Clipping tikz and raster pictures zigzag style

Question

1. Is there a way to achieve this kind of clipping for raster images as well, not only for tikz pictures? -- Solved, see Edit #1.
2. Are there any easier solution to achieve this with TikZ? -- Solved, see Edit #1.
3. How to smooth the random step line a little more?

Here is my minimal working example:

\documentclass{article}

\usepackage{tikz}
\usepgflibrary{decorations.pathmorphing}
\usetikzlibrary{decorations.pathmorphing}

\begin{document}

\begin{tikzpicture}
    \fill[black](0, 0) rectangle (72pt, 72pt);
\end{tikzpicture}
\hspace{72pt}
\begin{tikzpicture}
    \clip (0, 0) rectangle (72pt, 36pt);
    \fill[black](0, 0) rectangle (72pt, 72pt);
    \filldraw[white] (0pt, 0pt) -- (72pt, 0pt) -- (72pt, 7pt)
    decorate [decoration={random steps,segment length=2pt,amplitude=1pt}] {-- (0pt, 7pt)}
    -- cycle;
\end{tikzpicture}

\end{document}

---

Edit #1: Found a better solution myself. I've merged the clip and filldraw commands, so clipping with a zigzag like random line at the bottom became a little easier.

Working code:

\begin{tikzpicture}
    \draw[clip] (0, 36pt)
    -- (0, 7pt)
    decorate [decoration={random steps,segment length=2pt,amplitude=1pt}] {-- (72pt, 7pt)}
    -- (72pt, 36pt)
    --cycle;
    \fill[black](0, 0) rectangle (72pt, 72pt);
\end{tikzpicture}

Clippnig a 72pt * 72pt png image (or any raster images):

\begin{tikzpicture}
    \draw[clip] (0, 36pt)
    -- (0, 7pt)
    decorate [decoration={random steps,segment length=2pt,amplitude=1pt}] {-- (72pt, 7pt)}
    -- (72pt, 36pt)
    --cycle;
    \node[anchor=south west,inner sep=0] (image) at (0,0) {\includegraphics[width=72pt]{black.png}};
\end{tikzpicture}

Still, there remain two unanswered questions. Let me rephrase my original third quiestion:

• How to round the edges of the random step line?

Clipping a picture in an easy way raises another question:

• Is there any way to set up a coordinate system at the beginning of the tikzpicture, so one shouldn't have to use exact lengths while clipping but scales instead?

Answer 1

First part of my question is answered already in the question itself, see Edit #1.

For rounding corners and using a [0,1] interval when addressing lengths during clipping I came up with this solution:

\begin{tikzpicture}[x=72pt,y=36pt]
    \draw[clip] (0, 1)
    -- (0, 0.2)
    decorate [decoration={random steps,segment length=4pt,amplitude=2pt},
              rounded corners=1pt] {-- (1, 0.1)}
    -- (1, 1)
    --cycle;
    \node[anchor=south west,inner sep=0] (image) at (0,0)
        {\includegraphics[width=72pt]{black.png}};
\end{tikzpicture}

First I added the rounded corners option to the decorated path. Second I specified the exact size of the tikzpicture at the start: \begin{tikzpicture}[x=72pt,y=36pt]. This may not be the most elegant solution but works like a charm, see the picture below (the blue box illustrates the size of the original picture).