Patent Publication Number: US-RE49149-E

Title: Method for detecting two dimensional sketch data from source model data for three dimensional reverse modeling

Description:
RELATED APPLICATION 
     This is a reissue application of U.S. Pat. No. 7,830,376, filed Nov. 10, 2006, issued Nov. 9, 2010, and which claims the benefit of priority to Korean Patent Application No. 10-2006-0040659, filed May 4, 2006. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to detection of two dimensional sketch data for three dimensional modeling, and more particularly to a method for detecting two dimensional sketch data for three dimensional modeling, which can produce three dimensional model data using two dimensional sketch data. 
     2. Description of the Prior Art 
     As is generally known in the art, in operation of a digital production system utilizing CAD/CAM/CAE, a digital model such as a three dimensional CAD model of a part is necessarily required. Specifically, representative fields in which reproduction work using reverse engineering is utilized typically includes a hand-made prototype, an old article, a competitor&#39;s product, human body scanning, and so on. 
     In order to cooperatively perform the reproduction work process with an existing work process, the reproduced model must be supported so as to be reutilized in a CAD system and must ensure product quality to allow mass production. 
     However, it is not easy to acquire digital data which can be utilized in an actual production site and can ensure product quality. 
     Also, the data acquired by actually scanning a product or a part have an error to some extent when compared to those of a CAD model designed through a production process. Moreover, surrounding circumstances or an error rate of a scanner itself may change data during a scanning operation. 
     For these reasons, it is very difficult to create a solid model from scanned data, such as that created by CAD, and in some cases, it may be impossible to create a solid model from scanned data. 
     Further, in the case that operational processes, in which a prototype model is scanned, simulated on a computer, physically processed and manually changed, are repeated, the precise shape information of a digital model may be damaged or distorted in the course of conducting modification work, and a difference is caused between a designed CAD model and the actual prototype model. 
     Furthermore, since the model data detected from the scanned data or by a method such as surface-fitting of a model, etc. are not parametrically connected with one another, when a user inputs new numerical information or corrects erroneous model data, only the corresponding specific portion of the data is changed or corrected, and other portions connected to the specific portion are not changed or corrected. 
     In addition, because the method such as surface-fitting, etc. detects only the outer surface of a model, it is difficult to acquire precise shape information. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention has been made to solve the above-mentioned problems occurring in the prior art, and an object of the present invention is to provide a method for detecting two dimensional sketch data from source model data for three dimensional reverse modeling, which detects, from optional model data, feature regions of a two dimensional section for three dimensional modeling, and causes sectional shapes and numerical information for the detected feature regions of the two dimensional section to be parametrically connected and displayed. 
     In order to achieve the above object, according to one aspect of the present invention, there is provided a method for detecting two dimensional sketch data from source model data for three dimensional reverse modeling, including the steps of a) detecting optional model data, establishing X-axis, Y-axis and Z-axis of the model data depending upon a reference coordinate system information inputted from a user, and setting a work plane for detecting two dimensional section data of the model data; b) projecting, on the work plane, two dimensional section data to be detected from the model data or polylines detected by designating a detection position; c) detecting two dimensional projected section data of the model data projected on the work plane, and dividing the two dimensional projected section data into feature segments depending upon a curvature distribution; and d) establishing a constraint and numerical information in accordance with connection of the divided feature segments of the two dimensional projected section data, and creating two dimensional sketch data. 
     According to another aspect of the present invention, the two dimensional section data of the step b) is established at a position which is vertically offset from the work plane or is determined by rotating the work plane about a measurement axis. 
     According to another aspect of the present invention, the two dimensional section data of the step b) is converted into a UV coordinate system by rotating the work plane and is then established in U and V directions thereof. 
     According to another aspect of the present invention, when the two dimensional section, data of the step b) has a draft, a measurement range having the draft is established so as to acquire a precise section from the measured model data, and a silhouette of the model data which is included in the established measurement range is projected on the work plane. 
     According to another aspect of the present invention, the measurement range corresponds to a region between two planes established by the user. 
     According to another aspect of the present invention, a value of the draft corresponds to a cosine of a triangle which is formed by a base corresponding to a distance between the two planes and a height corresponding to a difference in size between two dimensional section data respectively projected on the two planes. 
