Argthtjtr

I was trying to replicate the figure in Box half filled color using TikZ.

I draw a rectangle from (0,0) to (4,3).

I locate two points (0,1) as A and (4,2) as B.

When I connect the points A and B, I was expecting that the line will touch the rectangle. But there is a gap when the two points are connected.

I wish to understand this behavior.

Code:

documentclass{article}

usepackage{tikz}

begin{document}

begin{tikzpicture}

  draw (0,0) rectangle (4,3);



  node (A) at (0,1) {};

  node (B) at (4,2) {};



  draw (A) -- (B);



end{tikzpicture}

end{document} 

It is because your path joins two nodes (A) and (B), not the coordinates themselves. This can be made clearer by adding draw option to the nodes:

documentclass[tikz]{standalone}

begin{document}

begin{tikzpicture}

  draw (0,0) rectangle (4,3);



  node[draw] (A) at (0,1) {};

  node[draw] (B) at (4,2) {};



  draw (A) -- (B);



end{tikzpicture}

end{document} 

Let's add some texts to make it even clearer:

documentclass[tikz]{standalone}

begin{document}

begin{tikzpicture}

  draw (0,0) rectangle (4,3);



  node[draw] (A) at (0,1) {A};

  node[draw] (B) at (4,2) {B};



  draw (A) -- (B);



end{tikzpicture}

end{document} 

So how to solve it? Of course, to join the coordinates, there is a standard solution with coordinate:

documentclass[tikz]{standalone}

begin{document}

begin{tikzpicture}

  draw (0,0) rectangle (4,3);



  coordinate (A) at (0,1);

  coordinate (B) at (4,2);



  draw (A) -- (B);



end{tikzpicture}

end{document} 

or you can have the coordinates directly

documentclass[tikz]{standalone}

begin{document}

begin{tikzpicture}

  draw (0,0) rectangle (4,3);



  draw (0,1) -- (4,2);



end{tikzpicture}

end{document} 

If you want to keep the nodes: you should use the coordinate (<node name>.center):

documentclass[tikz]{standalone}

begin{document}

begin{tikzpicture}

  draw (0,0) rectangle (4,3);



  node (A) at (0,1) {};

  node (B) at (4,2) {};



  draw (A.center) -- (B.center);



end{tikzpicture}

end{document} 

or set option coordinate for the nodes (suggested by Gregory Puleo):

documentclass[tikz]{standalone}

begin{document}

begin{tikzpicture}

  draw (0,0) rectangle (4,3);



  node[coordinate] (A) at (0,1) {};

  node[coordinate] (B) at (4,2) {};



  draw (A) -- (B);



end{tikzpicture}

end{document}

Output of the above four codes: