tikz-cd: extra gray line with Rightarrow

Question

At first I thought this was a problem of my pdf viewer. For example, with the code

\documentclass{article}
\usepackage{tikz-cd}
\begin{document}
\begin{tikzcd}
    A & B
    \arrow[Rightarrow, from=1-1, to=1-2]
\end{tikzcd}
\end{document}

The resulting Rightarrow seems to contain two small extra gray lines:

The problem has been addressed in this question a few months ago, where the author provided a complicated new style nRightarrow which appears to fix the problem (Rightarrow above, nRightarrow below):

However, the need for this new style nRightarrow mainly seems to come from other more complicated issues. Thus I would like to know that just for the problem of these two extra gray lines, is there an existing simpler solution by now?

Below is a MWE, in which I have copied the code of nRightarrow from the linked question.

\documentclass{article}
\usepackage{tikz-cd}
\usetikzlibrary{calc}
\newlength{\eqoffset}
\makeatletter
% relative coordinates: (0,0) is the arrow's tail, x points towards the head,
% y points perpendicular, unit distance is \eqoffset
\newcommand{\relptstart}[2]{($($(k0)!#1\eqoffset+\pgf@shorten@start@additional!0:(k1)$)!#2\eqoffset!90:(k1)$)}
% (0,0) is the arrow's tip, rest is the same
\newcommand{\relptend}[2]{($($(k1)!#1\eqoffset-\pgf@shorten@end@additional
    -2\eqoffset-.5\pgflinewidth!180:(k0)$)!#2\eqoffset!-90:(k0)$)}
\tikzcdset{
    nRightarrow/.style={line join=round,
    no head,
    /tikz/commutative diagrams/@shiftabletopath,
    execute at begin to = {
        % Do not use tikzcd@noda or tikzcd@x here, it causes interference.
        % Use new names instead
        \path (\tikztostart) -- (\tikztotarget) coordinate[pos=0] (k0) coordinate[pos=1] (k1);
        \pgfpointnormalised{\pgfpointdiff{\pgfpointanchor{k1}{center}}{\pgfpointanchor{k0}{center}}}
        \pgfgetlastxy{\kdx}{\kdy}
        \tikzset{
        to path={
            % arrow body
            % the .06 is from \pgftransformxshift{.06\pgfutil@tempdima}
            \relptstart{0}{1}
            -- \relptend{-.06}{1}
            {
                % correct vertical position, more central horizontal position
                %               [xshift=-\kdy\eqoffset, yshift=\kdx\eqoffset]
                % matches original Rightarrow more closely
                [xshift=-\kdy\eqoffset-\kdx(\eqoffset+.25\pgflinewidth),
                yshift=\kdx\eqoffset-\kdy(\eqoffset+.25\pgflinewidth)] 
                \tikztonodes}
            \relptstart{0}{-1}
            -- \relptend{-.06}{-1}
            % arrow tip
            % fake the round cap by using round joins and drawing the path twice with a turnaround at the caps
            \relptend{2}{0}         % tip to top end
            .. controls \relptend{1}{0.05} and \relptend{-0.75}{1.25} ..
            \relptend{-1.4}{2.65}   % top end back to tip
            .. controls \relptend{-0.75}{1.25} and \relptend{1}{0.05} ..
            \relptend{2}{0}         % tip to bottom end
            .. controls \relptend{1}{-0.05} and \relptend{-0.75}{-1.25} ..
            \relptend{-1.4}{-2.65}  % bottom end back to tip
            .. controls \relptend{-0.75}{-1.25} and \relptend{1}{-0.05} ..
            \relptend{2}{0}
            % Add a degenerate path segment at the end so shorten < and shorten > are not applied again
            (k1)
        }}
    }}
}
\setlength{\eqoffset}{.225ex}
\makeatother
\begin{document}
\begin{tikzcd}[column sep=large]
    A & B
    \arrow[Rightarrow, from=1-1, to=1-2]
\end{tikzcd}
\begin{tikzcd}[column sep=large]
    A & B
    \arrow[nRightarrow, from=1-1, to=1-2]
\end{tikzcd}
\end{document}

The simplest solution would be to use another PDF renderer/viewer that doesn't let that sliver of black shine through. — Qrrbrbirlbel, Mar 12 '23 at 08:34
Have you noticed that the linked questions spawned off a lengthy discussion about this topic with the author of tikz-cd and implementation of how to draw parallel curves? — Qrrbrbirlbel, Mar 12 '23 at 11:29

Qrrbrbirlbel · Accepted Answer · 2023-03-12T12:01:37.560

The linked question spanwed off a lengthy discussion with the author of TikZ-CD on how to re-implement parallel lines. The code seems to be almost finished on a PGF level.

Short of reimplementing the way PGF draws double lines here is a proof of concept on how to slightly alter the way PGF draws the double lines

either by lengthening the inner line
or by shortening the outer line.

This is far from perfect but works best for

straight lines
that consists of one segment (only one move to at the start),
aren't dashed and use the normal rect line cap.

In addition it'd be best if the arrows don't touch drawn nodes.

The spath3 library could help in making the (un)shortening not distort the curve and could also help in applying this to paths that have more than one segment.