     According to another aspect of the present invention, the step b) includes the step of inserting a polyline optionally selected by the user into a feature region projected on the work plane or of overlapping a polyline detected from the model data and having a third specific shape on the feature region projected on the work plane. 
     According to another aspect of the present invention, the feature segment projected on the work plane in the step c) is one selected from the group consisting of a straight line, a circle, an arc, a free curve, a rectangle, a polygon and a slot. 
     According to another aspect of the present invention, the constraint and numerical information of the step d) are conditions which are fitted from the feature segments of the two dimensional projected section data including a straight line, a circle, an arc, a free curve, a rectangle, a polygon and a slot, or which are set by the user to satisfy at least one of verticality, horizontality, parallelism, slopeness, junction, fixing, correspondence, the same straight line, concentricity, the same radius, and the same distance between the feature segments. 
     According to another aspect of the present invention, the constraint and numerical information are automatically established in accordance with an internal tolerance between the feature segments of the two dimensional projected section data or are manually established or corrected by data inputted from the user. 
     According to another aspect of the present invention, the feature segment to be manually established or corrected is selected through snapping for automatically fixing a cursor of a mouse in the vicinity of a node of points constituting the feature segment of the two dimensional projected section data to be established or corrected. 
     According to another aspect of the present invention, the feature segment of the two dimensional projected section data is displayed by being highlighted. 
     According to another aspect of the present invention, the feature segments, constraint and numerical information of the two dimensional projected section data projected on the work plane are parametrically connected with one another. 
     According to still another aspect of the present invention, the created two dimensional sketch data is displayed as one element of a feature tree. 
     According to a still further aspect of the present invention, the model data is one selected from the group consisting of scanned data, point cloud data, mesh data, general CAD data, and CAD data having curved surfaces. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the present invention will be more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a block diagram illustrating the construction of a system for detecting two dimensional sketch data from source model data for three dimensional reverse modeling in accordance with the present invention; 
         FIG. 2  is a flow chart illustrating a method for detecting two dimensional sketch data from source model data for three dimensional reverse modeling in accordance with the present invention; 
         FIG. 3  is a perspective view illustrating optional model data in accordance with the present invention; 
         FIG. 4  is a view illustrating a process for setting a reference coordinate system for the model data shown in  FIG. 3 ; 
         FIG. 5  is a view illustrating a process for projecting a two dimensional section silhouette of the model data shown in  FIG. 3  on a work plane; 
         FIG. 6  is a view illustrating the two dimensional section silhouette projected on the work plane; 
         FIG. 7  is a view illustrating another embodiment of detecting a two dimensional section silhouette; 
         FIG. 8  is a view illustrating a process for automatically setting the two dimensional section silhouette projected on the work plane; 
         FIG. 9  is a view illustrating an embodiment for detecting a two dimensional section silhouette having a draft; 
         FIG. 10  is a view illustrating a process for setting the straight line of the two dimensional section silhouette; 
         FIG. 11  is a view illustrating a process for setting the curved line of the two dimensional section silhouette; and 
         FIG. 12  is a view illustrating a process for setting the fillet of the two dimensional section silhouette. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, a preferred embodiment of the present invention will be described with reference to the accompanying drawings. In the following description and drawings, the same reference numerals are used to designate the same or similar components, and so repetition of the description on the same or similar components will be omitted. 
       FIG. 1  is a block diagram illustrating the construction of a system for detecting two dimensional sketch data of model data for three dimensional modeling in accordance with the present invention. Referring to  FIG. 1 , the system for detecting two dimensional sketch data of model data for three dimensional modeling includes a scanner  10  for measuring a modeling object, a reverse modeling control unit  20  for controlling the entire modeling system and performing a modeling function, a display unit  30  for outputting scan data detected from the scanner  10  or model data produced by transforming the scan data to allow a user to confirm the data, a user input unit  40  for detecting setting data inputted from the user, and a data storage unit  50  for storing the three dimensional scan data or the model data and two dimensional sketch data detected from the scan data or the model data. 
     The scanner  10  is a component part for acquiring scan data through measuring the object, and preferably, includes a three dimensional scanner. 
