Decoration at end of line after “shorten >” command

Question

I have a style that puts an arrow at the end of a line for me:

\makeatletter
\tikzset{nomorepostaction/.code={\let\tikz@postactions\pgfutil@empty}}
\makeatother
\tikzset{mystyle/.style={
    shorten >=0.5pt,
    postaction={
        nomorepostaction,
        decorate,
        decoration={
            markings,
            mark=at position 1 with {\arrow[semithick]{>}}
        }
    }
}}

I need to do it like this as it's exactly this "semithick" arrowhead that I want. But if I shorten a line, the arrowhead is drawn in the wrong place:

\draw [mystyle, shorten >=5pt] (0,0) to (2,0);

It seems to me that the problem is caused by the "at position 1" part, which refers to the end of the line without taking into effect any shortening.

So, my question is: how can I define a style to reliably put a decoration at the end of a line (or some fixed offset from the end of the line), which is robust against possible shortening or lengthening of the line arising from "shorten <" and "shorten >"?

Answer 1

Here's an answer to the actual question (I still think that defining a new arrowhead would be more elegant). This uses Caramdir's answer to Placement of arrowtips with TikZ’s markings which extends the mark=at position to be able to handle arithmetic. That, together with the knowledge that shortenings are held in the lengths \pgf@shorten@end@additional (and \pgf@shorten@start@additional) seems to do the trick (at least with this example, I don't know if it would more generally).

Incidentally, the shorten > key is not cumulative so the shorten >=0.5pt in the mystyle decoration gets overriden by the shorten >=5pt in your actual usage. I wasn't sure if you wanted the arrowhead to be pulled back by an additional 0.5pt to whatever the line was already shortened by so I've given you various alternatives in the below. Hopefully there's enough there for you to figure out what fits your actual usage best.

\documentclass[border=10]{standalone}
% \url{https://tex.stackexchange.com/q/25830/86}
\usepackage{tikz}
\usetikzlibrary{decorations.markings}

\makeatletter
% overwrite the number parsing macro from pgflibrarydecorations.markings
\def\pgf@lib@dec@parsenum#1{%
    \gdef\pgf@lib@dec@computed@width{0 pt}%
    \tsx@pgf@lib@dec@parsenum#1+endmarker+%
    \ifdim\pgf@lib@dec@computed@width<0pt\relax%
        \pgfmathparse{\pgfdecoratedpathlength\pgf@lib@dec@computed@width}
        \edef\pgf@lib@dec@computed@width{\pgfmathresult pt}%
    \fi%
}

\def\tsx@pgf@lib@dec@parsenum@endmarker{endmarker}

% this is iterated over all numbers that are summed
\def\tsx@pgf@lib@dec@parsenum#1+{
    \def\temp{#1}%
    \ifx\temp\tsx@pgf@lib@dec@parsenum@endmarker%
    \else%
        \tsx@pgf@lib@dec@parsenum@one{#1}%
        \expandafter\tsx@pgf@lib@dec@parsenum%
    \fi%
}

% calculate the length for each number
\def\tsx@pgf@lib@dec@parsenum@one#1{%
  \pgfmathparse{#1}%
  \ifpgfmathunitsdeclared%
    \pgfmathparse{\pgf@lib@dec@computed@width + \pgfmathresult pt}%
  \else%
    \pgfmathparse{\pgf@lib@dec@computed@width + \pgfmathresult*\pgfdecoratedpathlength*1pt}%
  \fi%
  \edef\pgf@lib@dec@computed@width{\pgfmathresult pt}%
}

\tikzset{nomorepostaction/.code={\let\tikz@postactions\pgfutil@empty}}
\tikzset{mystyle 1/.style={
    shorten >=0.5pt,
    postaction={
        nomorepostaction,
        decorate,
        decoration={
            markings,
            mark=at position 1 with {\arrow[semithick]{>}}
        }
    }
  },
  mystyle 2/.style={
    postaction={
        nomorepostaction,
        decorate,
        decoration={
            markings,
            mark={at position 1 + - \pgf@shorten@end@additional with {\arrow[semithick]{>}}}
        }
    }
  },
    mystyle 3/.style={
    postaction={
        nomorepostaction,
        decorate,
        decoration={
            markings,
            mark={at position 1 + - \pgf@shorten@end@additional + -0.5pt with {\arrow[semithick]{>}}}
        }
    }
  },
  mystyle 4/.style={
    shorten >=0.5pt,
    postaction={
        nomorepostaction,
        decorate,
        decoration={
            markings,
            mark={at position 1 + - \pgf@shorten@end@additional with {\arrow[semithick]{>}}}
        }
    }
  },
}
\makeatother

\begin{document}
\begin{tikzpicture}
\foreach \st in {1,...,4} {
\begin{scope}[yshift=-\st cm]
\draw [mystyle \st,shorten >=5pt] (0,0) to (2,0);
\draw [mystyle \st] (0,-.5) to (2,-.5);
\end{scope}
}
\end{tikzpicture}
\end{document}

