PHIGS and PHIGS+, the Programmer's Hierarchical Interactive Graphics System. Both systems support graph- ics primitives, such as lines, polygons, and character strings, and their attributes. The GKS system allows for groupings into segments with no nesting capabilities. PHIGS supports geometrical transformations(i.e, scaling, anslating, and rotating )and a database structure that allows for selective editing and manipulation of the picture. PHIGS runs best when there is hardware support for the transformation, clipping, and rendering features. Other software include the cross-platform OpenGL, which is a low-level graphics rendering and imaging library, and Inventor, which is object-oriented and built on top of Open GL [ Reynolds and Danielson, 1996) In traditional graphical systems development, image data are stored either as Cartesian coordinates rectors. These data are manipulated through the geometrical transformations of scaling, translating, and rotating in a reference system known as the world coordinate system(wCS). The units of the WCS system might be inches, millimeters, or miles. Physical devices use their own coordinate systems known as screen coordinate systems(SCS). In order to ready the image for display, a viewing transformation takes place, which changes the image data in the wCS to its corresponding device-specific screen coordinates in SCS. A window or portion of the world picture is chosen to be shown in an area of the display known as the viewport. Because some of the data in the world could be outside the window, a clipping operation is necessary. Clipping will eliminate any data points outside the window. These values are then converted to an intermediate coordinate system known as the normalized device coordinate system(NDC). Values in this system are in the range of 0 to 1. Because a iewport may be any portion of the display area and the image could be displayed on more than one device, the NDC values are easily adjusted to screen coordinates. In 3-D, the clipping volume uses the viewing transformation which must take into account the view reference point (i.e, the position from which an object is to be viewed) and the perspective or parallel projection(i. e, the conversion from the object's 3-D coordinates to the screens 2-D coordinates Solid Modeling Packages Feature-based systems such as solid or geometric modeling rather than mathematical-based systems form the basis of some CAD systems. Solid modeling(SM)systems use constructive solid geometry to build complicated bjects. These systems have a descriptive language which uses a database of 3-D primitive objects such as block, cylinder, sphere, wedge, cone, and torus. These solids are combined to form other solids using the set operators of union, intersection, and difference. The resultant object can then be named, saved, and positioned into picture or drawing. Attributes stored with the objects allow them to be displayed in wire-frame format or as a completely rendered image. Representative SM systems are MAGI (Mathematical Applications Group, Inc. ynthavision, PADL-2(Production Automation Project), GM Solid(a proprietary package of General Motors) and McDonnell Douglas's UNISOLID [Teicholz, 1985] Object-Oriented Pl Object-oriented programming is the paradigm for designing and implementing software and is particularly important in computer graphics. An engineering approach, these languages allow software to be constructed from reusable, interchangeable, and extensible parts [Cunningham et al., 1992]. Class libraries of graphical objects are being developed. Classes of objects are defined in terms of what an object can do(i.e, what actions and reactions it might produce) and communicate via messages. Subclasses inherit actions or characteristics of the superclass. For example, a robot could be constructed from instances of such classes as legs, arms, and head Each class would have actions defined for it(e.g, a head would be able to nod up and down or shake from de to side). An instance of a head in the object robot would preserve these characteristics. Representative object-oriented languages are Smalltalk, C++, Objective-C, Actor, and Object Pascal Plotting and Page Description Languages Plotting packages, such as ISSCO's DISSPLA and Precision Visuals'DI-3000, consist of routines that are callable from a high-level program. These packages handle 2-D and 3-D images and generally display them in a wire- e 2000 by CRC Press LLC© 2000 by CRC Press LLC PHIGS and PHIGS+, the Programmer’s Hierarchical Interactive Graphics System. Both systems support graph￾ics primitives, such as lines, polygons, and character strings, and their attributes. The GKS system allows for groupings into segments with no nesting capabilities. PHIGS supports geometrical transformations (i.e., scaling, translating, and rotating) and a database structure that allows for selective editing and manipulation of the picture. PHIGS runs best when there is hardware support for the transformation, clipping, and rendering features. Other software include the cross-platform OpenGL, which is a low-level graphics rendering and imaging library, and Inventor, which is object-oriented and built on top of Open GL [Reynolds and Danielson, 1996]. In traditional graphical systems development, image data are stored either as Cartesian coordinates or as vectors. These data are manipulated through the geometrical transformations of scaling, translating, and rotating in a reference system known as the world coordinate system (WCS). The units of the WCS system might be inches, millimeters, or miles. Physical devices use their own coordinate systems known as screen coordinate systems (SCS). In order to ready the image for display, a viewing transformation takes place, which changes the image data in the WCS to its corresponding device-specific screen coordinates in SCS. A window or portion of the world picture is chosen to be shown in an area of the display known as the viewport. Because some of the data in the world could be outside the window, a clipping operation is necessary. Clipping will eliminate any data points outside the window. These values are then converted to an intermediate coordinate system known as the normalized device coordinate system (NDC). Values in this system are in the range of 0 to 1. Because a viewport may be any portion of the display area and the image could be displayed on more than one device, the NDC values are easily adjusted to screen coordinates. In 3-D, the clipping volume uses the viewing transformation which must take into account the view reference point (i.e., the position from which an object is to be viewed) and the perspective or parallel projection (i.e., the conversion from the object’s 3-D coordinates to the screen’s 2-D coordinates). Solid Modeling Packages Feature-based systems such as solid or geometric modeling rather than mathematical-based systems form the basis of some CAD systems. Solid modeling (SM) systems use constructive solid geometry to build complicated objects. These systems have a descriptive language which uses a database of 3-D primitive objects such as block, cylinder, sphere, wedge, cone, and torus. These solids are combined to form other solids using the set operators of union, intersection, and difference. The resultant object can then be named, saved, and positioned into a picture or drawing. Attributes stored with the objects allow them to be displayed in wire-frame format or as a completely rendered image. Representative SM systems are MAGI (Mathematical Applications Group, Inc.), Synthavision, PADL-2 (Production Automation Project), GM Solid (a proprietary package of General Motors), and McDonnell Douglas’s UNISOLID [Teicholz, 1985]. Object-Oriented Programming Object-oriented programming is the paradigm for designing and implementing software and is particularly important in computer graphics. An engineering approach, these languages allow software to be constructed from reusable, interchangeable, and extensible parts [Cunningham et al., 1992]. Class libraries of graphical objects are being developed. Classes of objects are defined in terms of what an object can do (i.e., what actions and reactions it might produce) and communicate via messages. Subclasses inherit actions or characteristics of the superclass. For example, a robot could be constructed from instances of such classes as legs, arms, and head. Each class would have actions defined for it (e.g., a head would be able to nod up and down or shake from side to side). An instance of a head in the object robot would preserve these characteristics. Representative object-oriented languages are Smalltalk, C++, Objective-C, Actor, and Object Pascal. Plotting and Page Description Languages Plotting packages, such as ISSCO’s DISSPLA and Precision Visuals’ DI-3000, consist of routines that are callable from a high-level program. These packages handle 2-D and 3-D images and generally display them in a wire￾frame format