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diff --git a/graphics/pgf/base/doc/pgfmanual-en-base-design.tex b/graphics/pgf/base/doc/pgfmanual-en-base-design.tex new file mode 100644 index 0000000000..fb7eacca2f --- /dev/null +++ b/graphics/pgf/base/doc/pgfmanual-en-base-design.tex @@ -0,0 +1,146 @@ +% 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} |