Abstract:
A technique for organizing graphical objects in a graphics database that facilitates the rapid selection of one or more graphical objects on a computer display screen. Graphical objects are stored in the graphics database using a spatially organized data structure. The spacially organized data structure is formed by recursively subdividing the graphics space until each subspace contains no more than a predetermined number of graphical objects. The spacially organized database is ideally suited for spacial queries required to select, based on visual criteria, graphical objects appearing on a display screen. Graphical objects may be selected in response to a cursor moving about a display screen under programmer control, or in response to a system request to identify one or more objects spacially located in a given portion of the graphics space.

Description:
This is a continuation of application Ser. No. 07/819,250, filed Jan. 10, 1992, now abandoned. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to graphical data organization in a computer system, and more particularly to improving the performance of graphical object selection and display. The invention has particular application in computer aided design (CAD) systems, but also can be advantageously applied to illustration systems and desktop publishing systems. 
     BACKGROUND OF THE INVENTION 
     Graphics Database Organization 
     Graphics database applications such as those used in CAD systems are traditionally organized on the basis of graphical object type, the order of graphical object creation, and the layer on which the graphical object exists. However, some of the most fundamental interactions with graphics databases require queries based on the spacial location of the graphical objects making up the database. Servicing such interactions under the paradigm of the prior art requires extensive searching through the underlying database, which is randomly organized vis-a-vis location-based queries. As the database becomes large, these queries exact a significant penalty on system performance, including noticeable delays in system response time. Thus, the typical graphics database organization does not lend itself to graphical object selection queries based on object location. 
     Important graphics database interactions requiring location-based queries include user selection of graphical objects appearing on a display screen and system selection of graphical objects located in a given region of a graphics space. Both of these types of interactions are discussed in more detail below. 
     User Selection of Graphical Objects 
     User selection of a single graphical object on a display screen is typically accomplished by positioning a pointing cursor near the desired object and pressing the select button. The graphics database is then searched and distance computations are performed to find the object that was closest to the cursor when the select button was pressed. This approach is adequate when there are only a few objects to pick from. However, as the number of objects increases the system&#39;s task becomes increasingly processor consumptive. 
     In some systems, as soon as an object is found that is within a predetermined distance from the cursor location, that object is selected and the search is terminated. This method has better average performance than a complete search of all graphical objects, but still may not be fast if the number of objects is large and the desired object is near the end of the list. Also, since the system may not select the correct object initially, the user may have to position the cursor very carefully or zoom in to make the desired selection. 
     Part of the problem is minimized if the system searches a data structure containing the graphical objects on the display each time the cursor is repositioned and identifies the object that would be selected if the select button were pressed (preselection). One known approach is to identify the object that would be selected by using various types of highlighting. Although this technique reduces the burden on a user, it dramatically increases the burden on the system because of the need for constant searching. Thus, as the number of objects increases, this approach suffers from the increased overhead associated with preselecting objects. In another approach for object selection, a user selected X,Y location on the display is used as an initial seed location and a radial search is conducted using the initial X,Y location as the center of the search. This technique is an example of a resource intensive solution that does not organize information to optimize selection of objects on a display. 
     The difficulty of quickly selecting among increasing numbers of objects increases linearly as the number of objects increases. Also, unless the feedback is nearly instantaneous from the user-perception point of view, the selection method hinders users. In a typical CAD drawing there may be thousands of objects; in such an environment, the object selection techniques contemplated by the prior art are not capable of providing nearly-instantaneous feedback. 
     Sometimes a user may need to select multiple objects. If many objects are to be selected, it is inconvenient to select the objects one-by-one as described above. Thus, it becomes desirable for a graphical editing system to provide support for multiple object selection. Examples include selecting all objects that are within a rectangle or selecting all objects intersecting a line. After the rectangle or line is defined by a user, the graphics database is searched and calculations are performed for each object on the display to determine whether the object is within the rectangle or intersects the line, respectively. Again, as is the case for single object selection, this can be a slow process if a large number of objects are involved. 