     The model data which is the modeling object in the present invention includes point cloud data detected from the scanner  10 , the scan data, the model data produced by transforming the scan data, general CAD data, CAD data having curved surfaces, and polylines produced in a variety of ways by connecting line segments through graphics in an external device such as a computer. Hereinbelow, these data will be referred to as model data. 
     The reverse modeling control unit  20  analyzes optional model data, establishes the X-axis, Y-axis and Z-axis of the model data depending upon the reference coordinate system information inputted from the user input unit  40 , and sets a work plane for detecting the two dimensional section data of the established model data. 
     Also, the reverse modeling control unit  20  analyzes the two dimensional projected section data of the model data projected on the work plane to produce and store two dimensional sketch data, and parametrically displays the produced two dimensional sketch data on the display unit  30 . Here, the two dimensional section data of the three dimensional model data projected on the work plane is defined as two dimensional projected section data, and the data produced by fitting the two dimensional projected section data is defined as two dimensional sketch data. 
     The reverse modeling control unit  20  analyzes the distribution of the two dimensional projected section data projected on the work plane depending upon a curvature distribution, extracts the feature segments (for example, a straight line, a circle, an arc, a free curve, a rectangle, a polygon, a slot, etc.) of the two dimensional projected section data depending upon the curvature distribution of the analyzed two dimensional projected section data, and detects the coordinate system information and numerical information of the feature segments. 
     Here, the curvature means a rate of change in the direction of a curved line, which is mediated by the length of an arc. Through the curvature distribution of the two dimensional projected section data projected on the work plane, the silhouette of the two dimensional projected section data which is composed of straight lines, circles, arcs, free curves, rectangles, polygons, slots, etc. can be displayed. 
     The reverse modeling control unit  20  parametrically establishes the connection relationships of the feature segments by using the numerical information of the detected feature segments as parameters, and displays the established feature segments and numerical information on the display unit  30 . 
     The display unit  30  parametrically displays and outputs, preferably through a feature tree, the model data, the information regarding the directions of the X-axis, Y-axis and Z-axis, which is inputted from the user input unit  40 , the work planes produced by the reverse modeling control unit  20 , the two dimensional projected section data projected on the work plane, and the feature segments and numerical information of the two dimensional projected section data detected in the course of forming the two dimensional sketch data, which are produced based on the two dimensional projected section data. 
     The user input unit  40  includes input means such as a keyboard, a mouse, etc. The user input unit  40  inputs establishment information for establishing the reference coordinate system of the model data displayed on the display unit  30 , the feature segments detected from the two dimensional projected section data of the model data, correction information for correcting the numerical information of the feature segments, and so on. 
     The data storage unit  50  stores the model data, the two dimensional sketch data produced by the reverse modeling control unit  20 , etc. 
       FIG. 2  is a flow chart illustrating a method for detecting two dimensional sketch data of the model data for three dimensional modeling in accordance with the present invention. The method for detecting two dimensional sketch data of the model data for three dimensional modeling will be described below with reference to  FIGS. 1 and 2 . 
     The reverse modeling control unit  20  for producing the two dimensional sketch data of the model data detects the model data through the scanner  10  or an external device (for example, a computer system) and displays the detected model data through the display unit  30  (S 100 ). 
     That is to say, as shown in  FIG. 3 , the reverse modeling control unit  20  detects the model data  100  of the modeling object through the scanner  10  and/or the external device. In this embodiment, the modeling object includes a regular hexahedron, fillets are formed on the corners of the modeling object, and the modeling object has a draft for injection molding. 
     Referring again to  FIGS. 1 and 2 , after implementing the step S 100 , the reverse modeling control unit  20  establishes the reference coordinate system including the X-axis, Y-axis and Z-axis of the model data  100  in accordance with the coordinate system establishment information which is inputted from the user input unit  40 , to thereby establish the coordinates of the model data  100  (S 110 ). 
     That is to say, as shown in  FIG. 4 , the user divides the model data  100  including the regular hexahedron into a first region  110 , a second region  120  and a third region  130 , and establishes an X-Y plane  200 , a Y-Z plane  210  and a Z-X plane  220  as work planes for detecting two dimensional section data of the divided model data  100 . 
