model using various styles. The style generating module  210  performs common functions for generating styles using information expressed using 3D model data such as vertexes, faces, and edges by the 3D model data manager  100 , and the detail style generating module  220  determines detail features of the generated styles. That is, the style generating module  210  provides a common interface for the detail style generating module  220 , so that a user (or a developer) can easily expand the style.  
         [0043]     The face painting manager  300  and the line drawing manager  400  perform uppermost managing functions for painting faces and drawing lines, respectively. That is, the face painting manager  300  and the line drawing manager  400  manage rendering works of the render processors  10 ,  20 , and  30 .  
         [0044]     The characteristic line extracting manager  500  is a processor including modules that extracts characteristic lines and performs management functions. That is, the characteristic line extracting manager  500  extracts characteristic lines collectively called contour. When the characteristic line extracting group  510  extracts characteristic lines, the characteristic line extracting module  520  provides a common interface according to each characteristic line. The detail characteristic line extracting module  530  extracts detail characteristic lines for the extracted characteristic lines through the common interface according to detail categories, thereby extracting characteristic lines called contour.  
         [0045]     That is, the framework of the present invention provides basic characteristic lines: silhouette, crease, boundary, and suggestive contour lines. Typical tools available in the market provide functions for extracting silhouette, crease, and boundary lines. The suggestive contour line extracting scheme has been recently developed. The framework of the present invention is configured such that a user (or a developer) can easily apply an additional structure to the framework for extracting characteristic lines of new types.  
         [0046]     When the characteristic line extracting manager  500  extracts a 3D contour from an object, the interior line extracting manager  600  generates interior lines for expressing the faces (defined between contour lines) of the object. The interior lines are defined using start points and directions. The start points of the interior lines are defined by the particle placer module  640 . Additionally, the start points of the interior lines can be manually defined by an end user through the line drawing control module  410  in a device configured based on the framework of the present invention. The interior lines are defined from the start points along directional fields generated by the 3D model data manager  100 . The interior lines may be straight or curved depending on the direction fields. Like the characteristic line extracting manager  500 , the interior line extracting manager  600  is configured such that a user (or a developer) can easily apply an additional structure for generating interior lines having a different type.  
         [0047]     The motion line extracting manager  700  generates motion lines for each frame of a rendering image according to motions obtained by animating the rendering image regardless of geometrical information of a 3D model. Like in the interior line extracting manager  600 , initial positions of the motion lines can be determined by receiving inputs from a used through the line drawing control module  410 .  
         [0048]     The interior painting brusher group  800  and the line drawing brusher group  900  are tools actually performing face painting and line drawing. Each brusher is coupled with a stroke expressing a style for drawing an image according to a predetermined style. That is, a stroke expressing a style generated by the style manager  200  is coupled to each brusher selected from the face painting brusher group  800  and the line drawing brusher group  900 , such that the number of styles can be N*M when the used numbers of strokes and brushers are N and M.  
         [0049]     Although the brushers of the framework of the present invention are classified into the face painting brusher group  800  and the line drawing brusher group  900 , common brushers can be used for face painting and line drawing. The brushers include a 3D polygon brusher, a 2D projection brusher, a 2D brusher, a color brusher, and a texture brusher in order to express a given style.  
         [0050]     Since the framework of the present invention is configured based on face painting and line drawing, the state manager  1000  manages the overall operation of the framework and the face painting and line drawing operations of the framework. The framework of the present invention has a very intuitive and easily understandable structure, such that the state of the framework can be simply performed.  
         [0051]     The non-photorealistic render  10  calls the face painting render  20  and the line drawing render  30  and performs line drawing and face painting by using data generated or extracted by the processors of the framework.  
         [0052]      FIG. 3  is a schematic view illustrating a set of a style and a brusher used in a non-photorealistic rendering framework as a basic component for style expression according to the present invention. That is, the brusher is coupled with a predetermined stroke expressing a particular style, and then it is used as a tool for painting or drawing faces and lines in an intuitive manner.  
         [0053]      FIG. 5  shows exemplarily images rendered using various styles generated by a non-photorealistic rendering system developed based on the framework of the present invention.  
         [0054]     As described above, according to the unified framework base on extensible styles for 3D non-photorealistic rendering, functions for generating various non-photorealistic rendering styles are included in the same structure (the style manager  200 ) to use the functions in the same way, and an intuitive painting tool (refer to  FIG. 3 ) for expressing a particular style is made by replacing a stroke coupled to a brusher with another stroke expressing the particular style, such that each required function can be effectively provided for non-photorealistic rendering. Further, every non-photorealistic rendering style is expressed by the unified concept of face painting and line drawing, so that easily understandable framework can be provided. As a result, tools and styles can be easily developed for non-photorealistic rendering and animation.  
         [0055]     It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.