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+% Copyright 2018 by Till Tantau
+%
+% This file may be distributed and/or modified
+%
+% 1. under the LaTeX Project Public License and/or
+% 2. under the GNU Free Documentation License.
+%
+% See the file doc/generic/pgf/licenses/LICENSE for more details.
+
+
+\section{Design Principles}
+
+This section describes the basic layer of \pgfname. This layer is built on top
+of the system layer. Whereas the system layer just provides the absolute
+minimum for drawing graphics, the basic layer provides numerous commands that
+make it possible to create sophisticated graphics easily and also quickly.
+
+The basic layer does not provide a convenient syntax for describing graphics,
+which is left to frontends like \tikzname. For this reason, the basic layer is
+typically used only by ``other programs''. For example, the \textsc{beamer}
+package uses the basic layer extensively, but does not need a convenient input
+syntax. Rather, speed and flexibility are needed when \textsc{beamer} creates
+graphics.
+
+The following basic design principles underlie the basic layer:
+%
+\begin{enumerate}
+ \item Structuring into a core and modules.
+ \item Consistently named \TeX\ macros for all graphics commands.
+ \item Path-centered description of graphics.
+ \item Coordinate transformation system.
+\end{enumerate}
+
+
+\subsection{Core and Modules}
+
+The basic layer consists of a \emph{core package}, called |pgfcore|, which
+provides the most basic commands, and several \emph{modules} like commands for
+plotting (in the |plot| module). Modules are loaded using the |\usepgfmodule|
+command.
+
+If you say |\usepackage{pgf}| or |\input pgf.tex| or |\usemodule[pgf]|, the
+|plot| and |shapes| modules are preloaded (as well as the core and the system
+layer).
+
+
+\subsection{Communicating with the Basic Layer via Macros}
+
+In order to ``communicate'' with the basic layer you use long sequences of
+commands that start with |\pgf|. You are only allowed to give these commands
+inside a |{pgfpicture}| environment. (Note that |{tikzpicture}| opens a
+|{pgfpicture}| internally, so you can freely mix \pgfname\ commands and
+\tikzname\ commands inside a |{tikzpicture}|.) It is possible to ``do other
+things'' between the commands. For example, you might use one command to move
+to a certain point, then have a complicated computation of the next point, and
+then move there.
+%
+\begin{codeexample}[]
+\newdimen\myypos
+\begin{pgfpicture}
+ \pgfpathmoveto{\pgfpoint{0cm}{\myypos}}
+ \pgfpathlineto{\pgfpoint{1cm}{\myypos}}
+ \advance \myypos by 1cm
+ \pgfpathlineto{\pgfpoint{1cm}{\myypos}}
+ \pgfpathclose
+ \pgfusepath{stroke}
+\end{pgfpicture}
+\end{codeexample}
+
+The following naming conventions are used in the basic layer:
+%
+\begin{enumerate}
+ \item All commands and environments start with |pgf|.
+ \item All commands that specify a point (a coordinate) start with
+ |\pgfpoint|.
+ \item All commands that extend the current path start with |\pgfpath|.
+ \item All commands that set/change a graphics parameter start with
+ |\pgfset|.
+ \item All commands that use a previously declared object (like a path,
+ image or shading) start with |\pgfuse|.
+ \item All commands having to do with coordinate transformations start with
+ |\pgftransform|.
+ \item All commands having to do with arrow tips start with |\pgfarrows|.
+ \item All commands for ``quickly'' extending or drawing a path start with
+ |\pgfpathq| or |\pgfusepathq|.
+ \item All commands having to do with matrices start with |\pgfmatrix|.
+\end{enumerate}
+
+
+\subsection{Path-Centered Approach}
+
+In \pgfname\ the most important entity is the \emph{path}. All graphics are
+composed of numerous paths that can be stroked, filled, shaded, or clipped
+against. Paths can be closed or open, they can self-intersect and consist of
+unconnected parts.
+
+Paths are first \emph{constructed} and then \emph{used}. In order to construct
+a path, you can use commands starting with |\pgfpath|. Each time such a command
+is called, the current path is extended in some way.
+
+Once a path has been completely constructed, you can use it using the command
+|\pgfusepath|. Depending on the parameters given to this command, the path will
+be stroked (drawn) or filled or subsequent drawings will be clipped against
+this path.
+
+
+\subsection{Coordinate Versus Canvas Transformations}
+\label{section-design-transformations}
+
+\pgfname\ provides two transformation systems: \pgfname's own \emph{coordinate}
+transformation matrix and \pdf's or PostScript's \emph{canvas} transformation
+matrix. These two systems are quite different. Whereas a scaling by a factor
+of, say, $2$ of the canvas causes \emph{everything} to be scaled by this factor
+(including the thickness of lines and text), a scaling of two in the coordinate
+system causes only the \emph{coordinates} to be scaled, but not the line width
+nor text.
+
+By default, all transformations only apply to the coordinate transformation
+system. However, using the command |\pgflowlevel| it is possible to apply a
+transformation to the canvas.
+
+Coordinate transformations are often preferable over canvas transformations.
+Text and lines that are transformed using canvas transformations suffer from
+differing sizes and lines whose thickness differs depending on whether the line
+is horizontal or vertical. To appreciate the difference, consider the following
+two ``circles'' both of which have been scaled in the $x$-direction by a factor
+of $3$ and by a factor of $0.5$ in the $y$-direction. The left circle uses a
+canvas transformation, the right uses \pgfname's coordinate transformation
+(some viewers will render the left graphic incorrectly since they do no apply
+the low-level transformation the way they should):
+
+\begin{tikzpicture}[line width=5pt]
+ \useasboundingbox (-1.75,-1) rectangle (14,1);
+
+ \begin{scope}
+ \pgflowlevel{\pgftransformxscale{3}}
+ \pgflowlevel{\pgftransformyscale{.5}}
+
+ \draw (0,0) circle (0.5cm);
+ \draw (.55cm,0pt) node[right] {canvas};
+ \end{scope}
+ \begin{scope}[xshift=9cm,xscale=3,yscale=.5]
+ \draw (0,0) circle (0.5cm);
+ \draw (.55cm,0pt) node[right] {coordinate};
+ \end{scope}
+\end{tikzpicture}