Why do lines drawn in TikZ add space equal to their width into the picture and how can this be prevented?

Question

Related: Horizontal space before TikZ picture

Consider the following MWE

\documentclass{article}
\usepackage{tikz}
\usepackage[showframe]{geometry}
\begin{document}
\noindent
\begin{tikzpicture}
  \draw [red] (0pt,0pt) -- ++(\linewidth,0pt);
  \node [anchor=south,font=\sffamily\footnotesize] at (current bounding box.north) {\textcolor{red}{\the\pgflinewidth} too wide};
\end{tikzpicture}
\noindent
\begin{tikzpicture}
  \draw [red,line width=1pt] (0pt,0pt) -- ++(\linewidth,0pt);
  \node [anchor=south,font=\sffamily\footnotesize] at (current bounding box.north) {\textcolor{red}{1pt} too wide};
\end{tikzpicture}
\noindent
\begin{tikzpicture}
  \draw [red,line width=10pt] (0pt,0pt) -- ++(\linewidth,0pt);
  \node [anchor=south,font=\sffamily\footnotesize] at (current bounding box.north) {\textcolor{red}{10pt} too wide};
\end{tikzpicture}
\noindent
\begin{tikzpicture}
  \draw [red,line width=100pt] (0pt,0pt) -- ++(\linewidth,0pt);
  \node [anchor=south,font=\sffamily\footnotesize] at (current bounding box.north) {\textcolor{red}{100pt} too wide};
\end{tikzpicture}
\end{document}

Naively, I would not expect this to produce overfull boxes, yet it does

Overfull \hbox (0.4pt too wide) in paragraph at lines 246--251
[]
Overfull \hbox (1.0pt too wide) in paragraph at lines 252--257
[]
Overfull \hbox (10.0pt too wide) in paragraph at lines 258--263
[]
Overfull \hbox (100.0pt too wide) in paragraph at lines 264--269
[] 

The output shows the displacement of the lines quite clearly

What causes this and how can I prevent it?

• Clearly, one option to prevent an overfull box is to simply reduce the length of the line by \pgflinewidth. This is noticeable even at smaller widths, but it is clearly not viable as lines get thicker.
• Another option would be to draw the line while not accounting for the bounding box and then repeat the path, without drawing it, to set the bounding box.
• Gonzalo Medina recommends using overlay, but this means the bounding box is wrong.

[Edited to remove misleading example.]

Is there a recommended way to address this issue?

Answer 1

What causes this and how can I prevent it?

PGF adds .5\pgflinewidth on all four sides to the bounding box (if the at this point established bounding box is smaller) when a path is stroked/drawn.

It does this to cover not only the line cap rect and round but also the line width itself.

Even in your examples, a simply horizontal line of width 100pt should result in a picture that's 100pt high, otherwise it will cover up text:

\documentclass{article}
\usepackage{tikz}
\usepackage[showframe, pass]{geometry}
\setlength\parindent{0pt}
\begin{document}
I'm covered by a red bar.
\tikz[line width=100pt, opacity=.5]
  \useasboundingbox[postaction={draw=red, overlay}]
    (0pt,0pt) -- +(\linewidth,0pt);
I cover a red bar.
\vspace{50pt}
I'm covered by a red bar.
\tikz[line width=100pt, opacity=.5]
  \useasboundingbox[postaction={draw=red, overlay}]
    (0pt,0pt) -- +(0pt,2cm);
I cover a red bar.
\end{document}

Not all miter line joins will be covered by this.

Clearly, one option to prevent an overfull box is to simply reduce the length of the line by \pgflinewidth. This is noticeable even at smaller widths, but it is clearly not viable as lines get thicker.

In combination with line cap=rect maybe:

\tikz[line width=100pt]
  \draw[red, line cap=rect]
    (0pt, 0pt) -- +(\linewidth-\pgflinewidth,0pt);

Another option would be to draw the line while not accounting for the bounding box and then repeat the path, without drawing it, to set the bounding box.

Not really, this still doesn't cover the actual line width in the vertical dimension. (Though, for a line width of 0.4pt, it's not going to be noticable really.) We can at least use postaction to not have to literally repeat the path again (and also not to parse the path and built the softpath).

Still covered.
\tikz[line width=100pt, opacity=.5]
  \path[postaction={draw=red, overlay}]
    (0pt, 0pt) -- +(\linewidth,0pt);
Still covering.

---

If all you want is to draw horizontal lines from 0pt to \linewidth don't use TikZ but \rule use trim left=0pt, trim right=\linewidth:

\tikz[line width=100pt, trim left=+0pt, trim right=+\linewidth]
  \draw[red] (0pt, 0pt) -- +(\linewidth,0pt);

These keys are useful if you know the width of your paths without the line width (or can determine it by two coordinates since it also allows trim left=(left coordinate of the picture), trim right=(right coordinate of the picture).

I've used these options before, especially when it comes to pictures spanning the whole \linewidth.

