Text Rendering Overview

(For textplacement branch)

Abstract

This page aims to give an explanation of how all the parts in rendering a TextSymbolizer play together. Only important functions and members are documented here.

TextSymbolizer

As it's only property contains a pointer to a text_placements object.

text_placements

(Abstract class) This object handles the management of all TextSymbolizer properties. It can be used as a base class for own objects which implement new processing semantics. Basically this class just makes sure a pointer of the right class is returned by the get_placement_info call.

Functions

• get_placement_info: (Abstract function) Get a text_ placement_info object to use in rendering. The returned object creates a list of settings which is used to try to find a placement and stores all information that is generated by the placement finder.

This function usually is implemented as

{
     return text_placement_info_ptr(new text_placement_info_XXX(this));
}

• get_all_expressions: Get a list of all expressions used in any placement. This function is used to collect attributes.

Members

All properties are grouped into a class text_symbolizer_properties accessible via public member properties. They are used by subclasses as their default settings.

Implemented subclasses

• text_placements_dummy Always takes the default value.
• text_placement_simple Parse a simple string and created placement based on this string.
• text_placement_list Take placements from a list

text_placement_list

(Abstract class) Generate a possible placement and store results of placement_finder. This placement has first to be tested by placement_finder to verify it can actually be used.

Functions

• Constructor: Takes the parent text_placements object as a parameter to read defaults from it.
• next: (Abstact function) Get next placement. This function is also called before the first placement is tried.
• init: Initialize values used by placement finder. These values are provided by the renderer and can't be derived from TextSymbolizer properties.

Members:

• properties: Properties actually used by placement finder and renderer. Values in here are modified each time next() is called.
• placements: List of text_paths returned by placement finder. One for each placement.
• Some values from the renderer.
• Values returned by placement_finder.

text_path

List of all characters and their positions and formats for a placement. Produced by placement_finder, consumed by the renderer.

text_processor

Stores formating information and uses this to produce formated text for a given feature.

Functions

• process: Takes a feature and produces formated text as output. The output processed_text object has to be created by the caller and passed in for thread safety.
• get_all_expressions: Get a list of all expressions used in any placement. This function is used to collect attributes.
• from_xml: Construct object from XML.
• to_xml: Write object to XML ptree.

text_symbolizer_properties

Contains all text symbolizer properties which are not directly related to text formating.

Members

• processor: text_processor object which contains everything related to text formating.

Functions

• set_values_from_xml: Load all values and also the processor object from XML ptree.
• to_xml: Save all values to XML ptree (but does not create a new parent node!).