\documentclass{article}
\title{Region Layering}
\author{}
\date{}

\usepackage{graphicx}
\begin{document}
\maketitle

\section{Introduction}

When regions overlap in time, we need to decide which one should be
played.  Ardour has a few options to set how this decision is made.


\subsection{Layers}

Each region on a playlist is on a \emph{layer}.  All overlapping regions
are on a unique layer, and when overlaps exist the highest-layered
region is played.  This is illustrated in Figure~\ref{fig:basic-layering}.

\begin{figure}[ht]
\begin{center}
\includegraphics{basic-layering.pdf}
\end{center}
\caption{Basic region layering}
\label{fig:basic-layering}
\end{figure}

Here we see that region $A$ overlaps $B$, $B$ overlaps $C$, and
$D$ overlaps nothing.  There are several ways in which these regions
could be arranged; in the drawing, $A$ is on layer~2, $B$ on layer~1,
$C$ and $D$ on layer~0.  If this area is played back, region $A$ will
play in its entirety, followed by the end part of region $B$, followed
by the end part of region $C$, followed by the whole of region $D$.
This follows the basic rule that, at any given point, the region on
the highest layer will be played.


\section{Choice of layering}

There are two main decisions to be made with regards to how a playlist should be layered:

\begin{itemize}
\item Given overlapping regions, what order should they be layered in?
\item When should layering be changed?
\end{itemize}


\subsection{Layering order}

Ardour provides three-and-a-half ways to decide on the order in which
regions are layered.  The most basic choice is:

\begin{itemize}
\item \emph{Later is higher} --- regions which are later in time will
  be on higher layers.
\item \emph{Most recently added is higher} --- regions which were more
  recently added to the playlist will be on higher layers.
\item \emph{Most recently edited or added is higher} --- regions which
  were more recently edited or added to the playlist will be on
  higher layers.
\end{itemize}

This choice can be set per-session from the \emph{Session Properties} dialogue
box.

\subsubsection{Explicit ordering}

There are also cases when none of these rules should apply.  If, for
example, you want to put a given region at the top of the stack (on
the highest layer), this is possible using the region `raise to top'
command.  Following such a command (called an `explicit layering'),
the regions on the playlist may no longer obey any of the standard
ordering rules.

This situation also arises when editing tracks using the `stacked' layer mode.
In this mode, almost all layering is explicit.  When starting a region drag,
the other regions on a track spread apart vertically to allow the dragged
region to be dropped in any position within the region stack.  The normal
layering rules will only be followed if a region is dropped on top of another;
in all other cases, explicit layering will be used to put the region wherever
it was dropped.

\subsection{When to update layering}

There are two distinct approaches to updating layering:

\begin{itemize}
\item Update whenever any region edit is performed.
\item Update only when a region is edited such that a new overlap has been set up.
\end{itemize}

The approach to use is optional, and can be set in \emph{Session Properties}.

This decision only has consequences when an explicit layering command has
been used.  Consider the case in Figure~\ref{fig:explicit-layering1}.

\begin{figure}[ht]
\begin{center}
\includegraphics{explicit-layering1.pdf}
\end{center}
\caption{Explicit layering: stage 1}
\label{fig:explicit-layering1}
\end{figure}

Given that arrangement, imagine that we perform a `raise to top' on region $C$.
This results in the arrangement in Figure~\ref{fig:explicit-layering2}.

\begin{figure}[ht]
\begin{center}
\includegraphics{explicit-layering2.pdf}
\end{center}
\caption{Explicit layering: stage 2}
\label{fig:explicit-layering2}
\end{figure}

Imagine now that region $C$ is moved very slightly to the left, so
that it still overlaps both $A$ and $B$.  If we are updating whenever
any region edit is performed, this will result in a relayer; the
regions' arrangement will go back to that in
Figure~\ref{fig:explicit-layering1}.

If, on the other hand, we only relayer when a new overlap is set up,
the region layering will remain as in
Figure~\ref{fig:explicit-layering2}.  Before the edit, regions $A$,
$B$ and $C$ overlapped; after the edit, the situation is the same, so
no relayering is performed.

Another, more complex, example is shown in Figure~\ref{fig:tricky-explicit-layering}.

\begin{figure}[ht]
\begin{center}
\includegraphics{tricky-explicit-layering.pdf}
\end{center}
\caption{More complex explicit layering}
\label{fig:tricky-explicit-layering}
\end{figure}

% XXX: this makes no sense

Here, imagine that $C$ has been moved to the top of the stack with an explicit
`raise to top' command.  Now consider an extension of $C$ so that its
right-hand edge overlaps $D$.  If we are relayering only on new overlaps, this
case presents one new overlap (that of $C$ with $D$).  In this case, $C$ is
moved according to the current layering rules so that it is correct with
respect to $D$.  In addition, $A$ and $B$ are re-layered so that the relation
of $C$ to $A$ and $B$ is preserved.

\end{document}