     At this time, in a usual way of establishing the reference coordinate system, three reference surfaces, for example, the X-Y plane  200 , the Y-Z plane  210  and the Z-X plane  220  are established, a local coordinate system is created at a position where the respective surfaces are crossed with one another, and the created local coordinate system is correlated with a global coordinate system. 
     As other ways of establishing the reference coordinate system, in the case where the CAD data of the modeling object exists, there are one way of minimizing the distance between the curved surface of the CAD data and the points of the model data, and another way of finding the reference points predefined in the CAD data and the corresponding points on the model data and minimizing the distance between these points. 
     Further, while it is possible to establish the X-Y plane  200 , the Y-Z plane  210  and the Z-X plane  220  as basic work planes, it is also possible to establish an optional plane selected by the user as a work plane. 
     Referring again to  FIGS. 1 and 2 , if the establishment of the reference coordinate system of the model data is completed in the step S 110 , the reverse modeling control unit  20  outputs the work planes for detecting the two dimensional section data of the model data along with the model data to the display unit  30 , and detects the two dimensional section data of the model data depending upon the detection information inputted from the user input unit  40  (S 120 ). 
     That is to say, as shown in  FIG. 5 , in order to extract the two dimensional section data of the model data, the first region  110  of the model data of which reference coordinate system is established in the step S 110  (see  FIG. 2 ) is projected on the work plane  300 . Accordingly, a projected part  310  is formed on the work plane  300  as two dimensional projected section data. 
     At this time, as the two dimensional projected section data being a detection object, the section data of the model data, which is obtained at a position vertically offset from the work plane  300  or determined by rotating the work plane  300  about a measurement axis, is projected on the work plane  300 . 
     Also, it is possible to convert the reference coordinate system into a UV coordinate system by rotating the work plane  300  and then establish U and V directions of the two dimensional section data. Also, the U and V directions of the specific two dimensional projected section data can be reestablished afterwards. 
     Further, in the case where the two dimensional projected section data has a draft for injection molding, it can be envisaged that a measurement range is established (for example, as a region between two planes determined by the user) so as to acquire a precise section from the detected model data, and the silhouette of the model data which is included in the established measurement range is projected on the work plane  300 . The detection of the two dimensional projected section data of the model data having the draft will be described in detail with reference to  FIG. 8 . 
     Referring again to  FIGS. 1 and 2 , after the step S 120  is implemented, the reverse modeling control unit  20  displays the two dimensional projected section data projected on the work plane  300  by dividing the two dimensional projected section data into feature regions including a straight line, a circle, an arc, a free curve, a rectangle, a polygon, a slot, etc. depending upon curvature distributions (S 130 ). 
     That is to say, the reverse modeling control unit  20  automatically divides the two dimensional sketch region projected on the work plane  300  into a straight line, a circle, an arc, a free curve, a rectangle, a polygon, a slot, etc., and performs a fitting operation. 
     As shown in  FIG. 6 , the projected part  310  projected on the work plane  300  is composed of four straight lines L 1  through L 4 , four arcs C 1  through C 4 , and eight representative points P 1  through P 8 . 
     In other words, as a result of the analyses of the curvature distribution of the two dimensional projected section data projected on the projected part  310 , the portion extending from the starting point P 8  of points over which the curvature of a plane curve is distributed to the starting point P 1  of points over which the curvature of a curved curve is distributed is represented by a straight line L 1 , the portion extending from the starting point P 1  of points over which the curvature of a curved curve is distributed to the starting point P 2  of points over which the curvature of a plane curve is distributed is represented by an arc C 1 , and the portion extending from the starting point P 2  of points over which the curvature of a plane curve is distributed to the starting point P 3  of points over which the curvature of a curved curve is distributed is represented by a straight line L 2 . 
     Also, the portion extending from the starting point P 3  of points over which the curvature of a curved curve is distributed to the starting point P 4  of points over which the curvature of a plane curve is distributed is represented by an arc C 2 , the portion extending from the starting point P 4  of points over which the curvature of a plane curve is distributed to the starting point P 5  of points over which the curvature of a curved curve is distributed is represented by a straight line L 3 , and the portion extending from the starting point PS of points over which the curvature of a curved curve is distributed to the starting point P 6  of points over which the curvature of a plane curve is distributed is represented by an arc C 3 . 
