How to split a shape created by hobby along a curve?

Question

I want to draw an arbitrary domain which I can split in two along an arbitrary curve so I can colour both subdomains using different colours. I tried drawing the domain using the hobby library, however I don't know how to colour it separately. I attach the code of what I tried below.

\documentclass[tikz]{standalone}
\usetikzlibrary{hobby}

\begin{document}
\begin{tikzpicture}[use Hobby shortcut,closed=true]
  \fill[draw=black, fill=orange] (-3.5,0.5) .. (-3,2.5) .. (-1,3.5).. (1.5,3).. (4,2.5).. (5,0.5) ..(2.5,-2).. (0,-1.5).. (-2.5,-2).. (-3.5,0.5);
  \draw[black] (-3.5, 0.5) to [out=0, in=-135] (-1.5,1) to[out=45, in=-118] (1.5,0) to[out=62, in=150] (5,0.5);
  \node[above] at (0,1.5) {Colour A};
  \node[above] at (0, -1) {Colour B};
\end{tikzpicture}
\end{document}

Thank you very much!

Answer 1

For example, clipping can be used without the need to modify the curves:

\documentclass[tikz]{standalone}
\usetikzlibrary{hobby}

\begin{document}
\begin{tikzpicture}[use Hobby shortcut,closed=true]
  %\fill[draw=black, fill=orange]
  \def\OuterCurve{%
    (-3.5,0.5) .. (-3,2.5) .. (-1,3.5) .. (1.5,3) .. (4,2.5) ..
    (5,0.5) ..(2.5,-2).. (0,-1.5).. (-2.5,-2).. (-3.5,0.5)
  }
  \coordinate (Left) at (-3.5, 0.5);
  \coordinate (Right) at (5, 0.5);
  \def\InnerCurve{%
    (Left) to [out=0, in=-135] (-1.5,1) to[out=45, in=-118]
    (1.5,0) to[out=62, in=150] (Right)
  }

  % Fill areas
  \begin{scope}
    \clip[use Hobby shortcut, closed=true] \OuterCurve;
    \begin{scope}
      \clip \InnerCurve
        -- (current bounding box.east |- Right)
        -- (current bounding box.south east)
        -- (current bounding box.south west)
        -- (current bounding box.west |- Left)
        -- cycle
      ;
      \fill[green] (current bounding box.south west)
        rectangle (current bounding box.north east)
      ;
    \end{scope}
    \clip \InnerCurve
      -- (current bounding box.east |- Right)
      -- (current bounding box.north east)
      -- (current bounding box.north west)
      -- (current bounding box.west |- Left)
      -- cycle
    ;
    \fill[orange] (current bounding box.south west)
      rectangle (current bounding box.north east)
    ;
  \end{scope}

  % Draw black curves
  \draw[use Hobby shortcut, closed=true] \OuterCurve;
  \draw \InnerCurve;

  \node[above] at (0,1.5) {Colour A};
  \node[above] at (0, -1) {Colour B};
\end{tikzpicture}
\end{document}

Remark:

• The clipping is additive, further \clip commands in the same scope reduce the clipped area by intersecting the current clipping path with the new path.

• The scope of the clipping can be controlled by environment scope. The \end{scope} restores the previous clipping path.