For displaying any map feature the system uses some style. There are two possibilities:
a feature can have its specific style specified for this particular feature and not depending on the style of other features of the same type;
there may be no specific style specified for a feature; in this case a feature inherits a common style specified for all features of the present component (layer or feature type).
In most cases style inheritance is used, which enables similar display of features of the same type. Specifying a specific style may be useful for unique features, which should differ from other features of the same type when they are displayed.
The style of area features, unlike point or line feature style, includes three elements: fill style, hatch style and style of the boundary line. Every style element is set not depending on others. For example, a feature may have its specific fill style, but inherit hatch and line styles.
The style of text features includes font and color of the text which are set independently from each other.
Map or theme component (that is, a layer or feature type) can also either have its specific style, or inherit the style of a more general component. Thus, it is possible to inherit the style from a component to a particular feature and from a more general component to the less general one.
Displaying styles specified for feature types and layers are stored in the system as a part of map structure description.
The most general style values for ObjectLand are the ones specified for map layers. These values can be inherited by map theme layers, then by the order of inheritance come feature types of map layer, feature types of theme layer, and at last – particular features.
For example, let the map “N-City” have a layer “Municipal Economy” and the layer “Municipal Economy” have feature type “Gas Pipeline”. Then let the browsed theme of this map include the layer “Municipal Economy” and the type “Gas Pipeline”. How will the style of the line displaying gas pipelines be determined?
First of all, if for a particular gas pipeline feature its specific line style is specified, this very style will be used for display. If the feature doesn't have its specific style, line style specified for feature type “Gas Pipeline” of the layer “Municipal Economy” of the browsed theme is inherited. If this type doesn't have its specific line style, the style set for feature type “Gas Pipeline” of the layer “Municipal Economy” of map “N-City” will be inherited. If this type doesn't have its specific style either, the style of the whole layer “Municipal Economy” of the browsed theme will be inherited. After that by order of inheritance comes line style of the whole layer “Municipal Economy” of map “N-City”.
Finally, if the style is not specified for the map layer, the system style chosen by the system developers is inherited. It includes a set of style elements of the most simple appearance.
The elements of the system style are not part of any libraries, they don't have names and cannot be modified.
When creating a new map layer or new feature type the system does not specify a specific style for it. For a new component inherited values of all style elements will be used until the user specifies this specific style.
Simplified style inheritance scheme is shown at figure 10-1. In chapter 37 “Work with filters” a more detailed scheme where filter themes influence is taken into account is presented.
Figure 10-1. Style inheritance scheme
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