     Further, the portion extending from the starting point P 6  of points over which the curvature of a plane curve is distributed to the starting point P 7  of points over which the curvature of a curved curve is distributed is represented by a straight line L 4 , and the portion extending from the starting point P 7  of points over which the curvature of a curved curve is distributed to the starting point P 8  of points over which the curvature of a plane curve is distributed is represented by an arc C 4 . In these ways, the feature segments of the two dimensional projected section data of the projected part  310  can be extracted. 
     Moreover, as shown in  FIG. 7 , it is possible to extract the feature segments of the two dimensional projected section data of the model data from at least two work planes  300 a and  300 b. 
     That is to say, in order to extract an optimal projected part, some feature segments (for example, straight lines L 1 a and L 2 a and arcs C 1 a and C 2 a) of the feature segments of the two dimensional projected section data of the model data are detected from the first work plane  300 a, and, in order to detect the remaining feature segments of the two dimensional projected section data of the model data, the remaining two dimensional projected section data (for example, straight lines L 3 a and L 4 a and arcs C 3 a and C 4 a) of the model data are projected on and extracted from the second work plane  300 b having the same axis as the first work plane  300 a. Then, the remaining feature segments of the two dimensional projected section data of the model data detected from the second work plane  300 b are coupled with the feature segments of the two dimensional projected section data of the model data detected from the first work plane  300 a, through overlapping on a third projection plane  300 c. In this way, the feature regions detected from a plurality of planes can be detected on one projection plane as an optimal projected part  310 a, that is, as the feature segments of the two dimensional projected section data. 
     Referring again to  FIG. 6 , differently from the intention of an actual designer, the feature segments produced by being automatically fitted on the work plane  300  may be produced at positions which are slightly deviated from the reference coordinate system, without any predefined constraint (for example, verticality, horizontality, parallelism, slopeness, junction, fixing, correspondence, the same straight line, concentricity, the same radius, the same distance, and so forth, between the feature segments of the two dimensional projected section data). 
     That is to say, the ending points between the respective feature segments of the two dimensional projected section data are separated from one another, and values such as a length, a radius, and the like, have some errors when compared to originally designed dimensions. This is to automatically or manually connect the ending points by internal tolerances and to allow the ending points to be automatically or manually established depending upon dimensions and constraint. 
     In other words, as shown in  FIG. 8 , if the ending point P 7  of a straight line and the ending point of an arc C 4 b projected on the projected part  310  approach within a predetermined distance, they are caused to meet each other. As the case may be, the ending points of a straight line and an arc can be extended to meet each other. 
     Also, it is possible for the user to establish a new arc C 4  using a snapping function at a node of the two dimensional projected section data and to produce a new sketch element. Namely, snapping is a skill of automatically fixing the cursor of a mouse in the vicinity of the node. In order for the user to properly establish the curve C 4 b, in the course of moving the cursor of the mouse including the user input unit  40  (see  FIG. 1 ) toward the point P 7  or P 8 , if the cursor of the mouse approaches one of the points P 7  and P 8  to be placed within a predetermined range, the cursor of the mouse is fixed to one of the points P 7  and P 8  through snapping. 
     Thereafter, as the user moves the cursor of the mouse from the fixed point (here, it is assumed that the cursor of the mouse is fixed to the point P 8 ) to the point P 7  to which the newly established arc C 4  will be connected, the cursor of the mouse is fixed to the point P 7  through snapping, and the new arc C 4  is established. 
     Further, it is possible to set a slope so that two lines are tangential to each other when the direction of a slope at an ending point approaches within a predetermined angle, and an angle, a length, a radius, and so forth, are automatically set based on rules such as rounding to the nearest whole number, rounding off, etc. 
     That is to say, if an angle between two straight lines of a projected feature region is 89.9°, the angle is automatically set to 90°. In this way, constraint between the respective feature regions is automatically invested, and the respective feature regions are parametrically connected with one another. 