     System Selection of Graphical Objects 
     There are a variety of circumstances in which it is desirable for a display system to select for itself certain graphical objects from a larger set of graphical objects, including: selection of all objects that are on the visible window for display; selection of all objects whose images are damaged when overlapping objects are erased from the display; and selection of all objects whose image must be re-drawn when an obscuring window is removed. The alternatives to selection--processing all objects in the graphics space or redrawing all visible objects--require unnecessary computations and I/O and hence suffer significant performance penalties. 
     The purpose of selection, then, is to improve performance in each of these examples. However, because prior art graphics databases are not spacially organized, the selection process itself is time consuming. Further, as the number of graphical objects in the graphics database increases, the time required to perform the selection increases, since the entire database must be searched to find all the sought-after objects. Thus, in modern graphics database applications (such as CAD applications) involving many objects, system-initiated selection of graphical objects can inhibit user productivity as much or more than user-initiated selection. 
     Objects of the Invention 
     It is therefore an object of the present invention to organize graphical objects in a graphics database according to their spacial location. 
     It is another object of the present invention to provide selection of graphical objects from a graphics database by performing a spacial query. 
     It is another object of the present invention to provide a method for selecting from a graphics database graphical objects pointed to by a cursor on a display. 
     It is another object of the present invention to provide a method for selecting from a graphics database graphical objects in a region of a graphics space. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a computer display system is provided with a scheme for organizing the graphical objects forming a graphics space in a graphics database such that single objects and groups of objects can be efficiently identified in response to spacial queries on the graphics database. A spacially organized data structure is created in a computer memory. The graphical objects making up a graphics space are mapped into the spacially organized data structure using a recursive subdivision of space integrated with a display list stored in the data structure. 
     Information on a display becomes readily accessible to a user by translating cursor pointings into a candidate object or objects to select. The translation is accomplished quickly and accurately by performing spacial queries on the spacial data structure previously formed in the computer memory. The candidate object or objects are communicated back to the user via highlighting or other user perceptible indicia such as blinking, inverse video, pointers, etc. 
     Information on a display also becomes readily accessible to a program by translating a region identified by the program into an object or objects to select for subsequent processing. The translation is likewise accomplished by performing spacial queries on the spacially organized graphics database. The object or objects selected are communicated back to the requesting program for subsequent manipulation such as display or removal of all objects within a visible window, redraw of objects damaged when overlapping objects are erased from the display, or redraw of overlapping objects when an obscuring window is removed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram illustrating a computer hardware system including a display screen on which is displayed several graphical objects in accordance with the present invention. 
     FIG. 2 is a diagram illustrating a data structure organized according to a recursive subdivision of space in accordance with the present invention. 
     FIG. 3e is a flow chart setting forth the logic for PROCEDURE subdivide in accordance with the present invention. 
     FIG. 4 is a flow chart setting forth the logic for PROCEDURE insert --  object in accordance with the present invention. 
     FIG. 5 is a flow chart setting forth the logic for PROCEDURE nearest --  object in accordance with the present invention. 
     FIG. 6 is a flow chart for PROCEDURE rectangle --  find. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Shown in FIG. 1 is a computer hardware system configured to implement the present invention. Processor means 10 executes programs and manipulates data (including the graphics database containing the spatially organized graphics space of the present invention) stored in a memory means, which may include any combination of processor memory 11 and disk subsystem 12. In the preferred embodiment, portions of the graphics database presently accessible for visual display or use by the processor are typically held in processor memory 11, while portions not in use are stored in disk subsystem 12. User input is entered through keyboard 13, mouse 14, and dial pad 15, all managed by input controller 16. Drawings are presented for user-viewing on display means 17, managed by display controller 18. In the preferred embodiment, display means 17 is an all-points-addressable graphics display screen. Shown in display space 19 is an exemplary CAD drawing including a group of graphical objects. Also shown is display cursor 20, manipulable by way of keyboard 13, mouse 14 or dial pad 15. It is to be noted that display 19 at any given time may contain an entire graphics space (e.g. an entire CAD drawing) or only part of a graphics space (e.g. where a CAD drawing is viewed at a resolution such that the entire drawing will not fit in the display space). 
