TikZ: rotating scope changes the shading, but not how you'd expect

Question

It is my understanding that the shading in the following code is applied after the scope transformation.

\documentclass[varwidth,margin=0.5cm]{standalone}
\usepackage{tikz}
\begin{document}
\pgfdeclareverticalshading{myshade}{2cm}{
    color(0cm)=(red);
    color(.3cm)=(orange);
    color(.6cm)=(yellow);
    color(.9cm)=(green);
    color(1.2cm)=(blue);
    color(1.5cm)=(purple);
    color(1.8cm)=(brown)
}

\begin{tikzpicture}
\begin{scope}[rotate=26.73]
    \draw [shading=myshade] (0, 0) circle (1);
\end{scope}
\end{tikzpicture}
\begin{tikzpicture}
\begin{scope}
    \draw [shading=myshade] (0, 0) circle (1);
\end{scope}
\end{tikzpicture}

\end{document}

Given that a circle is symmetrical by rotation, the result of both tikzpictures should be the same. Yet the shading is different. Why is this the case?

The shading is done on the basis of the bounding box. A circle is constructed from Bezier curves, and this means that its bounding box changes under rotations. As you can see, the circles are not vertically aligned either, for the same reason. — , Apr 17 '20 at 16:25

score 3 · Answer 1 · 2020-04-17T16:39:46.657

The shading is done on the basis of the bounding box, see e.g. here. A circle is constructed from Bezier curves, and this means that its bounding box changes under rotations. As you can see, the circles are not vertically aligned either, for the same reason. Once you fix the bounding box, the effect is gone.

\documentclass[varwidth,margin=0.5cm]{standalone}
\usepackage{tikz}
\usetikzlibrary{bbox}
\begin{document}
\pgfdeclareverticalshading{myshade}{2cm}{
    color(0cm)=(red);
    color(.3cm)=(orange);
    color(.6cm)=(yellow);
    color(.9cm)=(green);
    color(1.2cm)=(blue);
    color(1.5cm)=(purple);
    color(1.8cm)=(brown)
}

\begin{tikzpicture}[bezier bounding box]
\begin{scope}[rotate=26.73]
    \draw [shading=myshade] (0, 0) circle (1);
\end{scope}
\end{tikzpicture}
\begin{tikzpicture}[bezier bounding box]
\begin{scope}
    \draw [shading=myshade] (0, 0) circle (1);
\end{scope}
\end{tikzpicture}

\end{document}

If you want to rotate the shading, change the shading angle. (You can also use transform canvas but this is sometimes a bit hard to tame.)

\documentclass[varwidth,margin=0.5cm]{standalone}
\usepackage{tikz}
\usetikzlibrary{bbox}
\begin{document}
\pgfdeclareverticalshading{myshade}{2cm}{
    color(0cm)=(red);
    color(.3cm)=(orange);
    color(.6cm)=(yellow);
    color(.9cm)=(green);
    color(1.2cm)=(blue);
    color(1.5cm)=(purple);
    color(1.8cm)=(brown)
}

\begin{tikzpicture}[bezier bounding box]
\path (0, 0) circle[radius=1];
\begin{scope}[transform canvas={rotate=26.73}]
    \draw [shading=myshade] (0, 0) circle[radius=1];
\end{scope}
\end{tikzpicture}
\begin{tikzpicture}[bezier bounding box]
\begin{scope}
    \draw [shading=myshade,shading angle=26.73] (0, 0) circle[radius=1];
\end{scope}
\end{tikzpicture}
\begin{tikzpicture}[bezier bounding box]
\begin{scope}
    \draw [shading=myshade] (0, 0) circle[radius=1];
\end{scope}
\end{tikzpicture}
\end{document}

I think the OP's question is about rotating the inside color and not about bounding box! Isn't? — , Apr 17 '20 at 16:34
@C.F.G The question reads: "Given that a circle is symmetrical by rotation, the result of both tikzpictures should be the same. Yet the shading is different. Why is this the case?". — , Apr 17 '20 at 16:35
Can bezier bounding box be used with a scope, or does it have to apply to the entire tikzpicture? — usernumber, Apr 17 '20 at 19:58
@usernumber It can (and should) be applied in a scope. The library is not perfect at this point. — , Apr 17 '20 at 20:12
Note that the bbox library was removed from TikZ with version 3.1.6 due to licensing issues: https://github.com/pgf-tikz/pgf/releases/tag/3.1.6 — Torbjørn T., Sep 28 '20 at 18:25

TikZ: rotating scope changes the shading, but not how you'd expect

1 Answers1