用一种颜色绘制圆柱体表面的一半，用另一种颜色绘制另一半

Question

该代码给出了圆柱体，但我希望圆柱表面的一半部分是一种颜色，另一半部分是另一种颜色（一半相对于顶部而不是中间，意味着当我们将顶部圆形横截面切成两半时）。

\documentclass[tikz]{standalone}
\begin{document}
\begin{tikzpicture}
\draw (0,0) ellipse (1.25 and 0.5);
\draw (-1.25,0) -- (-1.25,-3.5);
\draw (-1.25,-3.5) arc (180:360:1.25 and 0.5);
\draw [dashed] (-1.25,-3.5) arc (180:360:1.25 and -0.5);
\draw (1.25,-3.5) -- (1.25,0);  
\fill [gray,opacity=0.5] (-1.25,0) -- (-1.25,-3.5) arc (180:360:1.25 and 0.5) -- (1.25,0) arc (0:180:1.25 and -0.5);
\end{tikzpicture}
\end{document}

我们如何才能实现它？

Answer 1

你是指这样的吗？

\documentclass[tikz]{standalone}
\begin{document}
\begin{tikzpicture}
  \draw (0,0) ellipse (1.25 and 0.5);
  \draw (-1.25,0) -- (-1.25,-3.5);
  \draw (-1.25,-3.5) arc (180:360:1.25 and 0.5);
  \draw [dashed] (-1.25,-3.5) arc (180:360:1.25 and -0.5);
  \draw (1.25,-3.5) -- (1.25,0);  
  % 
  \fill [left color = blue, right color = red, opacity=0.5] (-1.25,0) -- (-1.25,-3.5) arc (180:360:1.25 and 0.5) -- (1.25,0) arc (0:180:1.25 and -0.5);
\end{tikzpicture}
\end{document}

如果你仍然想要一个急剧的过渡，你可以fill用以下方法替换路径

\fill [opacity=0.5, color = blue] (-1.25,0) -- (-1.25,-3.5) arc
  (180:270:1.25 and 0.5) -- (0,-0.5) node[midway, left, color =
  black]{blue} arc(270:180:1.25 and 0.5) ;

\fill [opacity=0.5, color = red] (1.25,0) -- (1.25,-3.5) arc
  (0:-90:1.25 and 0.5) -- (0,-0.5) node[midway, right, color =
  black]{red} arc(270:360:1.25 and 0.5);