Abstract:
The invention provides a method of improving silhouette appearance in bump mapping, which not only reduces the operation overhead of applying displacement mapping to a whole model but also retains the truly geometric shape in displaying the object silhouette. The invention comprises the following steps: receiving, checking vertex, checking subdivision, subdividing, repeating, displacing and bump mapping. The invention also discloses a system employing the method of improving silhouette appearance in bump mapping.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates generally to a method and system of improving silhouette appearance, applied to a bump mapping technique, thereby rendering a three-dimensional and true object silhouette.  
           [0003]    2. Description of the Related Art  
           [0004]    Bump mapping roughens surfaces of a object by modifying the normals without displacing surfaces. Although there seems to be many bumps in the surfaces, it is no more than a camouflage.  
           [0005]    As shown in FIG. 1( a ), there are parallel, upward normals on a smooth surface. In contrast, a rough surface features disorderly directions of surface normals in FIG. 1( b ). After bump mapping is applied, a smooth surface whose normal directions should be parallel mimics a rough surface with disorderly surface normals as shown in FIG. 1( c ).  
           [0006]    In contrast with bump mapping modifying surface normals only, displacement mapping actually varies surfaces; for example, a screw is created from a cylinder.  
           [0007]    However, the main problem of bump mapping is that real concaves or convexes does not exist in a model. Thus, silhouette edges that seem to pass through a concave does not produce an expected cross section. In other words, silhouette edges still comply with the original geometry of the module. Silhouette appearance seems like flat, even if the bump mapping technique has been applied to emulate a rugged surface.  
         SUMMARY OF THE INVENTION  
         [0008]    In view of the above-mentioned problems, it is therefore an important object of the invention to provide a method and system of improving silhouette appearance in bump mapping so that the appearance of the bumpy object silhouette approaches the true geometry.  
           [0009]    A method of improving silhouette appearance in bump mapping comprises the following steps: receiving, checking vertex, checking subdivision, subdividing, repeating, displacing and bump mapping. In the receiving step, a triangle of a plurality of triangles is received. In the checking vertex step, whether there is a vertex near silhouette in the triangle is checked. In the checking subdivision step whether the triangle meets a predetermined criterion of subdivision is checked, if the result of the checking vertex step is “YES”. In the subdividing step, the triangle is subdivided if the result of the checking subdivision step is “YES”. In the repeating step, the receiving step the checking vertex step, the checking subdivision step and the subdividing step are repeated until the triangle does not meet the predetermined criterion. In the displacing step, the vertex positions near silhouette along their normals are displaced according to the displacements obtained from displacement mapping if the result of the checking subdivision step is “NO”. Finally, in a bump mapping step, the bump mapping technique is implemented if the result of the checking vertex step is “NO”.  
           [0010]    A system of improving silhouette appearance in bump mapping comprises the following devices. A receiving device receives a triangle of a plurality of triangles. A checking vertex device checks whether there is a vertex near silhouette in the triangle. A checking subdivision device checks whether the triangle meets a predetermined criterion of subdivision if there is a vertex near silhouette in the triangle. A subdividing device subdivides the triangle. A repeating device conducts the receiving device, the checking vertex device, the checking subdivision device and the subdividing device to repeat their own operations until the triangle does not meet the predetermined criterion, and then inputs the triangle into the displacing device. A displacing device of displacing the vertex positions near silhouette along their normals according to the displacements obtained from displacement mapping. And a bump mapping device implements bump mapping if there is no vertex near silhouette in the triangle or after the triangle has been processed by the displacing device.  
           [0011]    In addition, there is provided a machine-readable record medium storing programs for instructing an MPU (Microprocessor Unit) etc. to execute the aforementioned method of improving silhouette appearance in bump mapping.  
           [0012]    Furthermore, there is provided a system of improving silhouette appearance in bump mapping, comprising a CPU and a memory storing instructions so that the CPU can access instructions stored in the memory and execute the aforementioned method of improving silhouette appearance in bump mapping.  
