libvips/doc/reference/using-C.xml

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<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>

  <refsect1 id="using-C">
    <title>Using VIPS from C</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>

    <para>
      When your program starts, use <function>VIPS_INIT()</function> to set up 
      the VIPS library. You should pass it the name of your program, usually 
      <literal>argv[0]</literal>. Use <function>vips_shutdown()</function> 
      when you exit. 
    </para>

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

    <para>
      The basic data object is the #VipsImage, see <link
      linkend="VipsImage">VIPS Image</link> for details on the
      image class. 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 in pixels. It delays reading
      any pixels until they 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>

    <para>
      VIPS is based on the GObject library and is therefore refcounted. 
      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>
      Use things like vips_invert() to manipulate your images. See <link
      linkend="VipsOperation">VIPS Operations</link> 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>

    <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 introduxction to writing your own operations. 
    </para>

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

<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 );
	g_option_context_add_group( context, vips_get_option_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>

  </refsect1>

</refentry>