在 tikz 中绘制四分之一封闭轮廓

Question

我正在尝试绘制以下内容：

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{decorations.markings}

\begin{document}

\begin{tikzpicture}[decoration={markings,
mark=at position 0.5cm with {\arrow[line width=1pt]{>}},
mark=at position 2cm with {\arrow[line width=1pt]{>}},
mark=at position 7.85cm with {\arrow[line width=1pt]{>}},
mark=at position 9cm with {\arrow[line width=1pt]{>}}
}
]
% The axes
\draw[help lines,->] (-3,0) -- (3,0) coordinate (xaxis);
\draw[help lines,->] (0,-1) -- (0,3) coordinate (yaxis);

% The path
\path[draw,line width=0.8pt,postaction=decorate] (1,0) node[below] {$\varepsilon$} -- (2,0) node[below] {$r$} arc (0:180:2) -- (-1,0) arc (180:0:1);

% The labels
\node[below] at (xaxis) {$x$};
\node[left] at (yaxis) {$y$};
\node[below left] {$O$};
\node at (0.5,1.2) {$C_{\varepsilon}$};
\node at (1.5,1.8) {$C_{r}$};
\end{tikzpicture}

\end{document}

除了只有半个圆盘的一半，换句话说，就是这条路径，但面积只有一半，沿着虚轴切开一半。我似乎无法在\path不破坏形状的情况下改变的参数。如果这是非常基础的，我很抱歉，我是 TiKz 的新手。

Answer 1

由于您绘制的范围是从 0 度到 180 度（即，半径为 2，范围从 0 度到 180 度），因此该圆弧是一个完整的半圆0:180:2。要使其成为四分之一圆，请将其绘制的范围从 0 度到 90 度（0:90:2即，半径为 2，范围从 0 度到 90 度）。

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{decorations.markings}

\begin{document}

    \begin{tikzpicture}[decoration={markings,
        mark=at position 0.5cm with {\arrow[line width=1pt]{>}},
        mark=at position 2cm with {\arrow[line width=1pt]{>}},
        mark=at position 7.85cm with {\arrow[line width=1pt]{>}},
        mark=at position 9cm with {\arrow[line width=1pt]{>}}
    }
    ]
    % The axes
    \draw[help lines,->] (-3,0) -- (3,0) coordinate (xaxis);
    \draw[help lines,->] (0,-1) -- (0,3) coordinate (yaxis);

更改了行

    % The path
    \path[draw,line width=0.8pt,postaction=decorate]
         (1,0) node[below] {$\varepsilon$} --
         (2,0) node[below] {$r$}
         arc (0:90:2) --
         (0,1) arc (90:0:1);

其余不变

    % The labels
    \node[below] at (xaxis) {$x$};
    \node[left] at (yaxis) {$y$};
    \node[below left] {$O$};
    \node at (0.5,1.2) {$C_{\varepsilon}$};
    \node at (1.5,1.8) {$C_{r}$};
    \end{tikzpicture}

\end{document}

或者

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{decorations.markings}

\begin{document}

    \begin{tikzpicture}[decoration={markings,
        mark=at position 0.5cm with {\arrow[line width=1pt]{>}},
        mark=at position 2.8cm with {\arrow[line width=1pt]{>}},
        mark=at position 4.75cm with {\arrow[line width=1pt]{>}},
        mark=at position 5.8cm with {\arrow[line width=1pt]{>}}
    }
    ]
    % The axes
    \draw[help lines,->] (-3,0) -- (1,0) coordinate (xaxis);
    \draw[help lines,->] (0,-1) -- (0,3) coordinate (yaxis);

    % The path
    \path[draw,line width=0.8pt,postaction=decorate]
    (0,1) node[right] {$\varepsilon$} --
    (0,2) node[right] {$r$}
    arc (90:180:2) --
    (-1,0)
    arc (180:90:1);
    \end{tikzpicture}

\end{document}

Answer 2

或者你想得到

代码：

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{decorations.markings}

\begin{document}


\begin{tikzpicture}[decoration={markings,
mark=at position 0.5cm with {\arrow[line width=1pt]{>}},
mark=at position 2cm with {\arrow[line width=1pt]{>}},
mark=at position 4.75cm with {\arrow[line width=1pt]{>}},% <- changed
mark=at position 6cm with {\arrow[line width=1pt]{>}}% <-changed
}
]
% The axes
\draw[help lines,->] (-3,0) -- (3,0) coordinate (xaxis);
\draw[help lines,->] (0,-1) -- (0,3) coordinate (yaxis);

% The path
\newcommand\qangle{45}
\path[draw,line width=0.8pt,postaction=decorate] 
  (\qangle:1)coordinate(e)
  -- ([turn]0:1)coordinate(r)
  arc (\qangle:{\qangle+90}:2)coordinate[midway](Cr)
  -- ([turn]90:1) 
  arc ({\qangle+90}:\qangle:1)coordinate[midway](Ce)
;

% The labels
\node[below] at (xaxis) {$x$};
\node[left] at (yaxis) {$y$};
\node[below left] {$O$};
\node[below right]at(e){$\varepsilon$};
\node[below right]at(r) {$r$};
\node[above right]at(Cr){$C_{r}$};
\node[above right]at(Ce){$C_{\varepsilon}$};
\end{tikzpicture}
\end{document}

也可以使用其他值来表示\qanglelike0或90。

有了\newcommand\qangle{0}它，\node[below]at(e){$\varepsilon$};\node[below]at(r) {$r$};你就会得到