Code

\documentclass[tikz]{standalone}
\usepackage{tikz-cd}
\makeatletter
\def\pgf@double@cheat#1{%
  \let\pgf@tips@mode\pgf@tips@mode@false
  \pgfsetarrowsstart{}\pgfsetarrowsend{}%
  \pgfsetshortenstart{#1}\pgfsetshortenend{#1}%
  \pgf@prepare@end@of@path\pgf@prepare@start@of@path}
\pgfset{
  shorten outer/.code=%
    \pgfmathsetlength\pgfutil@tempdima{#1}%
    \pgfkeyssetevalue{/pgf/shorten outer}{\the\pgfutil@tempdima},
  shorten outer=+0pt,
  unshorten inner/.code=%
    \pgfmathsetlength\pgfutil@tempdima{#1}%
    \pgfkeyssetevalue{/pgf/unshorten inner}{\the\pgfutil@tempdima},
  unshorten inner=+0pt}
\usepackage{etoolbox}
\patchcmd{\pgf@stroke@inner@line}{\pgfsyssoftpath@invokecurrentpath}{%
    \ifdim\pgfkeysvalueof{/pgf/unshorten inner}>0pt
      \pgf@double@cheat{-\pgfkeysvalueof{/pgf/unshorten inner}}%
    \fi\pgfsyssoftpath@invokecurrentpath}{}{\PatchFailed}
\patchcmd{\pgfusepath}{\fi\pgfsyssoftpath@invokecurrentpath\pgf@up@action}{%
  \fi \ifdim\pgfinnerlinewidth>0pt
        \ifdim\pgfkeysvalueof{/pgf/shorten outer}>0pt
          \pgfgetpath\pgf@temppath
          \pgf@double@cheat{\pgfkeysvalueof{/pgf/shorten outer}}\fi\fi
      \pgfsyssoftpath@invokecurrentpath%
      \ifdim\pgfinnerlinewidth>0pt
        \ifdim\pgfkeysvalueof{/pgf/shorten outer}>0pt
          \pgfsetpath\pgf@temppath\fi\fi\pgf@up@action}{}{\PatchFailed}
\makeatother
\newcommand*\everycell[1]{%
  \draw[cd=Rightarrow,#1]node(A){$A$}node(B)at(2,0){$B$}(A)to(B);}
\begin{document}
\tikz[
  cd/.code=\tikzcdset{#1}, anchor=base,
  cd dbl/.style={cd={background color=green}},
  cells={nodes={behind path, help lines, draw, shape=asymmetrical rectangle}},
  column 1/.append style={anchor=base west, nodes=path only},
  rows/.style args={#1 => #2}{row #1/.append style={#2}},
  rows/.list={2 => unshorten inner = .05pt,
              3 => shorten outer = .05pt,
              4 => {cd dbl=green, unshorten inner = 2pt},
              5 => {cd dbl=green, shorten outer = 2pt}}
]\matrix[column sep=\tabcolsep]{
 \node{normal:};        & \everycell{} & \everycell{bend left} \
 \node{inner:};         & \everycell{} & \everycell{bend left} \
 \node{outer:};         & \everycell{} & \everycell{bend left} \
 \node{extreme inner:}; & \everycell{} & \everycell{bend left} \
 \node{extreme outer:}; & \everycell{} & \everycell{bend left} \};
\begin{tikzcd}[unshorten inner=+0.01pt]
    A & B
    \arrow[Rightarrow, from=1-1, to=1-2]
\end{tikzcd}
\end{document}

Output

In this case, it is best to compile yourself and look at the output in various zoom levels and with different PDF viewers/renderers. Still, you can click in the first two images to view a bigger version of it.

Thank you for spreading the word! The discussion has now concluded in me publishing tikz-nfold, I have updated my question here as well. — Jonathan, Mar 28 '23 at 13:22
@Jonathan I've got a GitHub notification about the issue on the tikz-cd repo. Feel free to add an answer with your library (doesn't need to be on CTAN) here and also self-answer your own question. — Qrrbrbirlbel, Mar 28 '23 at 23:53

score 5 · Answer 2 · answered Mar 29 '23 at 15:10

I wrote tikz-nfold specifically to solve this problem:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{cd}
\usetikzlibrary{nfold}
\begin{document}
\begin{tikzcd}
    A & B
    \arrow[Rightarrow, from=1-1, to=1-2, nfold]
\end{tikzcd}
\end{document}

On a side note, the author of tikz-cd recently informed me that \usepackage{tikz-cd} is a legacy feature, though there is nothing wrong with using it.

score 1 · Answer 3 · answered Mar 12 '23 at 11:57

1

If you use xy package for commutative diagram there is not vertical line or parallel line in the tail of the arrow.

\documentclass[a4paper,12pt]{article}
\usepackage[all,cmtip]{xy}
\begin{document}
\xymatrix@C=3pc{A \ar@{=>}[r]& B}
\end{document}

answered Mar 12 '23 at 11:57

Sebastiano

54,118

tikz-cd: extra gray line with Rightarrow

3 Answers3

Code

Output

Linked