How to alter the bond length of a double bond in chemfig?

Question

With the chemfig package, is it possible to alter the bond length of the pi-bond in a double-bond? I know about the \setbondoffset{}-command, but in this case, this does not work. Hope the below example clarifies what i want to accomplish.

Input:

\documentclass{scrartcl}
\usepackage{chemfig}

\begin{document}
\chemfig{-[:120]=^[:180]-[:240]}
\end{document}

Output:

Desired ouput:

Answer 1

I defined TikZ decorations:

\documentclass{article}
\usepackage{chemfig}
\usetikzlibrary{decorations}
\makeatletter
\newdimen\mystartshorten
\newdimen\myendshorten
\mystartshorten0pt
\myendshorten0pt
\pgfdeclaredecoration{sdbond}{initial}{
  \state{initial}[width=\pgfdecoratedremainingdistance,next state=final]
  {
    {
      \pgftransformyshift{\CF@double@sep}
      \pgfpathmoveto{\pgfqpoint{\mystartshorten}{0pt}}
      {
        \pgftransformxshift{\pgfdecoratedremainingdistance}
        \pgfpathlineto{\pgfqpoint{-\myendshorten}{0pt}}
      }
    }
    \pgfpathmoveto{\pgfpointorigin}
    \pgfpathlineto{\pgfqpoint{\pgfdecoratedremainingdistance}{0pt}}
  }
  \state{final}
  {}
}
\tikzset{
  lsdbond/.code 2 args={%
    \tikzset{decorate, decoration=sdbond}%
    \pgfmathsetlength\mystartshorten{\CF@double@sep*cot((#1)/2)}%
    \pgfmathsetlength\myendshorten{\CF@double@sep*cot((#2)/2}%
  },
  rsdbond/.code 2 args={%
    \tikzset{decorate, decoration={sdbond, mirror}}%
    \pgfmathsetlength\mystartshorten{\CF@double@sep*cot((#1)/2)}%
    \pgfmathsetlength\myendshorten{\CF@double@sep*cot((#2)/2}%
  }
}
\setdoublesep{0.4em}
\setatomsep{2em}
\makeatother
\begin{document}
\chemfig{[:60]--[::-60,,,,,rsdbond={120}{120}]-[::-60]}
\chemfig{[:90]--[::-90,,,,,rsdbond={90}{60}]-[::-120]}
\end{document}

Two decorations lsdbond (left short double bond) and rsdbond (right short double bond) are defined. lsdbond puts the shortened (pi) bond on the left of the middle (sigma) bond, and rsdbond on the right. They take two arguments: the first is the bond angle on the departure atom of the bond; the second on the arrival atom. The angles are in degrees and should be in the range of (0,180].

Answer 2

Six years later the accepted answer above does not work anymore with Chemfig in version 1.5 ... Use the following code

\documentclass{article}
\usepackage{chemfig}
\usetikzlibrary{decorations}
\makeatletter
\catcode`\_=11
\newdimen\mystartshorten
\newdimen\myendshorten
\mystartshorten0pt
\myendshorten0pt
% shorter double bond
\pgfdeclaredecoration{sdbond}{initial}{
  \state{initial}[width=\pgfdecoratedremainingdistance,next state=final]
  {
    {
      \pgftransformyshift{\CF_doublesep}%
      \pgfpathmoveto{\pgfqpoint{\mystartshorten}{0pt}}
      {
        \pgftransformxshift{\pgfdecoratedremainingdistance}
        \pgfpathlineto{\pgfqpoint{-\myendshorten}{0pt}}
      }
    },decorations.markings
    \pgfpathmoveto{\pgfpointorigin}
    \pgfpathlineto{\pgfqpoint{\pgfdecoratedremainingdistance}{0pt}}
  }
  \state{final}
  {}
}
\tikzset{
  lsdbond/.code 2 args={%
    \tikzset{decorate, decoration=sdbond}%
    \pgfmathsetlength\mystartshorten{\CF_doublesep*cot((#1)/2)}%
    \pgfmathsetlength\myendshorten{\CF_doublesep*cot((#2)/2}%
  },
  rsdbond/.code 2 args={%
    \tikzset{decorate, decoration={sdbond, mirror}}%
    \pgfmathsetlength\mystartshorten{\CF_doublesep*cot((#1)/2)}%
    \pgfmathsetlength\myendshorten{\CF_doublesep*cot((#2)/2}%
  }
}
\setchemfig{
  double bond sep=0.4em,%
  atom sep=2em%
}
\catcode`\_=8
\makeatother
%
\begin{document}
  \chemfig{[:60]--[::-60,,,,,rsdbond={120}{120}]-[::-60]}
  \chemfig{[:90]--[::-90,,,,,rsdbond={90}{60}]-[::-120]}
\end{document}

The chemfig options have changed and the catcode statements seem to be essential.

Edit: Experimenting with the answers given so far I detected that scaling of the formulas gave only poor results. Inspired from another thread here I propose the following

\documentclass[a5paper]{article}
\usepackage{geometry}
\usepackage{chemfig}
\usetikzlibrary{decorations.markings}
\newcommand{\actualscale}{2.0}% give here your wished scale
\setchemfig{
  atom style={scale=\actualscale},
  double bond sep=3.8pt,% bond spacing
}
%
\catcode`\_=11
\pgfmathsetmacro{\currentscale}{\actualscale}
\tikzset{
  shrtdbl/.code 2 args={
   \tikzset{,shorten >= 0pt,shorten <= 0pt}\global\CF_addtomacro\CF_currentbondstyle{,shorten >= #1*\currentscale,shorten <= #2*\currentscale}}
}
\catcode`\_=8
%
\begin{document}
\chemfig{[:60]-=_[::-60,,,,shrtdbl={2pt}{2pt}]-[::-60]}
\chemfig{[:90]-=_[::-90,,,,shrtdbl={4pt}{2pt}]-[::-120]}
\end{document}

The amount of shortening the double bond must be given in points for scale 1.0 ... You will find proper values for your purposes by trial and error.