Once the structure network has been produced, the next stage consists of displaying the set of structures in the network. The PPOST function links a structure network to a workstation. No structure in the network can be displayed until this link is set up.
PPOST includes 3 parameters : the workstation identifier (WKID) the network root structure identifier (STRID) and an order of priority if several networks are to be displayed. When the PPOST (WKID, STRID, priority), function is called, the structure STRID is said to be posted on the workstation WKID.
In order to display all the structures in a network, it is sufficient to post the root structure of that network (it is not necessary to post all the structures in the network). If the CSS is made up of several networks (when the CSS is unconnected), it is necessary to post the roots of all the networks to display them. The order in which these networks are displayed is determined by the priority (3rd PPOST function parameter).
There is still a link between the structure network and the workstation, even after the network has been modified. It is not necessary to call PPOST again after a network has been modified. To cancel a link between a network and a workstation, call PUPOST (which breaks a network link) or PUPAST (which breaks all the links on a workstation) or change the network root structure identifier.
Traversal and attribute scope
The fact that a structure has been posted does not usually imply that it is displayed on the workstation. The PRST (WKID, ...) function must be called to display the structure. This function displays all the structures in all the posted networks on the workstation.
PRST activates a process called traversal which works through a structure network, generating a display of the graphics elements on the workstation.
Traversal is the process which interprets and executes the structure element, applies the geometrical transformations, associates attributes with primitives, and displays the resulting image. Traversal of a network starts at element 1 in the root structure, elements are then processed sequentially, and the process ends with the last element in the root structure.
When a network is made up of several structures (the root has at least one PEXST element) traversal interprets the first elements in the root structure as far as the first PEXST(S) element. Traversal of the root structure is interrupted, and traversal continues with the structure identifier S. When this is completed, traversal of the root structure resumes at the next element.
The following diagram shows an example of a CSS with (elements e**) which may be of any type except PEXST
If the PPOST (WKID, S3, 0.) function is called, traversal, set off by calling PRST interprets the structure element e** in the following order :
e31 - e41 - e42 - e43 - e44 - e45 - e33 - e21- e22 - e41 - e42 - e43 - e44 - e45 - e24 - e25 - e35
Note that, in the preceding example, if certain e4* elements are graphics primitives, they will be displayed twice on the workstation WKID.
If the PPOST (WKID, S2, 1.) function is then called, traversal set off by calling PRST interprets the structure element e** in the following order :
e31 - e41 - e42 - e43 - e44 - e45 - e33 - e21- e22 - e41 - e42 - e43 - e44 - e45 - e24 - e25 - e35
e21- e22 - e41 - e42 - e43 - e44 - e45 - e24 - e25
Two structure networks (one with the root S3 and the other root S2) have been posted on the workstation WKID, so, in this case, traversal will work through structure S4 3 times. N.B. : traversal starts by displaying the network with the lowest priority (the 3rd PPOST function parameter).
During traversal , the system keeps track of the current value attributes. The current value of an attribute may be a default value, an inherited value, or an assigned value which is the result of an attributre assign element. Each time traversal analyzes a graphics primitive, it applies the current value and transformation attributes.
When traversal of a parent structure meets a PEXST (child) element, traversal is suspended, current attributes are saved in a pile and traversal of the child structure is initiated. When traversal is finished, the attributes in the pile are restored and traversal of the parent structure starts again, from the point at which it was suspended.
This concept of inherited attributes makes it possible to call a structure from several parent structures with different attribute values each time. The child structure may also modify the value and transformation attributes without affecting the environment of the parent structure.
Example of inherited PSTXCI text colr index and PSPLCI line colr index attributes in the following CSS :
the primitive PPL (nump1,...) is displayed with a line color index =1
the primitive PTX (...,'text1') is displayed with a text color index =3
the primitive PPL (nump3,...) is displayed with a line color index =1
the primitive PPL (nump5,...) is displayed with a line color index =2
the primitive PTX (...,'text2') is displayed with a text color index =3
the primitive PPL (nump6,...) is displayed with a line color index =4
the primitive PPL (nump7,...) is displayed with a line color index =4
the primitive PPL (nump4,...) is displayed with a line color index =2
the primitive PPL (nump2,...) is displayed with a line color index =1
N.B. : In this example, as the PSTXCI function is only present at the start of the S3 root structure, the text color attribute is taken into account by all the text primitives in the 3 structures.