     Also, it is possible to draw on the projected part  310  of the model data a polyline optionally selected by the user, or to establish a third feature shape detected from another model data, that is, two dimensional sketch data of a quadrangular shape, on the projected part  310 , through overlapping. 
       FIG. 9  illustrates an embodiment of detecting two dimensional sketch data having a precise section in the case of model data  100 a having a draft. 
     Here, an original designer designs the model data  100 a as a regular square which is represented by dashed lines A 1 . However, in the course of actually forming a product through injection molding, a draft is provided to the designed data of the regular square. 
     That is to say, while the initial design data is designed to be represented by the dashed lines A 1 , in the case where the designer places a draft for injection molding, the regular square is changed to a tapered shape represented by straight lines A 2 . In general, the angle of the draft is set to 0.5° to 2°. 
     As a consequence, since the model data measured using the scanner, etc. is detected as the regular square represented by the straight lines A 2 , it is necessary to detect the initially designed shape represented by the dashed lines A 1 . 
     At this time, while the two dimensional projected section data projected on a fourth projection plane  300 d has an initially designed shape which is two dimensional section data to be detected, since the two dimensional projected section data projected on the fourth projection plane  300 d is provided with a fillet or is rounded, it is difficult to detect precise shape and dimensions. 
     In consideration of this fact, the fourth projection plane  300 d is first established on the lowermost region of the model data  100 a, and two projection planes established by the user or automatically, that is, a fifth projection plane  300 e and a sixth projection plane  300 f are located at optional positions on the model data  100 a. 
     Here, because it is possible to detect the distance D 1  between the located fifth projection plane  300 e and the sixth projection plane  300 f and the lengths D 2  and D 3  of the two dimensional projected section data respectively projected on the fifth projection plane  300 e and the sixth projection plane  300 e, the value of the draft can be calculated using the detected distance D 1  and the length difference (D 2 -D 3 ). That is to say, when knowing the length of a base and a height of a triangle, the value of the draft can be calculated using sine, cosine and tangent laws of the triangle. 
     Hence, using the value of the draft which is calculated from, the fifth projection plane  300 e and the sixth projection plane  300 f, the value of the draft set to the model data  100 a can be detected, and based on the detected value of the draft, the precise two dimensional projected section data projected on the fourth projection plane  300 d can be detected. 
     The unexplained reference numeral  300 g designates a projection plane which is established on the upper end of the model data  100 a. 
     Referring again to  FIGS. 1 and 2 , the reverse modeling control unit  20  extracts the numeral information of the feature segments of the two dimensional projected section data which is extracted from the step S 130  and displays the numerical information on the display unit  30  (S 140 ). 
     After the step S 140  is implemented, the two dimensional projected section data inputted from the user input unit  40  may significantly be deviated from the original model data. The feature segments of the two dimensional projected section data acquired from the work plane or some feature segments which are established and required to be corrected by the user are automatically fixed in their established constraint and dimensions in the step S 130 . The reverse modeling control unit  20  determines whether correction request from the user for the numerical information, etc. is detected (S 150 ). 
     The user can correct the constraint and numerical information of the respective feature regions which are automatically established. 
     This procedure will be described in detail with reference to  FIGS. 10 through 12 . 
     As shown in  FIG. 10 , if the user requests to correct the straight line L 1  among the constraint and numerical information of the projected part which are automatically established, for example, sketch straight line 1″  420  is displayed on the display unit  30  (see  FIG. 1 ), and the X-axis coordinate  430  and Y-axis coordinate  432  of the starting point P 1  of the straight line L 1  and the X-axis coordinate  440  and Y-axis coordinate  442  of the ending point P 8  of the straight line L 1  are detected and displayed on the display unit  30 . 
     At this time, as shown in  FIG. 10 , the straight line L 1  to be corrected can be represented by a thick solid line or by another color and be highlighted. Excluding the straight line L 1  to be corrected, the remaining straight line L 2  between the points P 2  and P 3 , straight line L 3  between the points P 4  and P 5  and straight line L 4  between the points P 6  and P 7  are represented by dashed lines or by a color which is different from that of the straight line L 1  to be corrected. 
     Also, other than the method in which the user directly inputs the coordinates of the starting point P 1  and the ending point P 8 , it is possible to implement correction using combo boxes  431  and  441 . 
