考虑以下一段代码：
\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{decorations.text}
\usetikzlibrary{decorations.markings}
\begin{document}
\begin{figure}[H]
\centering
\begin{tikzpicture}
\begin{scope}[very thick,decoration={
markings,
mark=at position 0.5 with {\arrow{<}}}
]
\draw [dashed,blue,very thick,domain=162:-54] plot ({1.5*cos(\x)}, {1.5*sin(\x)});
\draw [very thick,blue,domain=-54:-198] plot ({1.5*cos(\x)}, {1.5*sin(\x)});
%-----------------------------------------------------------------
\path[draw=red, -stealth,
postaction={decorate,decoration={text effects along path,
text={\ Positive direction\ }, text align=center,
text effects/.cd,
text along path,
every character/.style={fill=white, yshift=-0.5ex}}}]
(0, 0) arc [start angle=-60, end angle=80, radius=4];
%--------------------------------------------------------------------
\draw[blue,very thick,postaction={decorate}] (0,1.5) -- (0,3) node[above] {\textcolor{black}{$ p_1 $}};
\draw[blue,very thick,postaction={decorate}] (1.43,0.46) -- (2.85,0.93)node[align=right, above] {\textcolor{black}{$ p_2 $}};
\draw[blue,very thick,postaction={decorate}] (0.88,-1.21) -- (1.76,-2.43)node[align=right, below] {\textcolor{black}{$ p_{N_c} $}};
\draw[blue,very thick,postaction={decorate}] (-0.88,-1.21) -- (-1.76,-2.43)node[align=left, below] {\textcolor{black}{$ p_{N_c+1} $}};
\draw[blue,very thick,postaction={decorate}] (-1.43,0.46) -- (-2.85,0.93)node[align=left, above] {\textcolor{black}{$ p_N $}};
\draw [dotted,very thick,domain=172:224] plot ({2*cos(\x)}, {2*sin(\x)});
\draw [dotted,very thick,domain=8:-44] plot ({2*cos(\x)}, {2*sin(\x)});
\draw [thick,very thick,domain=162:90,->] plot ({1.3*cos(\x)}, {1.3*sin(\x)});
\node[] at ({1*cos(126)},{1*sin(126)}) (a) {$ q $};
\node[] at ({1.8*cos(126)},{1.8*sin(126)}) (a) {$ m_0 $};
\node[] at ({1.8*cos(54)},{1.8*sin(54)}) (a) {$ m_1 $};
\node[] at ({1.8*cos(90)},{-1.8*sin(90)}) (a) {$ m_{N_c} $};
\end{scope}
\end{tikzpicture}
\end{figure}
\end{document}
在生成的图片中，红色圆弧的中心偏离了，尽管中心已设置为 (0,0)。问题是什么？
答案1
arc操作从给定的点绘制圆弧。该点不是圆弧的中心。因此，如果您想以某个点为中心绘制圆弧，您可能需要calc（但calc在这种特殊情况下不是必需的）极坐标。
固定代码：
\documentclass[tikz]{standalone}
\usetikzlibrary{decorations.text}
\usetikzlibrary{decorations.markings}
\begin{document}
\begin{tikzpicture}
\begin{scope}[very thick,decoration={
markings,
mark=at position 0.5 with {\arrow{<}}}
]
\draw [dashed,blue,very thick,domain=162:-54] plot ({1.5*cos(\x)}, {1.5*sin(\x)});
\draw [very thick,blue,domain=-54:-198] plot ({1.5*cos(\x)}, {1.5*sin(\x)});
%-----------------------------------------------------------------
\path[draw=red, -stealth,
postaction={decorate,decoration={text effects along path,
text={\ Positive direction\ }, text align=center,
text effects/.cd,
text along path,
every character/.style={fill=white, yshift=-0.5ex}}}]
(-60:4) arc [start angle=-60, end angle=80, radius=4];
%--------------------------------------------------------------------
\draw[blue,very thick,postaction={decorate}] (0,1.5) -- (0,3) node[above] {\textcolor{black}{$ p_1 $}};
\draw[blue,very thick,postaction={decorate}] (1.43,0.46) -- (2.85,0.93)node[align=right, above] {\textcolor{black}{$ p_2 $}};
\draw[blue,very thick,postaction={decorate}] (0.88,-1.21) -- (1.76,-2.43)node[align=right, below] {\textcolor{black}{$ p_{N_c} $}};
\draw[blue,very thick,postaction={decorate}] (-0.88,-1.21) -- (-1.76,-2.43)node[align=left, below] {\textcolor{black}{$ p_{N_c+1} $}};
\draw[blue,very thick,postaction={decorate}] (-1.43,0.46) -- (-2.85,0.93)node[align=left, above] {\textcolor{black}{$ p_N $}};
\draw [dotted,very thick,domain=172:224] plot ({2*cos(\x)}, {2*sin(\x)});
\draw [dotted,very thick,domain=8:-44] plot ({2*cos(\x)}, {2*sin(\x)});
\draw [thick,very thick,domain=162:90,->] plot ({1.3*cos(\x)}, {1.3*sin(\x)});
\node[] at ({1*cos(126)},{1*sin(126)}) (a) {$ q $};
\node[] at ({1.8*cos(126)},{1.8*sin(126)}) (a) {$ m_0 $};
\node[] at ({1.8*cos(54)},{1.8*sin(54)}) (a) {$ m_1 $};
\node[] at ({1.8*cos(90)},{-1.8*sin(90)}) (a) {$ m_{N_c} $};
\end{scope}
\end{tikzpicture}
\end{document}