     Spacial Organization of a Graphics Database 
     In accordance with the present invention, the graphical objects contained in a graphics space are organized into a database according to their spacial locations in the graphics space. Initially, the graphics space is divided into a plurality of smaller regions; in the preferred embodiment, quadrants are used to simplify calculations. For each quadrant, a list is formed of the objects that are wholly or partly within that quadrant. If there are more than N objects in the quadrant (where N is the predetermined maximum number of objects for which spacial calculations can be performed without impacting user response time), the quadrant is recursively subdivided into four additional quadrants until the new quadrants contain no more than N objects. When the entire process is complete, the graphics space is organized into a database containing lists of objects (display lists), each display list identifying all objects intersecting a particular quadrant, each quadrant accessible according to its physical position in the graphics space. 
     It is to be noted that N is a value determined empirically as a function of the processing power of the computer on which the present invention is to be implemented. In the preferred embodiment, a value of N=32 has been found to produce optimal results for subsequent spacial queries. 
     FIG. 2 illustrates a view of an exemplary graphics space with quadrants subdivided into additional quadrants. The graphics space is in the form of a square having sides 30. Each initial quadrant is in the form of a square having sides 31. The finest granularity recursive subdivision is exemplified by square 32. 
     The preferred embodiment logic for recursively subdividing a graphics space and simultaneously inserting each object in a box (quadrant) or boxes having appropriate granularity and spacial coordinates is illustrated in PROCEDURE subdivide, set forth below. Procedure insert --  object, which is called by PROCEDURE subdivide, is set forth following PROCEDURE subdivide. 
     Both PROCEDURE subdivide and PROCEDURE insert --  object are most readily understood with reference to FIGS. 3 and 4, respectively. As shown in FIG. 3, PROCEDURE subdivide begins at 41 by computing the midpoint of the rectangle in which it will attempt to place an object. At 42-45, the coordinate description of the object is compared to the midpoint to determine whether any portion of the object resides in any of the four quadrants (boxes) making up the rectangle. At 46-49 PROCEDURE insert --  object is called to insert the object into the appropriate box or boxes. 
     As shown in FIG. 4, PROCEDURE insert --  object begins at 61 by determining whether the present box has been subdivided already. If so, the object is subdivided at 62 by calling PROCEDURE subdivide recursively. If not, the present box is checked at 63 to determine if it is full. If not, the object is simply added to the box at 64 and PROCEDURE insert --  object is exited. If the box is full, then each constituent object (each object in the box) is extracted in rum at 65 and subdivided at 66 by calling PROCEDURE subdivide recursively. Once all the constituent objects have been subdivided, the object is subdivided at 67 and marked &#34;subdivided&#34; at 68 to prepare for test 61 to be executed on a future call of PROCEDURE insert --  object. 
