pgfdeclareshape-demo.tex

\documentclass[tikz]{standalone}
\usetikzlibrary{shapes.multipart}

% AUTHOR: Bjarne C. Hiller
% DESCRIPTION: short example that demonstrates how to declare a new tikz multipart shape with pgfdeclare

% Bookmarks:
%   https://tikz.dev/
%   https://ctan.org/pkg/pgf

% pgfdeclare defines a new node shape like rect and circle on the base layer
% new shape can be used via: \node[shape=myshape] {};
% see: https://tikz.dev/base-nodes#sec-106.5

% for repeating shapes and figures, you can also use tikz's Pics
% that can be instantiated anywhere on a path
% see: https://tikz.dev/tikz-pics

% so when should you consider using pgfdeclareshape?
% - shapes can be referenced later and can declare its own anchors for line connectors
% - while nodes within a tikz picture can be referenced, the picture itself cannot
% - custom shapes are faster to construct compared to tikz pictures

% our shape should have some custom properties that we need to define
% pgf uses a key-value system to store parameters
% use \pgfkeys{key=value} to write new keys
% use \pgfkeysvalueof{key} to read a key's value
% the key-value system has a tree structure similar to UNIX file system's
% e.g., variables relevant for tikz are prefixed with the path \pgfkeysvalueof{/tikz/variable}
% actually, \tikzset{} internally uses \pgfkeys, but prepends every key with /tikz/
% there are some special key handlers, see the section Key Handlers in the pgf/tikz documentary
% e.g., variable/.initial=3 stores 3 as the initial value of the variable
% also, \pgfkeys{/path/.cd, a=3, b=4} "changes directory" to path to define /path/a and /path/b

% which color to fill the triforce with
\pgfkeys{
    /tikz/.cd, 
    triforce side/.initial=3cm,
}

% makeatletter is essential here
\makeatletter

% declaring new shapes with \pgfdeclareshape:
%   - required:
%       - shape of the node
%       - code for computing saved anchors and saved dimensions
%       - code for computing anchor positions from saved anchors
%   - optional:
%       - code for background and forground path
%       - code for things to be drawn before/behind background/foreground path
%       - list of nodeparts

% define new length and set values
% latex will handle lengths a little bit different
% e.g., 0.5 * 10cm will be replaced with 5 cm for lengths
\newlength{\triforcea}
\newlength{\triforceh}
\newlength{\triforcemidbot}
\newlength{\triforcemidtop}
\newlength{\triforcemidhalf}
\newlength{\triforcemidbotsmall}
\newlength{\triforcemidtopsmall}

\pgfdeclareshape{triforce}{
    % setup macros within triforce shape
    % e.g., to simplify parameter access
    \savedmacro\triforceparams{%
        % difference between def and edef:
        % def defines replacement of macro without expansion
        % e.g., \def\mymacro{\abc} will replace \mymacro with \abc
        % edef expands replacement at definition time
        % e.g., \edef\mymacro{\abc} will replace \mymacro with the replacement of \abc at time of definition
        % for some reason, fill color is not stored in /tikz/fill
        \edef\triforcefill{\tikz@fillcolor}
        %\edef\triforcefill{\pgfkeysvalueof{/tikz/triforce fill}}
        \setlength{\triforcea}{\pgfkeysvalueof{/tikz/triforce side}}
        % \pgfmathparse will evaluate math expressions
        % and store the result (without units!) as text in \pgfmathresult
        % compute triforce height given side length a
        %\pgfmathparse{sin(60)*\triforcea}
        %\setlength{\triforceh}{\pgfmathresult pt}
        % instead can use \pgfmathset to avoid redundancy
        \pgfmathsetlength{\triforceh}{sin(60)*\triforcea}
        % compute bot to center point
        \pgfmathsetlength{\triforcemidbot}{tan(30) * \triforcea / 2}
        % compute trigorce height above center point
        \pgfmathsetlength{\triforcemidtop}{\triforceh - \triforcemidbot}
        % compute center to half height
        \pgfmathsetlength{\triforcemidhalf}{\triforcemidtop - 0.5\triforceh}
        % compute bot to center for small triangle
        \pgfmathsetlength{\triforcemidbotsmall}{tan(30)*0.25\triforcea}
        % compute center to top for small triangle
        \pgfmathsetlength{\triforcemidtopsmall}{0.5\triforceh - \triforcemidbotsmall}
    }

    % next, we need to define saved anchors and our anchors!
    % anchors set the values of \pgf@x and \pgf@y to a specific position of the shape
    % however, there is a difference between SAVED anchors and anchors:
    % a \savedanchor is computed once when a node is declared
    % an \anchor is computed every time the anchor is requested
    % Notice that saved anchors are not automatically anchors!
    % If you get an error like this, you probably missed to declare the center anchor, which sould always be present:
    %   Package PGF Math Error: Unknown function `center' (in 'center')
    % 

    % by default, each node only has the part "text"
    % using \nodeparts, we can define new node parts by passing a list of part names
    % each nodepart XYZ requires an anchor XYZ and a textbox \pgfnodepartsXYZbox
    % the textbox for each nodepart is automatically placed on the corresponding anchor
    \nodeparts{tritop,trileft,triright}

    \savedanchor\centerpoint{
        % \pgfpointorigin sets \pgf@x and \pgf@y to the origin point
        % equivalent to \pgf@x=0pt and \pgf@y=0pt
        \pgfpointorigin%
    }

