Line with transparence changing with TikZ

Question

I'd like to have a colored line that starts with transparent 100 and ends with trasparent, let's say, 30.

I tried this code:

\documentclass{standalone}

\usepackage{tikz}
\usetikzlibrary{calc,fadings,decorations.markings}


\makeatletter

\pgfkeys{/pgf/decoration/.cd,
         start shade/.store in = \startshade,
         end shade/.store in   = \endshade,
}

\pgfdeclaredecoration{shade change}{initial}{
\state{initial}[
width                     = \pgflinewidth,
next state                = line,
persistent precomputation = {\pgfmathdivide{(\startshade-\endshade)/100}{\pgfdecoratedpathlength}%
                             \let\@increment=\pgfmathresult
                             \pgfmathsetmacro\@pitch{\pgfdecoratedpathlength/100}
                             \def\@@n{0}
                             \def\@sh{\startshade}}
]{}

\state{line}[
width                      =  \pgflinewidth,
persistent postcomputation = {\pgfmathsetmacro\@@n{\@@n+1}
                              \pgfmathsetmacro\@sh{\startshade/100-\@increment*\@@n}}]
{
\pgfsetstrokeopacity{\@sh}
\pgfpathmoveto{\pgfpointorigin}
\pgfpathlineto{\pgfqpoint{\@pitch pt}{0pt}}
\pgfusepath{stroke}}

\state{final}{
\pgfpathmoveto{\pgfpointorigin}%
\pgfsetstrokeopacity{\@sh}
 \pgfusepath{stroke}%
}
}

\makeatother


\begin{document}
\centering
\begin{tikzpicture}
\draw[
line width = 2pt,
decoration = {shade change,
              start shade = 100,
              end shade   = 30},
decorate] (0,0)--++(3,3)node[midway,above]{$I$};

\end{tikzpicture}
\end{document}

but the result is the following:

Where do I do wrong?

Answer 1

Something like that :

\documentclass{standalone}

\usepackage{tikz}
\usetikzlibrary{calc,fadings,decorations.markings}


\makeatletter

\pgfkeys{/pgf/decoration/.cd,
         start shade/.store in = \startshade,
         end shade/.store in   = \endshade,
}

\pgfdeclaredecoration{shade change}{initial}{
\state{initial}[
width                     = \pgflinewidth,
next state                = line,
persistent precomputation = {\pgfmathdivide{(\startshade-\endshade)/100}%
 {\pgfdecoratedpathlength}%
                             \let\@increment=\pgfmathresult
 \pgfmathsetmacro\@pitch{\pgfdecoratedpathlength/100}
                             \def\@@n{0}
                             \def\x{\@@n}
 \def\@sh{\startshade}}
]{}

\state{line}[
width                      =  \pgflinewidth,
persistent postcomputation = {\pgfmathsetmacro\@@n{\@@n+1}
                     \pgfmathsetmacro\@sh{\startshade/100-\@increment*\@@n}}]
{%
\pgfsetstrokecolor{black!\@@n!white}%  
\pgfpathmoveto{\pgfpointorigin}
\pgfpathlineto{\pgfqpoint{\@pitch pt}{0pt}}
\pgfusepath{stroke}}

\state{final}{
\pgfpathmoveto{\pgfpointorigin}%
\pgfsetstrokecolor{black!\@@n!white}%  
 \pgfusepath{stroke}%
}
}

\makeatother


\begin{document}
\centering
\begin{tikzpicture}
\draw[
line width = 2pt,
decoration = {shade change,
              start shade = 100,
              end shade   = 30},
decorate] (0,0)--++(3,3)node[midway,above]{$I$};

\end{tikzpicture}
\end{document}