     
         __________________________________________________________________________PROCEDURE subdivide(object, xmin, ymin, xmax, ymax)*************************************************** PURPOSE:  Compute which of the four quadrants this object*      overlaps and insert the object into each of those*      quadrants that the object overlaps* INPUT:  The object and the boundary of the current*      rectangle* OUTPUT:  None*************************************************** Compute the midpoint of the rectanglexmid = (xmin + xmax) / 2ymid = (ymin + ymax) / 2* For each quadrant, see if object overlaps it; if so insert the* object into the quadrantIF (object overlaps rectangle [xmin, ymin, xmid, ymid]) THENinsert.sub.-- object(object, xmin, ymin, xmid, ymid)ENDIFIF (object overlaps rectangle [xmid, ymin, xmas, ymid]) THENinsert.sub.-- object(object, xmid, ymin, xmax, ymid)ENDIFIF (object overlaps rectangle [xmid, ymid, xmax, ymax]) THENinsert.sub.-- object(object, xmid, ymid, xmax, ymax)ENDIFIF (object overlaps rectangle [xmin, ymid, xmid, ymax]) THENinsert.sub.-- object(object, xmin, ymid, xmid, ymax)ENDIFEND PROCEDURE subdividePROCEDURE insert.sub.-- object(object, xmin, ymin, xmax, ymax)*************************************************** PURPOSE:  Insert the object into box [xmin, ymin, xmax, ymax]* INPUT:  the object and the boundary of the box* OUTPUT:  none*************************************************** determine if this box has been subdivided, if so subdivide the objectIF (box already subdivided) THENCALL subdivide(object, xmin, ymin, xmax, ymax)* If not subdivided, then determine if the box is full. If full, then* subdivide all the constituent objects and the object to be addedELSEIF (box is full) THENREPEAT for each constituent objectget constituent.sub.-- objectCALL subdivide(constituent.sub.-- object, xmin, ymin, xmax, ymax)ENDREPEATCALL subdivide(object, xmin, ymin, xmax, ymaxMark box as &#34;subdivided&#34;* if the box is not full, simply add the object to this boxELSEAdd object to this boxENDIFEND PROCEDURE insert.sub.-- object__________________________________________________________________________ 
    
     Spacial Queries 
     Once the graphics database has been spacially organized, searches based on object location (spacial queries) can be conducted with great efficiency. To perform a spacial query, the graphics space is successively subdivided, starting from the level of the undivided graphics space, comparing the midpoint of the current box to the coordinates of a query target (e.g. a cursor position) or a target characteristic (e.g. all objects residing in a given region of the graphics space) after each subdivision, until an indivisible box containing the query target or target characteristic is reached. If the query target or target characteristic encompasses more than one point, the above technique is applied to each quadrant containing part of the query target or target characteristic. 
     The preferred embodiment logic for finding the object nearest a given point (query target) is set forth below in PROCEDURE nearest --  object. A flowchart representation of PROCEDURE nearest --  object is set forth in FIG. 5. Referring to FIG. 5, PROCEDURE nearest --  object begins at 81 by initializing the current box to the entire graphics space. This ensures that, absent error, the query target is some place within the current box. At 82, subdivision of the current box into the smallest (indivisible) box containing the query target is begun. The subdivision process is designated generally by 83, and includes: finding the midpoint of the current box at 84; reducing the current box to the quadrant in which the query target lies at 85-89; and iterating on 82-89 until the smallest box containing the query target is found. 
     Having determined which box the query target is in, PROCEDURE nearest --  object continues by picking the closes object within the box. Designated generally by 90, this process includes initializing the nearest --  distance to infinity at 91 and then checking the distance between each object in the quadrant and the query target against nearest --  distance, retaining the lesser of nearest --  distance and the checked distance at each comparison (92-95). 
     
         __________________________________________________________________________PROCEDURE nearest.sub.-- object(x,y)*************************************************** PURPOSE:  Find the nearest object to the point (x,y)* INPUT:  the point (x,y)* OUTPUT:  the nearest object*************************************************** Initialize current box to the top level box; assumes that* point is inside the top level boxcur.sub.-- xmin = root.sub.-- xmincur.sub.-- ymin = root.sub.-- ymincur.sub.-- xmax = root.sub.-- xmaxcur.sub.-- ymax = root.sub.-- ymax* refine box until we get to a non-subdivided boxDO WHILE (current box is not subdivided)* find midpoint of the current boxxmid = (cur.sub.-- xmin + cur.sub.-- xmax) / 2ymid = (cur.sub.-- ymin + cur.sub.-- ymax) / 2* reduce the current box to the quadrant in which the point liesIF (x &gt; xmid) THENcur.sub.-- xmin = xmidELSEcur.sub.-- xmax = xmidENDIFIF (y &gt; ymid) THENcur.sub.-- ymin = ymidELSEENDIFcur.sub.-- ymax = ymidENDWHILE* at this point, the box has objects in it; pick the closest objectnearest.sub.-- distance = infinityREPEAT for each constituent objectget constituent.sub.-- objectIF      (distance to constituent.sub.-- object &lt; nearest.sub.-- distance.s   ub.--  THEN   nearest.sub.-- distance = distance to constituent.sub.-- object   nearest.sub.-- object = constituent.sub.-- object     ENDIFENDREPEATRETURN nearest.sub.-- objectEND PROCEDURE nearest.sub.-- object__________________________________________________________________________ 
    
     The preferred embodiment logic for finding all objects within the bounds of a given rectangle (target characteristic) and applying a designated procedure to them is set forth below in PROCEDURE rectangle --  find. A flowchart representation of PROCEDURE rectangle --  find is set forth in FIG. 6. Referring to FIG. 6, PROCEDURE rectangle --  find begins at 101 with the subdivision of the current box into the smallest box containing at least one of the sought-after objects. The subdivision process is designated generally by 102, and includes: finding the midpoint of the current box at 103; reducing the current box to a quadrant in which at least one of the sought-after objects lies at 104-111; and recursing on PROCEDURE rectangle --  find until the smallest box containing at least one of the sought-after objects is found. 
