GPHIGS is a graphic system for storing and handling data organized as structures within the CSS.
The CSS is the core of GPHIGS implementation. 2D or 3D graphics objects are generated in this graphics data base for display on the workstation.
The basic unit is called an element. Elements are put together in structures. A special element also makes it possible to connect structures in a hierarchical network. This type of element makes it possible for a structure to refer to another structure. This reference is used when a structure is plotted on a workstation, as explained below.
A structure is called a parent if it refers to other structures. A structure is called a child if it is referred to by other structures.
The elements in a structure are one of the following types :
Graphics primitives : e.g. the ppolyline primitive is defined by a list of points joined by straight segments when the element is plotted.
Primitive attributes : e.g. the pset_line_colr_ind attribute which specifies the color index to be used during plotting the following ppolyline primitives in the structure
Transformation : e.g. a pset_local_tran element introducing a transformation matrix (e.g. a translation) which is used to transform all the points defining a graphics primitive when the primitives are plotted
View index : this element indicates which view should be used to plot the graphics primitives following this element in the structure
Modelling clipping : this type of element clips graphics primitives in one or more planes in space. This clipping is carried out when the structure is plotted
Pick identifier : this type of element is used to recover an identifier during a graphics primitive pick operation (e.g. by a mouse)
Label : this element is useful for structure management : marking, modification, or destruction of structure elements
Name set : when a structure is plotted, this element is used to make certain primitives in the structure visible or invisible (and detectable or indetectable) even if they are in the "field of vision" of the current view
Application data : this element stores information specific to the application using PHIGS in a given structure.
Execute structure this element makes it possible for one structure to refer to another structure. The only parameter of the pexec_struct function is the identifier of the structure to be refered. Therefore, this element makes it possible to build a CSS corresponding to a structure network. Without this element, the CSS is merely a set of unconnected structures.
The following diagram shows the correspondence between an image of the contents of several structures and an image of the CSS in graph form. Each * represents any structure element
Elements such as label, pick identifier, or application data have no effect on the process that generates structure plotting, so they will not change the image corresponding to the structure.
These elements are, however, quite useful. In particular, label elements are very useful in structure construction and modification phases.