106 lines
2.7 KiB
TeX
106 lines
2.7 KiB
TeX
\section{Introduction}
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\mylabel{sec:cpp}
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This chapter describes the C++ API for the VIPS image processing library,
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version 7.12. The C++ API is as efficient as the C interface to VIPS, but is
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far easier to use: almost all creation, destruction and error handling issues
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are handled for you automatically.
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The Python interface is a very simple wrapping of this C++ API generated
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automatically with SWIG. The two interfaces are identical, except for language
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syntax.
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\subsection{If you've used the C API}
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To show how much easier the VIPS C++ API is to use, compare \fref{fg:negative}
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to \fref{fg:invert-c++}. \fref{fg:invert-py} is the same thing in Python.
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A typical build line for the C++ program might be:
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\begin{verbatim}
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g++ invert.cc \
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`pkg-config vipsCC-7.12 \
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--cflags --libs`
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\end{verbatim}
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The key points are:
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\begin{itemize}
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\item
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You just include \verb+<vips/vips>+ --- this then gets all of the
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other includes you need. Everything is in the \verb+vips+ namespace.
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\item
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The C++ API replaces all of the VIPS C types --- \verb+IMAGE+ becomes
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\verb+VImage+ and so on. The C++ API also includes \verb+VDisplay+,
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\verb+VMask+ and \verb+VError+.
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\item
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Image processing operations are member functions of the \verb+VImage+ class
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--- here, \verb+VImage( argv[1] )+ creates a new \verb+VImage+ object using
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the first argument to initialise it (the input filename). It then calls the
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member function \verb+invert()+, which inverts the \verb+VImage+ and returns a
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new \verb+VImage+. Finally it calls the member function \verb+write()+, which
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writes the result image to the named file.
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\item
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The VIPS C++ API uses exceptions --- the \verb+VError+ class is covered
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later. If you run this program with a bad input file, for example, you get the
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following output:
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\begin{verbatim}
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example% invert jim fred
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invert: VIPS error: im_open:
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"jim" is not a supported
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format
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\end{verbatim}
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\end{itemize}
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\begin{fig2}
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\begin{verbatim}
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#include <iostream>
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#include <vips/vips>
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int
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main( int argc, char **argv )
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{
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if( argc != 3 ) {
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std::cerr << "usage: " << argv[0] << " infile outfile\n";
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exit( 1 );
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}
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try {
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vips::VImage fred( argv[1] );
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fred.invert().write( argv[2] );
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}
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catch( vips::VError e ) {
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e.perror( argv[0] );
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}
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return( 0 );
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}
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\end{verbatim}
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\caption{\texttt{invert} program in C++}
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\label{fg:invert-c++}
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\end{fig2}
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\begin{fig2}
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\begin{verbatim}
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#!/usr/bin/python
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import sys
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from vipsCC import *
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try:
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a = VImage.VImage (sys.argv[1])
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a.invert ().write (sys.argv[2])
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except VError.VError, e:
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e.perror (sys.argv[0])
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\end{verbatim}
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\caption{\texttt{invert} program in Python}
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\label{fg:invert-py}
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\end{fig2}
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