Diamond backgroundpath and anchorborder are wrong if diamond is not a rotated square

[hammondkd]

Brief outline of the bug

The diamond shape behaves as expected if used without either the minimum width or minimum height attributes (or if the two are set to the same value), but if they are different, the anchor border is wrong. Basically, if the diamond is not a rotated square, the anchor border is in the wrong place relative to the background path.

I am willing to fix this myself, but I have one question: should the solution be to force the object to have right-angle corners, or should a non-square diamond be allowed? I would assume the latter.

Minimal working example (MWE)

\documentclass[tikz]{standalone}
\usetikzlibrary{shapes.geometric}
\begin{document}
\begin{tikzpicture}
  \node [shape=diamond,draw=gray,line width=0.25cm,minimum height=3cm,
    minimum width=6cm] (s) {};
  \foreach \x in {0,...,359}
    \draw [red,shift=(s.\x)] plot [mark=x] coordinates {(0,0)};
\begin{scope}[yshift=-4cm]
  \node [shape=diamond,draw=gray,minimum size=4cm,line width=0.25cm] (t) {};
  \foreach \x in {0,...,359}
    \draw [orange,shift=(t.\x)] plot [mark=x] coordinates {(0,0)};
\end{scope}
\end{tikzpicture}
\end{document}

[muzimuzhi]

Surely a non-square diamond is allowed, and that's why /pgf/aspect=<ratio> is first documented for the shape diamond.

Further tests show the unexpected shifting of anchorborder is relevant to the line width.

\documentclass[tikz, margin=10pt]{standalone}
\usetikzlibrary{shapes.geometric, spy}

\NewDocumentCommand{\testDiamond}{}{%
  \begin{tikzpicture}[spy using outlines={circle, magnification=6, size=1.5cm, connect spies}]
    \node [shape=diamond,draw=gray,minimum height=3cm,minimum width=6cm] (s) {};
    \foreach \x in {north, west, south, east}
      \draw [red,opacity=.5,shift=(s.\x)] plot [mark=x] coordinates {(0,0)};
    \spy[green] on (s.north) in node[left] at ([shift={(-2cm, 1cm)}]s.west);
    \spy[blue] on (s.west) in node[left] at ([shift={(-2cm, -1cm)}]s.west);
  \end{tikzpicture}
}

\begin{document}

\tikzset{every diamond node/.style={line width=1pt}}
\testDiamond
\tikzset{every diamond node/.style={line width=10pt}}
\testDiamond

\end{document}

[hammondkd]

@muzimuzhi It is not just the line width: in your first example, the red X should go right through the outer corners, but it does not. If you zoom way in, particularly on the west and east corners, you can see that they do not line up.

I will work on a solution to this that allows non-square diamonds.

[muzimuzhi]

@hammondkd You're right. I added \spys to my examples in #1396 (comment). Also line width defaults to .4pt so 1pt is already not the default.

[muzimuzhi]

The magic number 1.414213 used in adjusting the size of \backgroundpath indicates that a static 1:1 ratio is assumed, see lines 334 and 335 in snippet below.

An other question: what should be the preference of aspect and minimum size?

pgf/tex/generic/pgf/libraries/shapes/pgflibraryshapes.geometric.code.tex

Lines 328 to 342 in 757f4c2

	\backgroundpath{
	\pgf@process{\outernortheast}%
	\pgf@xc=\pgf@x%
	\pgf@yc=\pgf@y%
	\pgfmathsetlength{\pgf@xa}{\pgfkeysvalueof{/pgf/outer xsep}}%
	\pgfmathsetlength{\pgf@ya}{\pgfkeysvalueof{/pgf/outer ysep}}%
	\advance\pgf@xc by-1.414213\pgf@xa%
	\advance\pgf@yc by-1.414213\pgf@ya%
	\pgfpathmoveto{\pgfqpoint{\pgf@xc}{0pt}}%
	\pgfpathlineto{\pgfqpoint{0pt}{\pgf@yc}}%
	\pgfpathlineto{\pgfqpoint{-\pgf@xc}{0pt}}%
	\pgfpathlineto{\pgfqpoint{0pt}{-\pgf@yc}}%
	\pgfpathclose%
	}%
	}%

[muzimuzhi]

This seems to work. I used \fpeval to do l3fp calculations for more accurate results.

Update: Hmm it may only "work" for the initial outer sep value (.5\pgflinewidth). I need to revise the relationship between current line width and general outer (x|y)sep values. It's expected behavior that in some cases the border anchors of a node won't lie right on the visual node border.

Update 2: Should we also multiply outer (x|y)sep by a ratio when adding them to get the final node size, in \outernortheast?

