\documentclass{standalone}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
\begin{scope}[x={(.7cm,-.3cm)}]
\path (1,0,0); \pgfgetlastxy{\cylxx}{\cylxy} \path (0,1,0); \pgfgetlastxy{\cylyx}{\cylyy} \path (0,0,1); \pgfgetlastxy{\cylzx}{\cylzy} \pgfmathsetmacro{\cylt}{(\cylzy * \cylyx - \cylzx * \cylyy)/ (\cylzy\cylxx - \cylzx * \cylxy)} \pgfmathsetmacro{\ang}{atan(\cylt)} \pgfmathsetmacro{\ct}{1/sqrt(1 + (\cylt)^2)} \pgfmathsetmacro{\st}{\cylt * \ct}
\fill[red] (\ct,\st,0) -- ++(0,0,-8) arc[start angle=\ang,delta angle=180,radius=1] -- ++(0,0,8) arc[start angle=\ang+180,delta angle=-180,radius=1];
\begin{scope}[every path/.style={ultra thick}]
\draw (0,0,0) circle[radius=1];
\draw[->] (0,0,0) -- (1,0,0);
\draw[->] (0,0,0) -- (0,1,0);
\draw (\ct,\st,0) -- ++(0,0,-8);
\draw (-\ct,-\st,0) -- ++(0,0,-8);
\draw (\ct,\st,-8) arc[start angle=\ang,delta angle=180,radius=1];
\draw[dashed] (\ct,\st,-8) arc[start angle=\ang,delta angle=-180,radius=1];
\end{scope}
\end{scope}
\end{tikzpicture}
\end{document}
答案1
\cylxx前六行定义了、\cylxy、\cylyx、 ...的值。
最后四行定义了\cylt、\ang、 ...的值。
