Lack of precision in the stroke of an arrow arc. A bug or an inaccuracy due to TeX calculations?

[quark67]

Brief outline of the bug

In a draw of an arc with the -> decoration, the path is slightly deformed. This is not the case when the draw has no decoration.

I don't know if this is a little bug or a lack of accuracy of TeX, used by TikZ.

In the next code, there is first a thick arc drawn with a thin arc drawn above it. The paths are the same, and the thin arc is perfectly centred.

In the second example, the thick arc has an arrow at the end. We can notice that in the right part of the draw, the thick arc is not exactly centred at the thin arc. This can be show by a zoom.

But in fact, this is more easily shown if we use double. Like above, the not decorated arc is perfectly centred. But the arc with the -> decoration is not perfectly centred.

Code below.

If we zoom in the second arc, we see a perfect centred arc at the begin of the draw:

But not near the end (here, approximately at 75% from the beginning). We see that the above part is greater than the below part.

It's largely more noticeable in the arc with the double:

At begin, it's perfect:

At end, it's less perfect:

Minimal working example (MWE)

\documentclass{article}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
\draw[line width=2pt, yellow!50!cyan] (120:3) arc [start angle=120, end angle=60, radius=3cm];
\draw[very thin, red] (120:3) arc [start angle=120, end angle=60, radius=3cm];
\end{tikzpicture}

\begin{tikzpicture}
\draw[line width=2pt, ->, cyan] (120:3) arc [start angle=120, end angle=60, radius=3cm];
\draw[very thin, red] (120:3) arc [start angle=120, end angle=60, radius=3cm];
\end{tikzpicture}

\begin{tikzpicture}
\draw[line width=2pt, double, yellow!50!cyan] (120:3) arc [start angle=120, end angle=60, radius=3cm];
\draw[very thin, red] (120:3) arc [start angle=120, end angle=60, radius=3cm];
\end{tikzpicture}

\begin{tikzpicture}
\draw[line width=2pt, double, ->, cyan] (120:3) arc [start angle=120, end angle=60, radius=3cm];
\draw[very thin, red] (120:3) arc [start angle=120, end angle=60, radius=3cm];
\end{tikzpicture}

\end{document}

[muzimuzhi]

Caused by the path shortening.

\documentclass{article}
\usepackage{tikz}

\begin{document}
% assume the arrow has length 5pt
\begin{tikzpicture}
\draw[line width=2pt, double, shorten >=5pt, cyan] (120:3) arc [start angle=120, end angle=60, radius=3cm];
\draw[very thin, red] (120:3) arc [start angle=120, end angle=60, radius=3cm];
\end{tikzpicture}
\end{document}

[muzimuzhi]

The bending library provides a more accurate path shortening algorithm, for curves.

It seems to work well for your case. The only work is to add \usetikzlibrary{bending} to your example.

Update: Unfortunately the bending library itself is imperfect, see #896 for example.

[quark67]

Very thanks for your workaround! Yes, I have seen #896, unfortunately. But I my case, I'm not affected by the issue shown at #896, so I can use directly \usetikzlibrary{bending}. Do you think the code of TikZ can easily be corrected in a future version so it take the shortening in account when the path is calculated?

[muzimuzhi]

If you meant to fix #896, no estimation.