Repeat nodes in tikz rectangle iteratively

Question

So I want to fill a rectangle, like below, but without all the hassle of forlooping and getting coordinates.

I want a magic command that places my symbol repeatedly inside a rectangle. Preferably given a start and end coordinate;

\automagicGrid[step size x=0.1, step size y=0.1]{(rect.south west)}{(rect.north east)}{\node{$\otimes$}}

Here is an example of a manual, terrible way to do it.

\documentclass[margin=10pt]{standalone}
\usepackage{tikz}
\usetikzlibrary{fit}

\begin{document}
  \begin{tikzpicture}
    \makeatletter
    \tikzset{autofit/.style={
          % autofit: automagically generate awesome anchorpoints
          fit={(\pgf@pathminx,\pgf@pathminy) (\pgf@pathmaxx,\pgf@pathmaxy)},
          inner sep=0pt
        }
    }
    \makeatother
    % Draw my rectangle
    \def\rectH{2}
    \def\rectW{2}
    \draw(0,0) rectangle (\rectW,\rectH) node[autofit](rect){};
    % I want this command
    % \automagicGrid[step size x=0.1, step size y=0.1]{(rect.south west)}{(rect.north east)}{\node{$\otimes$}}

    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
    %%  Below is what I DON'T want  %%
    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
    \def\xStart{.2}
    \def\xStepSize{.3}
    \edef\paddingX{.1}
    \pgfmathsetmacro\xStep{\xStart+\xStepSize}
    \pgfmathsetmacro\xEnd{\rectH-\paddingX}

    \def\yStart{.2}
    \def\yStepSize{.3}
    \edef\paddingY{.1}
    \pgfmathsetmacro\yStep{\yStart+\xStepSize}
    \pgfmathsetmacro\yEnd{\rectH-\paddingY}

    \foreach \x in {\xStart,\xStep,...,\xEnd}{
        \foreach \y in {\yStart, \yStep,...,\yEnd}{
            \node at (\x+\paddingX/2,\y+\paddingY/2){$\otimes$};
          }
      }
  \end{tikzpicture}
\end{document}

And the result:

I will be happy though if someone just find a clean way to fetch said coordinates, because that's the real hassle of this.

Answer 1

I do not see how one can get rid of the \foreach but one could put them into a style. Using a path picture we now how large the thing is we want to fill, and can dial the loop parameters accordingly.

\documentclass[margin=10pt]{standalone}
\usepackage{tikz}
\usetikzlibrary{fit,calc}
\pgfkeys{/automagic/.cd,
  grid/.style={draw=gray!50,thin},
  x distance/.initial=3mm,
  y distance/.initial=3mm,
  x padding/.initial=2.5mm,
  y padding/.initial=2.5mm,
  distance/.code={\pgfkeys{/automagic/x distance=#1,/automagic/y distance=#1}},
  symbol/.initial= $\otimes$
}
\tikzset{automagic grid/.style={/utils/exec=\pgfkeys{/automagic/.cd,#1},
      path picture={
          \path[/automagic/grid]
          let
          \p1=($(path picture bounding box.north east)-(path picture bounding box.south west)$),
          \n1={int(0.5+(\x1-\pgfkeysvalueof{/automagic/x padding})/\pgfkeysvalueof{/automagic/x distance})},
          \n2={int(0.5+(\y1-\pgfkeysvalueof{/automagic/y padding})/\pgfkeysvalueof{/automagic/y distance})}
          in
          foreach \XX in {1,...,\n1}
            {
              foreach \YY in {1,...,\n2} {
                  ([
                      xshift={(\XX-1)*\pgfkeysvalueof{/automagic/x distance}+\pgfkeysvalueof{/automagic/x padding}},
                      yshift={(\YY-1)*\pgfkeysvalueof{/automagic/y distance}+\pgfkeysvalueof{/automagic/y padding}}
                    ]%
                  path picture bounding box.south west%
                  ) node{\pgfkeysvalueof{/automagic/symbol}}
                }
            };
        }}}
\begin{document}
  \begin{tikzpicture}
    % Draw my rectangle
    \def\rectH{2}
    \def\rectW{2}
    \draw[automagic grid](0,0) rectangle ++(\rectW,\rectH);
  \end{tikzpicture}
\end{document}