    % notice how savedanchor expects the name as command, but \anchor expects the name as argument!
    \anchor{center}{\centerpoint}

    \savedanchor\southpoint{
        \triforceparams
        \pgfpointorigin
        \advance\pgf@y by -\triforcemidbot
    }

    \anchor{south}{\southpoint}
    
    \savedanchor\northpoint{
        \triforceparams
        \pgfpointorigin
        \advance\pgf@y by \triforcemidtop
    }

    \anchor{north}{\northpoint}

    \savedanchor\westpoint{
        \triforceparams
        \pgfpointorigin
        \advance\pgf@x by -0.25\triforcea
        \advance\pgf@y by \triforcemidhalf
    }

    \anchor{west}{\westpoint}

    \savedanchor\eastpoint{
        \triforceparams
        \pgfpointorigin
        \advance\pgf@x by 0.25\triforcea
        \advance\pgf@y by \triforcemidhalf
    }

    \anchor{east}{\eastpoint}

    \anchorborder{
        \triforceparams
        \@tempdima=\pgf@x
        \@tempdimb=\pgf@y
        \pgfpointborderrectangle{\pgfpoint{\@tempdima}{\@tempdimb}}{\pgfpoint{0.5\triforcea}{0.5\triforceh}}
    }

    \savedanchor\southeastpoint{
        \triforceparams
        \pgfpointorigin
        \advance\pgf@x by 0.5\triforcea
        \advance\pgf@y by -\triforcemidbot
    }
    \anchor{southeast}{\southeastpoint}

    \savedanchor\southwestpoint{
        \triforceparams
        \pgfpointorigin
        \advance\pgf@x by -0.5\triforcea
        \advance\pgf@y by -\triforcemidbot
    }
    \anchor{southwest}{\southwestpoint}

    \anchor{tritop}{
        \northpoint
        \advance\pgf@x by -0.5\wd\pgfnodeparttritopbox
        \advance\pgf@y by -\triforcemidtopsmall
        \advance\pgf@y by -0.5\ht\pgfnodeparttritopbox
    }

    \anchor{trileft}{
        \southwestpoint%
        \advance\pgf@x by 0.25\triforcea
        \advance\pgf@x by -0.5\wd\pgfnodeparttrileftbox
        \advance\pgf@y by -0.5\ht\pgfnodeparttrileftbox
        \advance\pgf@y by \triforcemidbotsmall
    }

    \anchor{triright}{
        \southeastpoint%
        \advance\pgf@x by -0.25\triforcea
        \advance\pgf@x by -0.5\wd\pgfnodeparttrirightbox
        \advance\pgf@y by -0.5\ht\pgfnodeparttrirightbox
        \advance\pgf@y by \triforcemidbotsmall
    }

    % finally, let's draw the outline of our shape
    \backgroundpath{
        \triforceparams
        % the fillcolor is actually already set, if tikz's fill option was used
        % \pgfsetfillcolor{\tikz@fillcolor}
        % top triforce of power
        \northpoint
        \pgfmoveto{\pgfpoint{\pgf@x}{\pgf@y}} 
        \westpoint
        \pgfpathlineto{\pgfpoint{\pgf@x}{\pgf@y}} 
        \eastpoint
        \pgfpathlineto{\pgfpoint{\pgf@x}{\pgf@y}} 
        \pgfusepath{fill}
        % left triforce of wisdom
        \westpoint
        \pgfmoveto{\pgfpoint{\pgf@x}{\pgf@y}} 
        \southwestpoint
        \pgfpathlineto{\pgfpoint{\pgf@x}{\pgf@y}} 
        \southpoint
        \pgfpathlineto{\pgfpoint{\pgf@x}{\pgf@y}} 
        \pgfusepath{fill}
        % right triforce of courage
        \eastpoint
        \pgfmoveto{\pgfpoint{\pgf@x}{\pgf@y}} 
        \southpoint
        \pgfpathlineto{\pgfpoint{\pgf@x}{\pgf@y}} 
        \southeastpoint
        \pgfpathlineto{\pgfpoint{\pgf@x}{\pgf@y}} 
        \pgfusepath{fill}
    }
}

% return back to normal
\makeatother

\begin{document}

% for each node part XYZ a custom shape needs to declare:
%   - an anchor XYZ where the text box will be placed
%   - its own text box \pgfnodepartXYZbox
\newbox\pgfnodeparttritopbox
\newbox\pgfnodeparttrileftbox
\newbox\pgfnodeparttrirightbox

\begin{tikzpicture}
    % draw triforce
    \node[draw, shape=triforce, triforce side=7cm, fill=yellow] at (0,2.5) (triforce) {\nodepart{tritop} Power \nodepart{trileft} Wisdom \nodepart{triright} Courage};
    \node[draw, circle, fill=blue!50!white, minimum height=2cm] at (triforce.center) {};
    % draw dark triforce
    \node[draw, shape=triforce, triforce side=7cm, fill=yellow!40!black, rotate=180] (darkforce) at (0,-2.5) {\nodepart{tritop} Oppression \nodepart{trileft} Apathy \nodepart{triright} Ignorance};
    \node[draw, circle, fill=red!50!black, minimum height=2cm] at (darkforce.center) {};
    % draw lines to connect anchors
    \draw (triforce.south) -- (darkforce.south);
    \draw (triforce.southwest) -- (darkforce.southeast);
    \draw (triforce.southeast) -- (darkforce.southwest);
\end{tikzpicture}

\end{document}