Result:

(And if you find this useful, go and vote for Caramdir's code in the link above! All I did was cut-and-paste.)

Answer 2

Here's a new thick arrow. It's just the normal arrow definition taken from pgfcorearrows.code.tex, with the line \pgfsetlinewidth{...} added.

Edit: Following Andrew Stacey's suggestions, here's the same arrow with the thickness as an option. If you don't specify a unit for the thickness, the value will be multiplied by the line thickness to get the thickness for the arrow. The line thickness used is the one in force when the arrow is drawn.

Issuing

\draw [thick arrow] (0,0) to (2,0);
\draw [thick arrow=2pt] (0,-1) to (2,-1);
\draw [line width=2pt,thick arrow=1,shorten >=5pt] (0,-2) to (2,-2);
\draw [line width=2pt,thick arrow=2,shorten >=5pt] (0,-3) to (2,-3);

will yield

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{decorations.markings}


\makeatletter
\pgfarrowsdeclare{thickarrow}{thickarrow}
{
  \pgfutil@tempdima=-0.84pt%
  \advance\pgfutil@tempdima by-1.3\pgflinewidth%
  \pgfutil@tempdimb=0.21pt%
  \advance\pgfutil@tempdimb by.625\pgflinewidth%
  \pgfarrowsleftextend{+\pgfutil@tempdima}
  \pgfarrowsrightextend{+\pgfutil@tempdimb}
}
{
  \pgfmathparse{\pgfgetarrowoptions{thickarrow}}%
  \ifpgfmathunitsdeclared%
    \pgfmathparse{\pgfmathresult pt}%
  \else%  
    \pgfmathparse{\pgfmathresult*\pgflinewidth}%
  \fi%
  \let\thickness=\pgfmathresult
  \pgfsetlinewidth{\thickness pt}
  \pgfutil@tempdima=0.28pt%
  \advance\pgfutil@tempdima by.3\pgflinewidth%
  \pgfsetlinewidth{0.8\pgflinewidth}
  \pgfsetdash{}{+0pt}
  \pgfsetroundcap
  \pgfsetroundjoin
  \pgfpathmoveto{\pgfqpoint{-3\pgfutil@tempdima}{4\pgfutil@tempdima}}
  \pgfpathcurveto
  {\pgfqpoint{-2.75\pgfutil@tempdima}{2.5\pgfutil@tempdima}}
  {\pgfqpoint{0pt}{0.25\pgfutil@tempdima}}
  {\pgfqpoint{0.75\pgfutil@tempdima}{0pt}}
  \pgfpathcurveto
  {\pgfqpoint{0pt}{-0.25\pgfutil@tempdima}}
  {\pgfqpoint{-2.75\pgfutil@tempdima}{-2.5\pgfutil@tempdima}}
  {\pgfqpoint{-3\pgfutil@tempdima}{-4\pgfutil@tempdima}}
  \pgfusepathqstroke
}
\makeatother

\tikzset{
    set thickarrow thickness/.code={
        \pgfsetarrowoptions{thickarrow}{#1}},
    thick arrow/.style={set thickarrow thickness=#1,-thickarrow},
    thick arrow/.default=0.6pt
}


\begin{document}

\begin{tikzpicture}[transform canvas={scale=3}]
\draw [thick arrow] (0,0) to (2,0);
\draw [thick arrow=2pt] (0,-1) to (2,-1);
\draw [line width=2pt,thick arrow=1,shorten >=5pt] (0,-2) to (2,-2);
\draw [line width=2pt,thick arrow=2,shorten >=5pt] (0,-3) to (2,-3);
\draw [thick arrow=2,shorten >=5pt,line width=2pt] (0,-3) to (2,-3);
\end{tikzpicture}
\end{document}