\subsubsection{PostScript}

Adobe's PostScript Language Reference Manual defines a turing complete language for producing graphics output on an abstract ``output device''\cite{plrm}. A PostScript document is treated as a procedural program; an interpreter executes instructions in the order they are written by the programmer. Each symbol is pushed onto a stack as it is read. Special symbols called ``operators'' can act upon this stack and/or the output device. An internal ``graphics state'' stack can be constructed to store styling information (such as colour, line thickness, the current cursor position). It is possible for the language to define new operators. Figure \ref{PS} shows a vector image and one possible way to express this image in PostScript. PostScript was and is still widely used in printing of documents onto paper; many printers execute postscript directly, and newer formats including PDFs must still be converted into PostScript by printer drivers\cite{pdfref17, cheng2002portable}.

There are some limitations in PostScript's model. As mentioned in Section \ref{Compositing}, since PostScript predates Porter and Duff Compositing, there is no concept of transparency. In fact, using tools to convert between the SVG image in Figure \ref{SVG} and PostScript will simply rasterise the image and embed the rastered image in PostScript\footnote{For Figure \ref{SVG} converted using the Inkscape SVG editor: \url{http://szmoore.net/ipdf/figures/shape-svg-converted-to.ps}} 

Another limitation of PostScript is that the model of a document as a static page, convenient for printers which literally produce static pages, is unable to include interactive or dynamic elements. Dynamic PostScript attempted to fix this problem, but ``never caught on''\cite{hayes2012pixels}.

\begin{figure}[H]
\begin{minipage}[t]{0.65\textwidth}
\begin{minted}{postscript}
%!PS-Adobe-3.0 EPSF-3.0
%%BoundingBox: 0 -1 85 150
% These lines are comments to aid in human understanding
% Define an operator to produce a rectangular path
/re { exch dup neg 3 1 roll 5 3 roll moveto 0 rlineto
      0 exch rlineto 0 rlineto closepath } bind def
% Operator to produce the path for the first rectangle
/re1 { 24.613 133.001 24 -120 re } bind def
% Operator to produce the path for the second rectangle
/re2 { 10.215 45.001 48 -16 re } bind def
% Operator which will produce the curved path
/curve { 46.215 1.001 moveto 
	46.215 1.001 91.812 11.399 71.812 35.399 curveto 
	51.812 59.399  29.414 33.802 51.812 59.399 curveto 
	74.215 85.001 93.414 45.802 74.215 85.001 curveto 
	55.016 125.001 61.414 49.802 46.215 75.399 curveto 
	31.016 101.001 56.613 126.598 56.613 126.598 curveto 
	56.613 126.598 88.613 166.598 56.613 137.802 curveto 
	24.613 109.001 -18.586 83.399 9.414 50.598 curveto 
	37.414 17.802 45.414 1.001 45.414 1.001 curveto 
closepath } bind def
% Set stroke properties
0.8 setlinewidth 0 setlinecap 0 setlinejoin [] 
	0.0 setdash 4 setmiterlimit 
% Draw the straight line
0 setgray 0.613 149.001 moveto 83.812 0.2 lineto fill 
% Fill and outline the first rectangular path
0 0 1 setrgbcolor re1 fill 0 setgray re1 stroke
% Fill and outline the curved shape
1 0 0 setrgbcolor curve fill 0 setgray curve stroke
% Fill and outline the second rectangle 
0 1 0 setrgbcolor re2 fill 0 setgray re2 stroke
showpage
\end{minted}
\end{minipage}
\begin{minipage}[t]{0.3\textwidth}
	\begin{figure}[H]
	\centering
	\includegraphics[width=1\textwidth]{figures/shape.eps}
	\end{figure}
\end{minipage}
	\caption{Vector image and a possible PostScript representation}\label{PS}
\end{figure}

\subsubsection{{\TeX}, METAFONT and {\LaTeX}}

Knuth's ``The {\TeX}book''\cite{knuth1984texbook} and ``The METAFONT book''\cite{knuth1983metafont} define two complementary programming languages for typesetting documents. Wheras PostScript may be considered an interpreted language, in that it can be produced in a human readable form which is also readable by an interpreter, {\TeX} is a compiled language; a program parses human readable {\TeX} to produce a machine readable format DVI (``DeVice Independent''). A DVI interpreter might be thought of as a virtual ``Display Processor'' for drawing vector graphics directly (as defined in the earlier editions of ``Computer Graphics''\cite{computergraphics2}). 

DVI itself is not a widely used format for sharing documents. However, an system based upon {\TeX} called {\LaTeX} which includes libraries for advanced typesetting and programs that ultimately produce PDF output is particularly popular for producing technical reports and papers\footnote{The site \url{http://tex.stackexchange.com} (accessed 2014-05-22) is devoted to {\TeX} and {\LaTeX}} --- this report itself has been produced using the CTAN {\LaTeX} packages\footnote{The complete {\TeX} source code to produce this document can be found at \url{http://szmoore.net/ipdf/sam/}}.