           [0013]    Both bump mapping and displacement mapping can mimic a rugged surface. However, if the surface is not perpendicular to the line of sight, bump mapping has fine visual quality and less mathematic operations, but can not display the closely true geometry of bumps near silhouette. Although the visual quality is quite satisfactory, displacement mapping is limited to use considerable quantity of triangles to mimic the true geometry after the base model has been moved. Thus, in order to combine the advantages of bump mapping and displacement mapping, the invention disclose a method and system: Apply bump mapping to a triangle (surface) near silhouette, or apply displacement mapping to a triangle (surface) not near silhouette. Therefore, the invention not only reduces the operation overhead of applying bump mapping to a whole model, but also retains the true geometry in displaying object silhouette.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1( a ) is a schematic diagram showing normals of a smooth surface; FIG. 1( b ) is a schematic diagram showing normals of a rugged surface; and FIG. 1( c ) is a schematic diagram showing normals after applying bump mapping.  
         [0015]    [0015]FIG. 2 is a flow chart showing the method of improving silhouette appearance in bump mapping of the invention.  
         [0016]    [0016]FIG. 3 is a schematic diagram showing the near silhouette criterion.  
         [0017]    [0017]FIG. 4 shows that the vertex normal of a vertex is an average of surface normals of all surfaces adjacent to the vertex.  
         [0018]    [0018]FIG. 5 is a schematic diagram that shows how to divide a triangle in the invention.  
         [0019]    [0019]FIG. 6 is a schematic diagram that shows how to divide a triangle having two vertexes near silhouette.  
         [0020]    [0020]FIG. 7 is a schematic diagram that applies displacement mapping.  
         [0021]    [0021]FIG. 8 is an architectural diagram showing the system of improving silhouette appearance in bump mapping.  
         [0022]    [0022]FIG. 9 is also another architectural diagram showing the system of improving silhouette appearance in bump mapping. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]    The method of improving silhouette appearance in bump mapping in accordance with the preferred embodiments of the invention will be described with reference to FIG. 2.  
         [0024]    First, in step  21 , a triangle of a plurality of triangles is received.  
         [0025]    Then, in step  22 , if there is a vertex near silhouette in the triangle is checked. If the result of this step is “YES”, step  23  is performed. If “NO”, step  26  is performed.  
         [0026]    Referring to FIG. 3, how to check the vertex in step  22  is set forth as follows:  
         [0027]    Assume that there are a vertex normal {right arrow over (N)}, a view vector {right arrow over (V)} and a known positive real number d, 0≦d≦1,  
         [0028]    if 0≦|{right arrow over (N)}{right arrow over (V)}|≦d, it is called that the vertex is near silhouette,  
         [0029]    wherein {right arrow over (N)} is derived from averaging all surface normals of all triangles adjacent to the vertex, i.e. {right arrow over (N)}=({right arrow over (N)} 1 +{right arrow over (N)} 2 + . . . +{right arrow over (N)} n )/n (In FIG. 4, n=6, for example). The vector from vertex P to eye position E is denoted as {right arrow over (V)}, i.e. {right arrow over (V)}={right arrow over (E)}−{right arrow over (P)}. Note that {right arrow over (N)} and {right arrow over (V)} have to be normalized respectively before the dot product is operated. And, the positive real number d is a user-defined constant.  
         [0030]    The value of {right arrow over (N)}{right arrow over (V)} implies the angle formed between {right arrow over (N)} and {right arrow over (V)}. For example, if {right arrow over (N)}{right arrow over (V)} is equal to 0, it means that they are perpendicular to each other; if {right arrow over (N)}{right arrow over (V)} is equal to 1, it means that they are parallel to each other. Therefore, the positive number d defines a threshold value, which is the user-defined near silhouette criterion.  
         [0031]    In step  23 , whether it is necessary to stop subdividing the first triangle is determined. If the result of this step is “YES”, step  25  is performed; if “NOT”, step  24  is performed.  
         [0032]    In step  23 , according to the predetermined number of subdivision (for example at most 3 times) or minimum dividable area, whether it is necessary to stop subdividing the first triangle is determined. Alternatively, there is another method that the number of subdivision is derived from the distance between an object and the screen. Accordingly, the further the triangle away from the screen, the less subdivision is derived. In contrast, the nearer the triangle close to the screen, the more subdivision is derived. However, no matter which method, there must be a maximum number of subdivision, or it will be trapped in endless loops.  
