TikZ decorations: How to draw a closed path little by little?

Question

To make an animation (here), I used a found code (here: @Jake answers). It works but not for all increments. It works when increments are 0.02 to 0.02, but not when increments are 0.04 to 0.04, 0.05 to 0.05 or 0.1 to 0.1.

Here with an increment of 0.04, the animation does not make the complete turn:

Here, with an increment of 0.02, the animation makes the complete turn but the path is not closed:

First problem:

• Why doesn't it work with any increment?
• How do I make the decoration work for any increment?

Edition: By replacing \foreach by \multido the increments do not pose any more problems, the complete turn is carried out.

\multido{\npos=0.00+0.05}{21}{
%\multido{\npos=0.00+0.04}{26}{
%\multido{\npos=0.00+0.1}{11}{
%\foreach \npos  in {0,.1,...,1}{% 
            \begin{tikzpicture}[scale=1]
                \fill[green!40](0,0) rectangle (4,4);
                \draw[start segment=\npos,blue,ultra thick] (0,0) -- (4,0) -- (4,4) -- (0,4) --cycle ;
            \end{tikzpicture} 
 } 

Edition 2: Ruixi Zhang's solution allows you to go around the entire path using a \foreach loop.

%\foreach \x [evaluate =\x as \npos using \x/50] in {0,1,...,50}{% increment of 0.02
%\foreach \x [evaluate =\x as \npos using \x/25] in {0,1,...,25}{% increment of 0.04
\foreach \x [evaluate =\x as \npos using \x/10] in {0,1,...,10}{% increment of 0.1
            \begin{tikzpicture}[scale=1]
                \fill[green!40](0,0) rectangle (4,4);
                \draw[start segment=\npos,blue,ultra thick] (0,0) -- (4,0) -- (4,4) -- (0,4) --cycle ;
            \end{tikzpicture} 

Second problem:

How to close the path?

\documentclass[tikz,border=5mm]{standalone}
\usetikzlibrary{decorations}

\pgfdeclaremetadecoration{start}{initial}{
    \state{initial}[width={\pgfmetadecoratedpathlength*\pgfdecorationsegmentlength},next state=final]{
        \decoration{lineto}
    }
    \state{final}{}
}

\tikzset{start segment/.style={decoration={start,raise=2mm,segment length=#1},decorate}} 

\begin{document}
\foreach \rpos  in {0,.02,.04,...,1}{% 
            \begin{tikzpicture}[scale=1]
                \fill[green!40](0,0) rectangle (4,4);
                \draw[start segment=\rpos,blue,ultra thick] (0,0) -- (4,0) -- (4,4) -- (0,4) --cycle ;
            \end{tikzpicture} 
 } 
\end{document}

• How do I close it completely?

Answer 1

You have to close the path when it is done. (Unfortunately there are some roundings in place. This explains why the check reads >0.999pt instead of =1pt.) One can check with \pgfkeys{/pgf/decoration automaton/if input segment is closepath=...} whether the last segment is the close path segment and then close the path if one is close to its end. This avoids closing paths that are not to be closed. Notice that 0.999pt is an empirical value. If you have a very extreme path in which the close path segment is very short, such as a path (0,0) -- (4,0) -- (4,4) -- (0,4) -- (0,0) --cycle ;, this answer may not work.

As for the question on "any increment": of course, if you use the above and try \foreach \rpos in {0,.03,...,1} ..., say, the contour won't close. However, this is because you instructed TikZ to draw 99% of the path in the last step as the last \rpos is 0.99 (roughly) in this loop. How would TikZ know that it is supposed to close? In this case, one has to add the closing point to the loop and say \foreach \rpos in {0,.03,...,1,0.9999}.

\documentclass[tikz,border=5mm]{standalone}
\usetikzlibrary{decorations}
\pgfkeys{pgf/.cd,
close/.code={\pgfpathclose}}

\pgfdeclaremetadecoration{draw part of a path}{initial}{
    \state{initial}[width={\pgfmetadecoratedpathlength*\pgfdecorationsegmentlength},next state=final]{
        \decoration{lineto}
    }
    \state{final}{\ifdim\pgfdecorationsegmentlength>0.999pt
        \pgfkeys{/pgf/decoration automaton/if input segment is closepath=/pgf/close}
    \fi 
    }
}
%
\tikzset{start segment/.style={decoration={draw part of a path,raise=2mm,segment length=#1},decorate}} 

\begin{document}
\foreach \rpos  in {0,.03,...,1,0.9999}{% 
            \begin{tikzpicture}[scale=1]
                \fill[green!40](0,0) rectangle (4,4);
                \draw[start segment=\rpos,blue,ultra thick] (0,0) -- (4,0) -- (4,4) -- (0,4) --cycle ;
                \begin{scope}[xshift=5cm]
                \fill[red!40](0,0) rectangle (4,4);
                \draw[start segment=\rpos,blue,ultra thick] (0,0) -- (4,0) -- (4,4) -- (0,4);
                \end{scope}
            \end{tikzpicture} 
 } 
\end{document}

This also works properly for \foreach \rpos in {0,.02,...,1,0.9999}{%.

And yes, the path is really closed at the end.

EDITS: I streamlined the code and removed the \typeouts.