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I want to generate hidden markov models using tikz. I have successfully generated nodes and connected them. But i want to generate dashed lines generating below the node to an empty space like this enter image description here. I tried to make it work using this code:

\begin{tikzpicture}[->,>=stealth',shorten >=1pt,auto,node distance=2.8cm,
                semithick,scale=1.5,transform shape]
\tikzstyle{every state}=[fill=red,draw=none,text=white]

\node[state]    (A)                    {$S_1$};
\node[state]    (B) [right of=A]       {$S_2$};


\path (A) edge [loop above] node {$a_{11}$} (A)
          edge [bend left]  node {$a_{12}$} (B)
      (B) edge [loop above] node {$a_{22}$} (B)
          edge [bend left]  node {$a_{21}$} (A);

\draw [dashed] (A) edge [bend below] node {$b_{11}$};
\end{tikzpicture}

But this code doesn't work for me. It generates something like thisenter image description here How do i draw dashed lines from the nodes exactly shown in the figure with nodes rainy and sunny?

share|improve this question
    
Welcome to TeX.SX! – Heiko Oberdiek 12 hours ago
    
@Sigur i tried something like this \draw [dashed] (A.west) edge node {$b_{11}$} (A.center) but didn't work. – MarKS 12 hours ago
    
@Sigur did this \draw [dashed] (A.west) node {$b_{11}$} (A.center);. I could see b11 but this time there was no dashed line. – MarKS 12 hours ago
    
Oh, sorry, now I understand what you wish. See answer by @HeikoOberdiek. – Sigur 12 hours ago
up vote 5 down vote accepted

Like this? The labels are not precise, you can change the labels to whatever you want.

enter image description here

\documentclass[border=2mm,tikz]{standalone}
\usetikzlibrary{arrows,automata,positioning}
\begin{document}

\begin{tikzpicture}[->,>=stealth',shorten >=1pt,node distance=2.8cm,
                semithick,scale=1.5,transform shape]
\tikzstyle{every state}=[fill=red,draw=none,text=white]

\node[state]    (A)                    {$S_1$};
\node[state]    (B) [right of=A]       {$S_2$};


\path[auto] (A) edge [loop above] node {$a_{11}$} (A)
          edge [bend left]  node {$a_{12}$} (B)
      (B) edge [loop above] node {$a_{22}$} (B)
          edge [bend left]  node {$a_{21}$} (A);

\node (01) at ([shift={(-3em,-4em)}]A.south) {$b12$};
\node (04) at ([shift={(0,-5em)}]A.south) {$b11$};
\node (05) at ([shift={(3em,-4em)}]A.south) {$b13$};

\draw [shorten >=0pt,dashed] (A.south) --node[pos=.7,fill=white,inner sep=1pt]{0.4} (04);
\draw [shorten >=0pt,dashed] (A.south) --node[fill=white,inner sep=1pt]{0.1} (01);
\draw [shorten >=0pt,dashed] (A.south) --node[fill=white,inner sep=1pt]{0.5} (05);

\end{tikzpicture}

\end{document}
share|improve this answer
    
thank you so much @AboAmmar it worked perfectly. I need to go through how the code works but it was really helpful. – MarKS 11 hours ago
up vote 5 down vote

The following example positions the node with $b_{11}$ below A.south and adds a dashed vector between A.south and the previously created node. 

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{arrows}
\tikzset{state/.style={circle, fill=red,text=white}}
\begin{document}
\begin{tikzpicture}[->,>=stealth',shorten >=1pt,auto,node distance=2.8cm,
                semithick,scale=1.5,transform shape]
\tikzstyle{every state}=[fill=red,draw=none,text=white]

\node[state]    (A)                    {$S_1$};
\node[state]    (B) [right of=A]       {$S_2$};


\path (A) edge [loop above] node {$a_{11}$} (A)
          edge [bend left]  node {$a_{12}$} (B)
      (B) edge [loop above] node {$a_{22}$} (B)
          edge [bend left]  node {$a_{21}$} (A);

\draw[dashed, ->, inner sep=0pt] (A.south) +(-90:2em) node (tmp) {$b_{11}$}
  (A.south) -- (tmp);
\end{tikzpicture}
\end{document}

Result

A more complicate example:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{arrows}
\tikzset{state/.style={circle, fill=red,text=white}}
\begin{document}
\begin{tikzpicture}[->,>=stealth',shorten >=1pt,auto,node distance=2.8cm,
                semithick,scale=1.5,transform shape]
\tikzstyle{every state}=[fill=red,draw=none,text=white]

\node[state]    (A)                    {$S_1$};
\node[state]    (B) [right of=A]       {$S_2$};


\path (A) edge [loop above] node {$a_{11}$} (A)
          edge [bend left]  node {$a_{12}$} (B)
      (B) edge [loop above] node {$a_{22}$} (B)
          edge [bend left]  node {$a_{21}$} (A);

\begin{scope}[densely dashed, ->, inner sep=.2em]
  \draw[shorten >=-2pt]
    (A.south) ++(-90:1.7em) node[font=\scriptsize] (tmp1) {0.4}
              ++(-90:1.8em) node[inner sep=0pt]      (tmp2) {$b_{11}$}
    (A.south) -- (tmp1) -- (tmp2);
  \draw[shorten >=4pt]
    (A.south) ++(-45:1.5em) node[font=\scriptsize] (tmp1) {0.5}
              ++(-45:2em)   node[inner sep=0pt]    (tmp2) {$b_{12}$}
    (A.south) -- (tmp1) -- (tmp2);
  \draw[shorten >=-3pt]
    (A.south) ++(-135:1.5em) node[font=\scriptsize] (tmp1) {0.1}
              ++(-135:2em) node[inner sep=0pt]      (tmp2) {$b_{10}$}
    (A.south) -- (tmp1) -- (tmp2);
\end{scope}
\end{tikzpicture}
\end{document}

Result

share|improve this answer
    
thank you very much @Heiko it works. But i wonder is it possible for b11 to be near the center of the dashed line? And how do i create angle to draw 2 more dashed lines? Unfortunately, i want exactly like shown in the image. – MarKS 11 hours ago
    
i wish i could mark 2 answers but thank you for all the help @HeikoOberdiek. – MarKS 11 hours ago

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