Delete part of drawing (or draw in transparent color)

Question

I'm looking for a way to delete part of a previously drawn picture.

An easy way is drawing in the background color, but that does not work if the background is transparent.

Here's an example file.tex:

\documentclass[convert={density=100,outext=.png}]{standalone}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
  % draw a black line   
  \draw (0,0) -- (2,1);
  % "delete" part of the line
  \draw[fill,color=white](0.5,0) rectangle (1.5,1);
\end{tikzpicture}
\end{document}

I use the standalone class to produce a single picture. When run with pdflatex -shell-escape file.tex, this builds both:

• file.pdf
• file.png

The pdf looks fine, since background is indeed white.

The png is built through the imagemagick convert tool and has a transparent background, and so the "deleted" part appears as a white rectangle.

I know I can use the -alpha remove -background white options for convert to turn the background white but I'd rather like the reverse: having transparent background, including the "deleting rectangle".

I thought about using a reverse clip (as in How can I invert a 'clip' selection within TikZ?) but I can't know in advance which part I don't want to be drawn, thus the need to delete rather than avoid drawing the unwanted part in the first place.

Is there any way to do that?

Answer 1

Taking the idea to save the path to the aux file so that it is available at the next run.

I didn't test it much but for this example it works. I might have experience with using the aux file but I'm no expert. Externalizing the picture with the external library won't work.

---

On any path of TikZ you can use the

• remember path = <name> key so that it will be available at the next compilation and the
• remembered path = <name> key that uses this path (if it is available).

The key will use \pgfpictureid as a prefix so that you can specify the same path name in different pictures.

---

I've also defined a reverse clip key that supports both a clockwise as well as a counter clockwise rectangle with maximum supported size. This is done on the PGF layer and with the quick commands for three reasons:

1. they are quicker,
2. they ignore transtormations and
3. they don't update the bounding box (though, your \path[remember path=…] might use overlay anyway.

---

This is how you use it:

\begin{tikzpicture}
\fill[left color=blue, right color=green] (0,0) rectangle (2,1);
\clip[remembered path=box];
\draw (0,0) -- (2,1);
\path[remember path=box] (0.5, 0) rectangle (1.5, 1) [reverse clip];
\end{tikzpicture}

Code

\documentclass[border=5pt, tikz, convert]{standalone}
% Part 1: Quick PGF versions of reverse clips.
%% * uses quick commands that
%%   * ignore transformations and
%%   * don't contribute to the bounding box (we use overlay though anyway)
%% * only uses 16000pt since this is what PGF does, too (TeX'd allow 16384pt)
%% * clockwise and counterclockwise for nonzero and even odd rule
%%
\tikzset{% quick versions of reverse clips
  reverse clip/.is choice,
  reverse clip/clockwise/.code={%
    \pgfpathqmoveto{16000pt}{16000pt}%
    \pgfpathqlineto{16000pt}{-16000pt}%
    \pgfpathqlineto{-16000pt}{-16000pt}%
    \pgfpathqlineto{-16000pt}{16000pt}%
    \pgfpathclose},
  reverse clip/counter clockwise/.code={%
    \pgfpathqmoveto{16000pt}{16000pt}%
    \pgfpathqlineto{-16000pt}{16000pt}%
    \pgfpathqlineto{-16000pt}{-16000pt}%
    \pgfpathqlineto{16000pt}{-16000pt}%
    \pgfpathclose},
  reverse clip/.default=counter clockwise}
% Part 2: The actual keys
%         that save/use the path to/from the aux file.
%% * remember path will does what ever you do on the path
%%   and save it to the aux file
%% * remembered path will set the remembers path (if available)
%%   and use it as specified
%%
\makeatletter
\tikzset{
  remember path/.code={%
    \tikz@addmode{%
      \expandafter\pgfsyssoftpath@getcurrentpath\csname qrr@tikzpath@\pgfpictureid @#1\endcsname
      \immediate\write\pgfutil@auxout{%
        \noexpand\expandafter\gdef\noexpand\csname qrr@tikzpath@\pgfpictureid @#1\endcsname{%
          \expandafter\expandafter\expandafter\unexpanded
          \expandafter\expandafter\expandafter{\csname qrr@tikzpath@\pgfpictureid @#1\endcsname}}}}%
  },
  remembered path/.code={%
    \pgfutil@IfUndefined{qrr@tikzpath@\pgfpictureid @#1}{}{%
      \tikz@addmode{%
        \expandafter\pgfsyssoftpath@setcurrentpath
        \expandafter{\csname qrr@tikzpath@\pgfpictureid @#1\endcsname}}}}}
\makeatother
\begin{document}
\begin{tikzpicture}
\fill[left color=blue, right color=green] (0,0) rectangle (2,1);
\clip[remembered path=box];
\draw (0,0) -- (2,1);
\path[remember path=box] (0.5, 0) rectangle (1.5, 1) [reverse clip];
\end{tikzpicture}
\end{document}

Output

First compilation

Second compilation