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{shapes.geometric, spy}

% patch the \backgroundpath of "diamond" shape
\makeatletter
\def\backgroundpath@new{
    \pgf@process{\outernortheast}%
    \pgf@xc=\pgf@x%
    \pgf@yc=\pgf@y%
    % <<< patch begin
    \edef\pgf@tempa{\fpeval{sqrt(\pgf@xc*\pgf@xc+\pgf@yc*\pgf@yc)}}
    \edef\pgfshapeoutersep@ratio@x{\fpeval{\pgf@tempa/\pgf@yc}}%
    \edef\pgfshapeoutersep@ratio@y{\fpeval{\pgf@tempa/\pgf@xc}}%
    \pgfmathsetlength{\pgf@xa}{\pgfkeysvalueof{/pgf/outer xsep}}%
    \pgfmathsetlength{\pgf@ya}{\pgfkeysvalueof{/pgf/outer ysep}}%
    \advance\pgf@xc by-\pgfshapeoutersep@ratio@x\pgf@xa%
    \advance\pgf@yc by-\pgfshapeoutersep@ratio@y\pgf@ya%
    % >>> patch end
    \pgfpathmoveto{\pgfqpoint{\pgf@xc}{0pt}}%
    \pgfpathlineto{\pgfqpoint{0pt}{\pgf@yc}}%
    \pgfpathlineto{\pgfqpoint{-\pgf@xc}{0pt}}%
    \pgfpathlineto{\pgfqpoint{0pt}{-\pgf@yc}}%
    \pgfpathclose%
  }%

\let\pgf@sh@bg@diamond=\backgroundpath@new
\makeatother

\newcounter{shape}
\NewDocumentCommand{\testDiamond}{O{diamond} m}{%
  \begin{tikzpicture}[spy using outlines={circle, magnification=6, size=1.5cm, connect spies}]
    \node [shape=#1,draw=gray,minimum height=3cm,minimum width=6cm,#2] (s) {};
    \foreach \x in {0,90,180,270}
      \draw [red,opacity=0.5,shift=(s.\x)] plot [mark=x] coordinates {(0,0)};
    \spy[green] on (s.north) in node[left] at ([shift={(-2cm, 1cm)}]s.west);
    \spy[blue] on (s.west) in node[left] at ([shift={(-2cm, -1cm)}]s.west);
  \end{tikzpicture}
}

\begin{document}

\testDiamond{line width=1pt}

\testDiamond{line width=10pt}

\testDiamond{line width=20pt}

\end{document}

[hammondkd]

Ha! I was going to sit down and work on this, and you already have it fixed! Excellent.

I don't know that it matters whether \outernortheast lines up, as it is only used to construct the other anchors (and those are fixed by this patch).

[muzimuzhi]

In tikz v3.1.10 diamond nodes, \outernortheast uses multiplier 1 when adding outer (x|y)sep to the final node size, while \backgroundpath uses multiplier 1.414213 (~= sqrt(2)) when subtracting outer (x|y)sep from the computed node size to for the background path. This way, the drawn background path may overlap with the node text, even for rotated square diamonds.

My patch generalizes the second multiplier, but doesn't solve this overlapping issue. Example below also shows that the bounding box of such diamond nodes are smaller than required such that the two tikzpictures overlap, which would never happen for rectangle nodes. (Of course this is more or less off-topic.)

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{shapes.geometric}

\begin{document}
\tikz \node[shape=diamond, draw=gray, line width=10pt] {Text text text};

\tikz \node[shape=diamond, draw=gray, line width=20pt] {Text text text};
\end{document}

[hammondkd]

Changing the two lines below "Add outer border" to "2*sqrt(2)*(outer xsep)" instead of "(outer xsep)" (and same for y) enlarges the box so it does not touch the text:

    %
    % Add outer border
    %
    \pgfmathsetlength\pgf@xa{\pgfkeysvalueof{/pgf/outer xsep}}%
    \advance\pgf@x by 2.828427\pgf@xa
    \pgfmathsetlength\pgf@ya{\pgfkeysvalueof{/pgf/outer ysep}}%
    \advance\pgf@y by 2.828427\pgf@ya
  }%
  \savedanchor\text{%

Example:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{shapes.geometric}

\begin{document}
\begin{tikzpicture}
  \node[shape=diamond, draw=gray, line width=10pt] {Text text text};
  \node[shape=diamond, minimum width=6cm,draw=gray, line width=10pt] at (8,0) (d) {Text text text};
\foreach \x in {north,south,east,west,outer north east}
    \draw [orange,shift=(d.\x)] plot [mark=x] coordinates {(0,0)};
\draw [blue] (d.north) -| (d.east);
  \node[shape=diamond, draw=gray, line width=0.4pt] at (0,-4) {Text text text};
  \node[shape=diamond, draw=gray, minimum width=6cm, line width=0.4pt] at (8,-4) {Text text text};
  \node[shape=diamond, draw=gray, line width=20pt] (x) at (0,-9) {Text text text};
\foreach \x in {north,south,east,west,outer north east}
    \draw [green,shift=(x.\x)] plot [mark=x] coordinates {(0,0)};
\draw [red] (x.north) -| (x.east);
  \node[shape=diamond, draw=gray, minimum width=6cm, line width=20pt] (y) at (8,-9) {Text text text};
\foreach \x in {north,south,east,west,outer north east}
    \draw [violet,shift=(y.\x)] plot [mark=x] coordinates {(0,0)};
\draw [yellow] (y.north) -| (y.east);

\end{tikzpicture}
\end{document}

Important: The above code also defines \outernortheast as the anchor outer north east so I can easily verify where it is.