     Having found an indivisible quadrant containing at least one of the sought-after objects, PROCEDURE rectangle --  find continues by applying the desired procedure &#34;f&#34; to each object that is a constituent of the indivisible quadrant. Designated generally by 112, this process includes: extracting a constituent object from the indivisible quadrant at 113; checking whether it has already been processed through procedure &#34;f&#34; at 114, and if not previously processed, applying procedure &#34;f&#34; at 115 and marking the constituent object &#34;processed&#34; at 116; and finally repeating 112 until all constituent objects have been processed and marked. 
     Once an indivisible quadrant has been found and processed according to the above procedure, one level of recursion is vacated and 104-111 is re-entered in search of another indivisible quadrant containing at least one of the sought-after objects. The process is continued, recursing on PROCEDURE rectangle --  find and vacating levels of recursion, until all indivisible quadrants containing at least one of the sought-after objects have been found and processed as required by procedure &#34;f&#34;. 
     
         __________________________________________________________________________PROCEDURE rectangle.sub.-- find(f, cur.sub.-- xmin, cur.sub.-- ymin,     cur.sub.-- xmax, cur.sub.-- ymax,     rct.sub.-- xmin, rct.sub.-- ymin     rct.sub.-- xmax, rct.sub.-- ymax)*************************************************** PURPOSE:  Apply the procedure &#34;f&#34; to all of the object within the*      bounds of the rectangle (rct.sub.-- xmin, rct.sub.-- ymin,  rct.sub.-- xmax, rct.sub.-- ymax).*      The bounds of the current node are (xmin, ymin, xmax, ymax).* INPUT:  the function, rectangle boundary, and node boundary* OUTPUT:  None* EFFECTS:  Applies &#34;f&#34; to all objects in all nodes that overlap the  rectangle.**************************************************IF (current box is a subdivision) THEN* find midpoint of current boxxmid = (cur.sub.-- xmin + cur.sub.-- xmax) / 2ymid = (cur.sub.-- ymin + cur.sub.-- Wax) / 2* recurse on each quadrant that overlaps the rectangleIF (the rectangle overlaps rectangle (xmin, ymin, xmid, ymid)) THENCALL rectangle.sub.-- find(f, xmin, ymin, xmid, ymid,   rct xmin, rct ymin, rct xmax, rct ymax)ENDIFIF (the rectangle overlaps rectangle (xmin, ymid, xmid, ymax)) THENCALL rectangle.sub.-- find(f, xmin, ymid, xmid, ymax   rct.sub.-- xmin, rct.sub.-- ymin, rct.sub.-- xmax, rct.sub.--   ymax)ENDIFIF (the rectangle overlaps rectangle (xmid, ymin, xmax, ymid)) THENCALL rectangle.sub.-- find(f, xmid, ymin, xmax, ymid,   rct.sub.-- xmin, rct.sub.-- ymin, rct.sub.-- xmax, rct.sub.--   ymax)ENDIFIF (the rectangle overlaps rectangle (xmid, ymid, xmax, ymax)) THENCALL rectangle.sub.-- find(f, xmid, ymid, xmax, ymax,   rct.sub.-- xmin, rct.sub.-- ymin, rct.sub.-- xmax, rct.sub.--   ymax)ENDIFELSE* this node is a leaf node, apply &#34;f&#34; to each element.*  Objects are marked to insure that &#34;f&#34; will be applied exactly once*  to each object in the selected nodes. (Initially, all objects are*  not marked.)REPEAT for each constituent objectget constituent.sub.-- objectIF (constituent.sub. -- object is not already marked) THEN   CALL f(constituent.sub.-- object)   mark constituent.sub.-- objectENDIFENDREPEATENDIFENDPROCEDURE rectangle.sub.-- find__________________________________________________________________________ 
    
     Dynamic Selection 
     The spacial query technique of the present invention has applicability to a variety of display management functions requiring dynamic (rapid) selection of graphical objects. This set of functions includes those requiring dynamic preselection of graphical objects, such as preselection of the object nearest the cursor prior to actual manipulation of the object by a user, where the purpose of such preselection is to speed user turnaround time in the event the nearest object is actually selected by the user. 
