Demystifying Pgfkeys: A Deep Dive Into Tikz And Pgf’S Style And Option Parsing System

What is pgfkeys and Why is it Useful?

The pgfkeys package provides a flexible and extensible method for organizing, storing, and manipulating graphics styles and options in TikZ and PGF. At its core, pgfkeys allows users to define key-value pairs that set graphical parameters and styles. It acts as a central dispatching system to process keys consistently, validate inputs, set defaults, and execute custom key handlers.

Using pgfkeys for TikZ graphics organization provides several key benefits:

  • Centralize style definitions instead of scattering options throughout a graphic
  • Reuse sets of keys easily for consistency across graphics
  • Modify multiple parameters at once by changing key values
  • Create custom styles, libraries, and options simply by defining new key handlers
  • Reduce duplication by using inheritance and key value defaults
  • Handle complex nested graphics parameters via hierarchical key structures

In essence, pgfkeys protocols streamline working with PGF/TikZ by codifying APIs for setting graphical styles and automatically handling the underlying parameter processing in a consistent way.

Anatomy of a pgfkeys Call

At their simplest, pgfkeys provide a mechanism to set key-value pairs that establish attributes and styles for TikZ graphics. The structure follows this format:


This sets key1 to value1 and key2 to value2. The \pgfkeys command dispatches processing of the keys to establish the graphic parameters. For example:

  line width=2pt,

Would set the line width to 2pt and color to red. Multiple keys can be set in one call and are processed sequentially. If no value is provided, the key is assumed to have a true boolean value.

In addition to simple key-value pairs, pgfkeys has robust support for defaults. A key can point to another key for its default using /.get:

  line width/.get=\mylinewidth,

Here line width would take on the value of \mylinewidth, establishing internal references to enforce consistency and reduce duplication.

Defining New Key Sets

Beyond setting individual graphic parameters, groups of keys can be organized into coherent “key sets” that function as reusable styles or libraries. This allows abstraction of low-level TikZ options into higher level semantic constructs.

New key sets are defined using:

  /myset/.is family, /tikzset

This establishes /myset as a family of keys in the /tikzset domain. Keys in /myset would then be accessible as:


Defining keys in this manner enables several powerful capabilities:

  • Grouping - Related keys can be organized together in a common set
  • Namespacing - Key groups' names can convey semantics describing their purpose
  • Sharing - Consistent key interfaces improve sharing of graphics libraries
  • Inheritance - Key value resolution respects hierarchical namespaces
  • Validation - Key handlers can validate data types or value ranges

By leveraging these facets of the pgfkeys system, sophisticated TikZ tools and graphic styles can be codified into simple key-based APIs.

Accessing and Manipulating Keys

In addition to setting keys, it is often useful to programmatically get or transform key values. This enables creating graphics that dynamically respond to user options.

Key values can be accessed in code using:

\pgfkeysvalueof{/tikz/line width}

This expands to the value of the /tikz/line width key. Computed expressions can rely on key data:

\def\linewidth{0.5*\pgfkeysvalueof{/tikz/line width}}

Here, \linewidth is defined based on the current line width. In addition to accessing absolute key values, relative adjustments are also possible:

\pgfkeys{/tikz/line width/.add=0.2pt}

This increments /tikz/line width by 0.2pt, modifying keys dynamically. Such capabilities enable TikZ graphics to have programmatic relationships to user options set via pgfkeys.

Advanced pgfkeys Techniques

Leveraging its underlying power, advanced pgfkeys techniques exist for creating extensible and customizable TikZ tools:

  • Inheritance - Key sets can inherit default values from hierarchical parents
  • Metadata - Extra data like help text can be encoded in keys
  • Interfaces - Key handlers enable complex logic and APIs
  • Dispatch - Key changes trigger custom callbacks into user code
  • Control - Looping and conditionals for complex key processing

By harnessing these advanced features, pgfkeys can drive complex TikZ graphics generation from simple user option specifications.

Putting pgfkeys to Work

Understanding pgfkeys’ capabilities is best supported by practical examples. Some usage areas that highlight effective applications of its core competencies include:

  • TikZ Libraries - Lightweight tools can use pgfkeys interfaces for simple access to complex algorithms. E.g. graph layout engines accessed via style and node placement keys.
  • Configurable Plots - Scientific plots with full style/theme control through hierarchical key groups controlling coordinate scaling, axes, colors, etc.
  • Graphics Engines - Keys hook into programmatic graphics generation routines, keeping rendering parameterized.

These types of demonstration cases help ground the understanding of pgfkeys facilities in tangible use cases that can inform additional TikZ tool and library development.

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