Anonymous 1123
I see this figure
This is not my code
```
\documentclass[12pt,tikz,border=3.14mm]{standalone}
\usepackage{amsmath}
\usetikzlibrary{arrows, calc}
\begin{document}
\begin{tikzpicture}[line cap=round,line join=round,>=triangle 45,x=1.0cm,y=1.0cm]
\path (1,0) coordinate (A) (3,3) coordinate (B) (0,2) coordinate (C) (3,1.5) coordinate (D) ;
\draw[black] (A) to [out=30,in=-90] (B);
\draw[black] (B) to [out=90,in=45] (C);
\draw[black] (A) to [out=0,in=-90] (D);
\draw[black, dashed] (D) to [out=90,in=15] (C);
\draw (B)--(D) (A)--(C);
\foreach \x/\g in
{A/-150, C/135}\fill[black]
(\x) circle (1pt)
($(\x)+(\g:3mm)$)node{$\x$};
\end{tikzpicture}
\end{document}
```
How to draw this with `tikz-3dplot` or `3dtools`?
Top Answer
marmot
There are two tests one might want to make:
1. the vertical line is on the left or on the right (`\itest` in the code below), and
2. the normal of the upper plane points into or out of the screen (`\jtest`).
Then one can draw the hidden and visible stretches accordingly.
```
\documentclass[tikz,border=3mm]{standalone}
\usetikzlibrary{3dtools}%https://github.com/marmotghost/tikz-3dtools
\begin{document}
\foreach \Angle in {5,15,...,355}
{\begin{tikzpicture}[3d/install view={phi=\Angle,theta=70},line cap=round,line join=round,
declare function={R=2;},same bounding box=A]
\pgfmathtruncatemacro{\itest}{screendepth(1,0,0)<0?0:1}
\pgfmathtruncatemacro{\jtest}{screendepth(0,-1,1)<0?0:1}
\begin{scope}[canvas is xy plane at z=0]
\ifnum\itest=0
\draw[3d/hidden] (0:R)
arc[start angle=0,end angle=-180+\pgfkeysvalueof{/tikz/3d/phi},radius=R];
\draw (180+\pgfkeysvalueof{/tikz/3d/phi}:R) coordinate (B)
arc[start angle=-180+\pgfkeysvalueof{/tikz/3d/phi},end angle=180,radius=R];
\else
\draw[3d/hidden] (\pgfkeysvalueof{/tikz/3d/phi}:R) coordinate (B)
arc[start angle=\pgfkeysvalueof{/tikz/3d/phi},end angle=180,radius=R];
\draw (R,0)arc[start angle=0,end angle=\pgfkeysvalueof{/tikz/3d/phi},radius=R];
\fi
\end{scope}
\ifnum\jtest=0
\ifnum\itest=0
\draw[3d/hidden]
plot[smooth,domain=-180+\pgfkeysvalueof{/tikz/3d/phi}:0] ({R*cos(\x)},{R*sin(\x)},{R*sin(\x)})
-- ({-R},0,0);
\draw
plot[smooth,domain=180:-180+\pgfkeysvalueof{/tikz/3d/phi}] ({R*cos(\x)},{R*sin(\x)},{R*sin(\x)})
-- (B);
\else
\draw[3d/hidden]
plot[smooth,domain=\pgfkeysvalueof{/tikz/3d/phi}:180] ({R*cos(\x)},{R*sin(\x)},{R*sin(\x)})
-- ({R},0,0);
\draw
plot[smooth,domain=0:\pgfkeysvalueof{/tikz/3d/phi}] ({R*cos(\x)},{R*sin(\x)},{R*sin(\x)})
-- (B);
\fi
\else
\draw
plot[smooth,domain=0:180] ({R*cos(\x)},{R*sin(\x)},{R*sin(\x)})
-- cycle;
\draw ({(\itest==0?-1:1)*R*cos(\pgfkeysvalueof{/tikz/3d/phi})},
{(\itest==0?-1:1)*R*sin(\pgfkeysvalueof{/tikz/3d/phi})},
{(\itest==0?-1:1)*R*sin(\pgfkeysvalueof{/tikz/3d/phi})})
-- (B);
\fi
\end{tikzpicture}}
\end{document}
```