     One application is for dynamic selection of the graphical object nearest the cursor (query target) as a user moves the cursor about the display screen by way of a mouse or other cursor control means. In this application, PROCEDURE nearest --  object is called to find the graphical object nearest the cursor&#39;s present position. Upon return, the object provided can be identified to the user by any of a number of means well known in the art, including highlighting, offset, blinking, thickness change, style change, outlining, inverse video, attachment of handles, appearance of name or attributes, arrows, pointing fingers, replication into a box, displacement to an offset location, halo, color change, or marquee (animating the pixels within the object, animating a marker or markers that move along the object, or outlining the object and animating the pixels of the outline). 
     Another application is for dynamic selection of the graphical object nearest a given point (query target) in response to a request from the computer system. The procedure is similar to that described above for dynamic selection in response to cursor movement, except the search point is provided by the system rather than the cursor. The object provided may be identified graphically as described above, or further processed (e.g. removed, enlarged, elongated, inverted) according to any system-provided function. 
     Another application is for dynamic selection of multiple objects crossed by the cursor (target characteristic) as a user moves the cursor about the display screen. In this application, PROCEDURE retangle --  find is called based on parameters identifying cursor movement over a given period of time. Upon return, the objects provided can be identified to the user or processed according to any system-provided function, as described above in connection with dynamic selection of single graphical objects. 
     Another application is for dynamic selection of all objects in a given region or window (target characteristic). In this application, PROCEDURE rectangle --  find is called based on parameters identifying the boundary of the window. Upon return, the objects provided can be identified to the user or processed according to any system-provided function, as described above in connection with dynamic selection of single graphical objects. 
     Another application is for dynamically regenerating portions of a raster image, such as regenerating objects when an obscuring window is removed, or regenerating objects damaged by the addition of an overlapping object or window (target characteristic). In this application, PROCEDURE retangle --  find is called based on parameters identifying the boundary of the obscuring window or the overlapping object or window. Upon return, the objects provided are selectively redrawn on the raster display; the needless redraw of unaffected objects is avoided. 
     Another application is for dynamic selection of intersections of objects (query target or target characteristic). In this application, PROCEDURE nearest --  object is called based on the location of a given object. Upon return, the nearest object provided is compared with the given object to determine whether the objects intersect. Alternatively, PROCEDURE rectangle --  find may be called to return all objects in the vicinity of the given object. Then each object provided is compared with the given object to determine whether the objects intersect. 
     Many additional applications exist for spacial queries. In fact, any display management process requiring dynamic selection of geometrical objects will benefit from spacial organization of the underlying graphics database and the resulting spacial query capability of the present invention. Thus, while the invention has been particularly described and illustrated with reference to a preferred embodiment, it will be understood by those skilled in the art that changes in the description or illustrations may be made with respect to form or detail without departing from the spirit and scope of the invention. Accordingly, the present invention is to be considered as encompassing all modifications and variations coming within the scope defined by the following claims.