---

For now, let's ignore the round line cap (the bounding box will always be perfect then anyway) and the butt line cap (I believe only horizontal and vertical lines would be good uses for it).

For a round line join the bounding box will be perfect as well, The

What determines the “real” bounding box?

For each path segment, we just need to calculate a path that is .5\pgflinewidth parallel to the segment. And for each connection between two path segments we need to calculate

That's a lot of parsing a math I don't want to do, but for butt line caps and miter line joins, the nfold library provides the algorithm. We use it to offset the original path half the line width to both sides and use that as the bounding box of the path.

Code

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{nfold}
\makeatletter
\tikzset{
  real bb/.style={% this is not perfect because it messes with the original
                  % postaction and softpath system
    overlay,      % ignore the original's path bb
    postaction={overlay=false, path only, qoffset=+1},
    postaction={overlay=false, path only, qoffset=-1}},
  qoffset/.code=\tikz@addoption{\pgfgetpath\tikz@temp\pgfsetpath\pgfutil@empty
                   \pgfoffsetpathqfraction\tikz@temp{.5\pgflinewidth}{#1}}}
\makeatother
\usepackage[showframe, pass]{geometry}
\setlength\parindent{0pt}
\tikzset{every picture/.append style={execute at end picture={
  \draw[help lines] ([xshift=+.5\pgflinewidth,yshift=+.5\pgflinewidth]
    current bounding box.south west) rectangle
    ([xshift=+-.5\pgflinewidth,yshift=+-.5\pgflinewidth]
    current bounding box.north east);}}}
\begin{document}
\section{Using only the path as bounding box}
Using only the path for the bounding box does not work
because a horizontal line still has a height and a
vertical line still has a width.
\vspace{35pt}
I'm covered by a red bar.
\tikz[line width=100pt, opacity=.5]
  \useasboundingbox[postaction={draw=red, overlay}]
    (0pt,0pt) -- +(\linewidth,0pt);
I cover a red bar.
\vspace{50pt}
I'm covered by a red bar.
\tikz[line width=100pt, opacity=.5]
  \useasboundingbox[postaction={draw=red, overlay}]
    (0pt,0pt) -- +(0pt,2cm);
I cover a red bar.
\bigskip
\section{Trimming left and right and \texttt{real bb}}
For a simple horizontal line, I'd use \verb|trim left| and \verb|trim right|:
\tikz[line width=100pt, trim left=+0pt, trim right=+\linewidth]
  \draw (0pt,0pt) --+(\linewidth,0pt);
Otherwise, we'll try \verb|real bb|:
\tikz[line width=100pt]
  \draw[real bb] (0pt,0pt) --+(\linewidth,0pt);\bigskip
\pagebreak
Another example where the default approach fails:\medskip
\tikz[line width=25pt, baseline=(current bounding box)]
  \draw [           rotate=45] (0,-1) rectangle (2,1);\qquad\qquad
\tikz[line width=25pt, baseline=(current bounding box)]
  \draw [yscale=.5, rotate=45] (0,-1) rectangle (2,1);\bigskip
And with \verb|real bb|:
\tikz[line width=25pt, baseline=(current bounding box)]
  \draw [           rotate=45, real bb] (0,-1) rectangle (2,1);
\tikz[line width=25pt, baseline=(current bounding box)]
  \draw [yscale=.5, rotate=45, real bb] (0,-1) rectangle (2,1);
\begingroup
\tikzset{every picture/.append style={line width=20pt, text=white, sloped}}
\section{Line Caps}
Only a round line cap will always fit perfectly.
\foreach \linecap in {butt, rect, round}
  \tikz[line cap=\linecap]
    \draw          (0,0) -- node{\linecap} +(30:2);\medskip
The \verb|real bb| key can be used to “fix” the default butt line cap.
The round cap does not need fixing.
Good luck with the rect line cap.\medskip
\foreach \linecap in {butt, rect, round}
  \tikz[line cap=\linecap]
    \draw[real bb] (0,0) -- node{\linecap} +(30:2);
\pagebreak
\section{Line Joins}
Once again, only a round line join will fit perfectly.
\foreach \linejoin in {miter, bevel, round}
  \tikz[line join=\linejoin]
    \draw (0,0) -- node{\linejoin} ++(30:1.5) -- +(-30:1.5);
With \verb|real bb| again:
Miter line join looks good.
The round line join didn't need it.
And the bevel does its own thing.
(And there's also the \verb|miter limit| changing things.)
\foreach \linejoin in {miter, bevel, round}
  \tikz[line join=\linejoin]
    \draw[real bb] (0,0) -- node{\linejoin} ++(30:1.5) -- +(-30:1.5);
\endgroup
\section{Hmm…}
\tikz[line width=1cm, trim left=+-.5\linewidth, trim right=+.5\linewidth]
  \draw (0,0) node {What about this?} circle[radius=.5\linewidth];
\end{document}