使用 `arrows.meta` TikZ 库自定义箭头

Question

本着这个问题，如何使用新的arrows.metaTikZ 库制作自定义箭头尖？我想制作一个半圆形的尖头，open理想情况下，它可以接受选项，也许还有其他选项，例如长度（或半径或圆形箭头尖接受的任何选项）。

Answer 1

声明新箭头的文档相当全面，但示例中似乎存在一些错误。我不是这方面的专家，arrows.meta但以下似乎是开始的最低要求：

\documentclass[tikz,border=0.25cm]{standalone}
\usetikzlibrary{arrows.meta}
\makeatletter
\pgfdeclarearrow{
  name=semicircle,
  parameters={ 
    \the\pgfarrowlength,%
    \ifpgfarrowopen o\fi%  
  },
  setup code={
    % The line end value:
    \pgfarrowssettipend{0pt}
    \pgf@x=-.25\pgfarrowlength
    \advance\pgf@x by-.5\pgflinewidth
    % The hull point:
    \pgfutil@tempdima=0.25\pgfarrowlength
    \advance\pgfutil@tempdima by.5\pgflinewidth
    \pgfarrowssetlineend{\pgf@x}
    \pgfarrowssetvisualbackend{-.25\pgfarrowlength}
    \pgfarrowssetbackend{-.75\pgfarrowlength}
    \pgfarrowssavethe\pgfarrowlength
    \pgfarrowlinewidth=\pgflinewidth
    \pgfarrowssavethe\pgfarrowlinewidth
    \pgfarrowshullpoint{0pt}{0pt}
    \pgfarrowshullpoint{0pt}{-\pgfutil@tempdima}
    \pgfarrowshullpoint{0pt}{\pgfutil@tempdima}
  },
  drawing code={
    \pgfpathmoveto{\pgfpoint{-.25\pgfarrowlength-.5\pgflinewidth}{.25\pgfarrowlength}}
    \pgfpathlineto{\pgfpoint{-.25\pgfarrowlength-.5\pgflinewidth}{-.25\pgfarrowlength}}
    \pgfpatharc{-90}{90}{0.25\pgfarrowlength}
    \pgfpathclose
    \ifpgfarrowopen\pgfusepathqstroke\else\ifdim\pgfarrowlinewidth>0pt\pgfusepathqfillstroke\else\pgfusepathqfill\fi\fi
  },
  defaults={ length=10pt }
}

\begin{document}

\begin{tikzpicture}
\draw [help lines] grid (3,2);
\draw [semicircle-{semicircle[open]}] (0,0) -- (3,2);
\draw [very thick, {semicircle[length=40pt,open]}-{semicircle[length=20pt]}] 
  (0,2) -- (3,0);
\end{tikzpicture}

\end{document}

Answer 2

我对马克的答案做了一些改进，以便半圆与圆完全一致。

\pgfdeclarearrow{
  name=semicircle,
  defaults={
    length  = +2.39365pt +3.191538,
    line width = +0pt 1 1,
  },
  setup code={
    % Cap the line width at 1/2th of the length
    \[email protected]\pgfarrowlength
    \ifdim\pgf@x<\pgfarrowlinewidth
      \pgfarrowlinewidth\pgf@x
    \fi
    \[email protected]\pgfarrowlength\advance\pgfutil@tempdima by-.5\pgfarrowlinewidth
    \[email protected]\pgfarrowlength\advance\pgfutil@tempdimb by.5\pgfarrowlinewidth
    \ifpgfarrowreversed
        \pgfarrowssetlineend{.5\pgfarrowlength\advance\pgf@x by-.5\pgfarrowlinewidth}
    \else
         \pgfarrowssetlineend{.5\pgfarrowlinewidth}
    \fi
    \pgfarrowssettipend{\pgfutil@tempdimb}
    % The hull:
    \pgfarrowsupperhullpoint{0pt}{.5\pgfarrowlength}
    \pgfarrowsupperhullpoint{0.20710678118\pgfarrowlength}{.5\pgfarrowlength}
    \pgfarrowsupperhullpoint{0.5\pgfarrowlength}{0.20710678118\pgfarrowlength}
    % The following are needed in the code:
    \pgfarrowssavethe\pgfarrowlinewidth
    \pgfarrowssavethe\pgfarrowlength
    \pgfarrowssavethe\pgfutil@tempdima
    \pgfarrowssavethe\pgfutil@tempdimb
  },
  drawing code = {
    \pgfsetdash{}{+0pt}
    \ifpgfarrowroundjoin\pgfsetroundjoin\else\pgfsetmiterjoin\fi
    \ifdim\pgfarrowlinewidth=\pgflinewidth\else\pgfsetlinewidth{+\pgfarrowlinewidth}\fi
    \pgfpathmoveto{\pgfpoint{.5\pgfarrowlinewidth}{\pgfutil@tempdima}}
    \pgfpathlineto{\pgfpoint{.5\pgfarrowlinewidth}{-\pgfutil@tempdima}}
    \pgfpatharc{-90}{90}{\pgfutil@tempdima}
    \pgfpathclose
    \ifpgfarrowopen\pgfusepathqstroke\else\ifdim\pgfarrowlinewidth>0pt\pgfusepathqfillstroke\else\pgfusepathqfill\fi\fi
  },
  parameters = {
    \the\pgfarrowlinewidth,%
    \the\pgfarrowlength,%
    \ifpgfarrowopen o\fi%
    \ifpgfarrowroundjoin j\fi%
  },
}