     Further, establishment information, such as “confirmation of fitting”  450  which causes the coordinates and length of the currently displayed straight line L 1  to be stored, “deletion of fitting”  460  which causes the coordinates and length of the currently displayed straight line L 1  to be deleted, and “deletion of section”  470  which causes the currently displayed straight line L 1  to be deleted, is detected from the user input unit  40  (see  FIG. 1 ) so that the straight line L 1  can be appropriately reestablished. 
     As shown in FIG. H, if the user requests to correct the curved line (arc) A 1  among the constraint and numerical information of the projected part which are automatically established, for example, “sketch curved line”  500  is displayed on the display unit  30  (see  FIG. 1 ), the X-axis coordinate  510  and Y-axis coordinate  512  of the center of the curved line A 1  are displayed, and the radius  520  of the curved line A 1  is displayed. 
     In addition, the X-axis coordinate  530  and Y-axis coordinate  532  of the starting point P 8 ′ of the curved line A 1  and the X-axis coordinate  540  and Y-axis coordinate  542  of the ending point P 1 ′ of the curved line A 1  are detected and displayed on the display unit  30 . 
     At this time, as shown in FIG. H, the curved line A 1  to be corrected can be represented by a thick solid line or by another color and be highlighted. Excluding the curved line A 1  to be corrected, the remaining straight line L 2 ′ between the points P 2 ′ and P 3 ′, straight line L 3 ′ between the points P 4 ′ and P 5 ′ and straight line L 4 ′ between the points P 6 ′ and P 7 ′ are represented by dashed lines or by a color which is different from that of the curved line A 1  to be corrected. 
     Also, other than the method in which the user directly inputs the coordinates of the center, the radius, the starting point and the ending point, it is possible to implement correction using combo boxes  511 ,  521 ,  531  and  541 . 
     Further, establishment information, such as “confirmation of fitting”  550  which causes the numerical information of the currently displayed curved line A 1  to be stored, “deletion of fitting”  551  which causes the numerical information of the currently displayed curved line A 1  to be deleted, and “deletion of section”  552  which causes the currently displayed curved line A 1  to be deleted, is detected from the user input unit  40  (see  FIG. 1 ) so that the curved line A 1  can be appropriately reestablished. 
     Moreover, as shown in  FIG. 12 , if the user requests to correct the fillets R 1  through R 4  among the constraint and numerical information of the projected part which are automatically established, for example, “sketch fillet”  600  is displayed on the display unit  30  (see  FIG. 1 ), and the constraint and radius  610  of the fillets are displayed on the display unit  30 . 
     At this time, as shown in  FIG. 12 , the fillets R 1  through R 4  to be corrected can be represented by a thick solid line or by another color and be highlighted. 
     Also, it is possible to set different constraint for the respective fillets, and unlike the method in which the user directly inputs the radius, it is possible to implement correction using a combo box  610 . 
     Referring again to  FIGS. 1 and 2 , the modification and the display of the feature segments of the two dimensional projected section data corrected as described above are corrected, and numerical information and constraint information of the feature segments which are corrected are displayed on the display unit  30  (S 160 ). 
     After the step S 160  is implemented, the reverse modeling control unit  20  establishes the two dimensional projected section data produced in the step S 160  as two dimensional sketch data and stores the two dimensional sketch data in the data storage unit  50  (S 170 ). 
     As is apparent from the above descriptions, the present invention provides advantages in that, since modeling work is conducted based on the model data of a modeling object, the efficiency of the modeling work is improved, and the time of modeling work is shortened. 
     Also, in the present invention, based on the model data of the modeling object, including point cloud data, mesh data, general CAD data, and CAD data having curved surfaces, which are obtained by a scanner, it is possible to automatically extract the two dimensional sketch feature regions of the model data. 
     Further, in the present invention, since two dimensional, sketch data are parametrically connected with one another, they can be automatically updated when a condition or information is newly set or changed. 
     Moreover, due to the fact that the information of the feature regions of the two dimensional sketch data is displayed in real time, a user can easily find and correct an error. 
     Although a preferred embodiment of the present invention has been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.