LaTeX でさまざまな形状のフロー図を描くにはどうすればよいでしょうか?

Question

下記のブロック図を描きたいのですが、オンラインヘルプを使って描こうとしたのですがうまくいきません。

コード：

\documentclass[border=10pt]{standalone}
\usepackage{tikz}
\usetikzlibrary{arrows,positioning,shapes.geometric}
\begin{document}
    \begin{tikzpicture}[>=latex']
        \tikzset{block/.style= {draw, rectangle, align=center,minimum width=2cm,minimum height=1cm},
        rblock/.style={draw, shape=rectangle,rounded corners=1.5em,align=center,minimum width=2cm,minimum height=1cm},
        input/.style={ % requires library shapes.geometric
        draw,
        trapezium,
        trapezium left angle=60,
        trapezium right angle=120,
        minimum width=2cm,
        align=center,
        minimum height=1cm
    },
        }
        \node [rblock]  (start) {Start};
        \node [block, right =2cm of start] (acquire) {Acquire Image and Modify};
        \node [block, right =2cm of acquire] (rgb2gray) {RGB to Gray};
        \node [block, right =2cm of rgb2gray] (otsu) {Localized OTSU \\ Thresholding};
        

%% paths
        \path[draw,->] (start) edge (acquire)
                    (acquire) edge (rgb2gray)
                    (rgb2gray) edge (otsu)
                    (otsu.east) -| (right) -- (left) |- (gchannel)
                    (gchannel) edge (closing)
                    (closing) edge (NN)
                    (NN) edge (limit)
                    ;
    \end{tikzpicture}
\end{document}