         [0033]    Take the triangle in FIG. 5( a ) as an example. Calculate three new vertex normals at middle points of three sides of the triangle by using interpolation. Then, four sub-triangles shown in FIG. 5( b ) are obtained after interconnecting three middle points. If there is another subdivision applied in the next loop, sixteen sub-triangles are finally presented in FIG. 5( c ). Moreover, as an alternative subdivision method, a triangle is subdivided into 3 sub-triangles by using the center of gravity of the triangle.  
         [0034]    Referring to FIG. 6( a ), the operations performed in step  21  through step  24  are described in the following. In triangle ACF, {overscore (AC)} is a silhouette, but {overscore (AF)} and {overscore (CF)} are not silhouette. Owing to vertexes A and C near silhouette, triangle ACF is divided into triangle ABD, triangle BCE, triangle DBE and triangle DEF. Also, triangles ABD, BCE and DBE are subdivided again because there is at least one vertex near silhouette in each triangle. Similarly, each of the resultant triangles are subdivided once more as shown in FIG. 6( b ).  
         [0035]    In step  25 , vertex positions near silhouette along their normal is displaced. According to the displacements along their vertex normals in displacement mapping, displace vertex positions. An original triangle in FIG. 7( a ) is divided into four sub-triangles in FIG. 7( b ) after the first division by using interpolation. After displacement mapping is applied to each vertex, the displacement of each vertex of the triangles is illustrated in FIG. 7( c ). Through interpolation, 16 sub-triangles in FIG. 7( d ) are obtained after the second subdivision. Similarly, after displacement mapping is applied to each vertex, the displacement of each vertex of the triangles is illustrated in FIG. 7( e ).  
         [0036]    Finally, in step  26 , the bump mapping technique is implemented.  
         [0037]    Besides, referring to FIG. 8, in this embodiment, a system of improving silhouette appearance in bump mapping comprises seven parts: a receiving device  81 , a checking vertex device  82 , a checking subdivision device  83 , a subdividing device  84 , a repeating device  85 , a displacing device  86  and a bump mapping device  87 . The aforementioned seven parts will be described hereafter.  
         [0038]    Receiving device  81  receives a first triangle of a plurality of triangles. Checking vertex device  82  checks whether there is a vertex near silhouette in the first triangle. Checking subdivision device  83  checks whether the first triangle meets a predetermined criterion of subdivision, if there is a vertex near silhouette in the first triangle. Subdividing device  84  subdivides the first triangle. Repeating device  85  directs the receiving, the checking vertex, the checking subdivision and the subdividing devices to repeat their own operations until the first triangle does not meet the predetermined criterion, then inputs the first triangle into the displacing device. Displacing device  86  displaces the vertex positions near silhouette along their normals according to the displacements obtained from displacement mapping. Bump mapping device  87  implements bump mapping if there is no vertex near silhouette in the first triangle, or after the first triangle has been processed by the displacing device.  
         [0039]    Further, the system of improving silhouette appearance in bump mapping can also be manufactured into hardware chip modules like ASICs for speeding up image processing by one of ordinary skill.  
         [0040]    Besides, FIG. 9 is another system of improving silhouette appearance in bump mapping, different from the architecture of FIG. 8, includes a CPU  91  and a memory storing instructions  92 . CPU  91  can access instructions stored in the memory and execute a method of improving silhouette appearance in bump mapping as described in FIG. 2.  
         [0041]    Note that CPU  91  can be any architecture, such as ALU (arithmetic logic unit) for mathematic and logic operations, registers for storing data or instructions temporarily and control units for controlling all operations of computers. Memory storing instructions  92  can be any computer-readable memory that can store data, such as dynamic random access memory (DRAM), read only memory (ROM), electrically erasable programmable read-only memory or the combination.  
         [0042]    The above-described embodiment should be considered in all respects as illustrative and not restrictive. Any modifications and changes made to the invention should be included in the appended claims without departing from the spirit and scope of the invention as set forth in the appended claims.