\chapter{Triangle Quality Metrics}

All the metrics in this section are defined on a triangular element
as illustrated in Figure~\ref{f:tri}.

\begin{figure}[bhp]
  \centering
  \includegraphics[width=2in]{tri}
  \caption{Numbering of vertices and edges on a triangular element.%
                                                                  \label{f:tri}}
\end{figure}

Note that unlike all the other elements that follow,
we name edge vectors of the triangle by the vertex opposite the edge so that
\begin{equation*}
\begin{array}{lcl}
  \vec L_0 &=& \vec P_2 - \vec P_1\\
  \vec L_1 &=& \vec P_0 - \vec P_2\\
  \vec L_2 &=& \vec P_1 - \vec P_0.
\end{array}
\end{equation*}

The triangle edge lengths are denoted as follows:
\[
L_0 = \normvec{L_0}\quad
L_1 = \normvec{L_1}\quad
L_2 = \normvec{L_2}
\]
and the largest and smallest edge lengths are, respectively,
\[
L_{\min} = \min\left(L_0, L_1, L_2\right)
  \rule{2em}{0pt}
L_{\max} = \max\left(L_0, L_1, L_2\right)
\]

The area of a triangle is one half the magnitude of the cross product of any pair of adjacent edge vectors:
\begin{equation*}
  A
    = \frac{1}{2}\normvec{L_0\times\vec L_1}
    = \frac{1}{2}\normvec{L_1\times\vec L_2}
    = \frac{1}{2}\normvec{L_2\times\vec L_0}
\end{equation*}

In addition, we will let $r$ be the inradius
\begin{equation*}
\label{eq:Arp}
  r = \frac{2A}{\normvec{L_0} + \normvec{L_1} + \normvec{L_2}}
\end{equation*}
and $R$ the circumradius
\[
  R = \frac{\normvec{L_0} \normvec{L_1} \normvec{L_2}}%
           {2 r \left(\normvec{L_0} + \normvec{L_1} + \normvec{L_2}\right)}
\]
of the triangle.
These are respectively the radii of the inscribed and circumscribed circles of this triangle.

We will frequently use $n$ to represent some arbitrary edge $L_n$ or vertex $P_n$ of the triangle.
When referring to the next counterclockwise entry $n+1$ (or clockwise entry $n-1$),
we take the result modulo $3$ so that, for example, if $n = 1$, $n+1 = 2$ and $n+2 = 0$.

% -------------------Metric Table-------------------
\newcommand{\trimetrictable}[8]{%
  \begin{center}
  \begin{tabular}{ll}
    \multicolumn{2}{r}{\textbf{\sffamily\Large triangle #1}}\\\hline
    Dimension:                           & #2\\ 
    Acceptable Range:                    & #3\\ 
    Normal Range:                        & #4\\ 
    Full Range:                          & #5\\ 
    $q$ for equilateral unit triangle:   & #6\\
    Reference:                           & #7\\
    \verd\ function:       & \texttt{#8}\\ \hline
  \end{tabular} 
  \end{center}
}

\newpage \input{TriArea}
\newpage \input{TriAspectRatio}
\newpage \input{TriAspectFrobenius}
\newpage \input{TriCondition}
\newpage \input{TriDistortion}
\newpage \input{TriEdgeRatio}
\newpage \input{TriMaximumAngle}
\newpage \input{TriMinimumAngle}
\newpage \input{TriScaledJacobian}
\newpage \input{TriRadiusRatio}
\newpage \input{TriRelativeSizeSquared}
\newpage \input{TriShape}
\newpage \input{TriShapeAndSize}