libvips/doc/using-C.xml

<?xml version="1.0"?>
<!-- vim: set ts=2 sw=2 expandtab: --> 
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
               "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
]>
<refentry id="using-from-c">
  <refmeta>
    <refentrytitle>VIPS from C</refentrytitle>
    <manvolnum>3</manvolnum>
    <refmiscinfo>VIPS Library</refmiscinfo>
  </refmeta>

  <refnamediv>
    <refname>Using VIPS</refname>
    <refpurpose>How to use the VIPS library from C</refpurpose>
  </refnamediv>

  <refsect3 id="using-C-intro">
    <title>Introduction</title>
    <para>
      VIPS comes with a convenient, high-level C API. You should read the API 
      docs for full details, but this section will try to give a brief 
      overview. The <command>vips</command> program is handy for getting a 
      summary of an operation's parameters.
    </para>
  </refsect3>

  <refsect3 id="using-C-startup">
    <title>Library startup</title>
    <para>
      When your program starts, use VIPS_INIT()
      to start up the VIPS library. You should pass it the name
      of your program, usually <code>argv[0]</code>. Use
      vips_shutdown() when you exit. VIPS_INIT() is a macro to let it check 
      that the libvips library you have linked to matches the libvips headers 
      you included. 
    </para>

    <para>
      You can add the VIPS flags to your %GObject command-line processing
      with vips_add_option_entries(), see below.
    </para>
  </refsect3>

  <refsect3 id="using-C-vipsimage">
    <title>The #VipsImage class</title>
    <para>
      The basic data object is the #VipsImage. You can create an
      image from a file on disc or from an area of memory, either
      as a C-style array, or as a formatted object, like JPEG. See
      vips_image_new_from_file() and friends.  Loading an
      image is fast: VIPS read just enough of the image to be able to get
      the various properties, such as width, but no decoding occurs until 
      pixel values are really needed.
    </para>

    <para>
      Once you have an image, you can get properties from it in the usual way. 
      You can use projection functions, like vips_image_get_width() or 
      g_object_get(), to get %GObject properties. All VIPS objects are
      immutable, meaning you can only get properties, you can't set them.
      See <link linkend="libvips-header">VIPS Header</link> to read about 
      image properties.
    </para>
  </refsect3>

  <refsect3 id="using-C-ref">
    <title>Reference counting</title>
    <para>
      VIPS is based on the %GObject library and is therefore reference counted. 
      vips_image_new_from_file() returns an object with a count of 1. 
      When you are done with an image, use g_object_unref() to dispose of it. 
      If you pass an image to an operation and that operation needs to keep a 
      copy of the image, it will ref it. So you can unref an image as soon as 
      you no longer need it, you don't need to hang on to it in case anyone 
      else is still using it. 
    </para>

    <para>
      See #VipsOperation for more detail on VIPS 
      reference counting conventions.
    </para>
  </refsect3>

  <refsect3 id="using-C-operations">
    <title>VIPS operations</title>
    <para>
      Use things like vips_invert() to manipulate your images. See 
      #VipsOperation 
      for information on
      running operations on images. When you are done, you can write
      the final image to a disc file, to a formatted memory buffer, or to
      C-style memory array. See vips_image_write_to_file() and friends.
    </para>
  </refsect3>

  <refsect3 id="using-C-pixels">
    <title>Getting pixels</title>
    <para>
      Use #VipsRegion to read pixels out of images. You can use 
      VIPS_IMAGE_ADDR() as well, but this can need a large amount of 
      memory to work. See <link linkend="extending">extending</link> 
      for an introduction to writing your own operations. 
    </para>
  </refsect3>

  <refsect3 id="using-C-errors">
    <title>Error handling</title>

    <para>
      VIPS keeps a log of error message, see  <link
      linkend="libvips-error">VIPS Error</link> to find out how to get and
      clear the error log.
    </para>
  </refsect3>

  <refsect3 id="using-C-example">
    <title>Example</title>

<example>
<title>VIPS from C example</title>
<programlisting language="C">
/* compile with:
 *
 *      gcc -g -Wall try211.c `pkg-config vips --cflags --libs`
 */

#include &lt;stdio.h&gt;
#include &lt;vips/vips.h&gt;

int
main( int argc, char **argv )
{
  GOptionContext *context;
  GOptionGroup *main_group;
  GError *error = NULL;
  VipsImage *in;
  double mean;
  VipsImage *out;

  if( VIPS_INIT( argv[0] ) )
    vips_error_exit( NULL ); 

  context = g_option_context_new( "hello infile outfile - VIPS demo" );

  main_group = g_option_group_new( NULL, NULL, NULL, NULL, NULL );
  g_option_context_set_main_group( context, main_group );
  vips_add_option_entries( main_group ); 

  if( !g_option_context_parse( context, &amp;argc, &amp;argv, &amp;error ) ) {
    if( error ) {
      fprintf( stderr, "%s\n", error-&gt;message );
      g_error_free( error );
    }

    vips_error_exit( NULL );
  }

  if( argc != 3 )
    vips_error_exit( "usage: %s infile outfile", argv[0] ); 
  
  if( !(in = vips_image_new_from_file( argv[1], NULL )) )
    vips_error_exit( NULL );

  printf( "image width = %d\n", vips_image_get_width( in ) ); 

  if( vips_avg( in, &amp;mean, NULL ) )
    vips_error_exit( NULL );

  printf( "mean pixel value = %g\n", mean ); 

  if( vips_invert( in, &amp;out, NULL ) )
    vips_error_exit( NULL );

  g_object_unref( in ); 

  if( vips_image_write_to_file( out, argv[2], NULL ) )
    vips_error_exit( NULL );

  g_object_unref( out ); 

  return( 0 );
}
</programlisting>
</example>

  </refsect3>

</refentry>