libvips/doc/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>

  <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. 
    </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>. If you call
      vips_shutdown() just before you exit, libvips will attempt to free all 
      resources. This can help leak checking, but is not required.
    </para>

    <para>
      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().
    </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. See the <link
      linkend="reference-pools-ref">Reference pools</link>
      section below for a way to automate reference counting in C.
    </para>

  </refsect3>

  <refsect3 id="using-C-operations">
    <title>VIPS operations</title>
    <para>
      Use things like vips_embed() to manipulate your images. You use it from 
      C like this:

<programlisting language="C">
const char *filename;
VipsImage *in = vips_image_new_from_file (filename, NULL); 
const int x = 10;
const int y = 10;
const int width = 1000;
const int height = 1000;
VipsImage *out;

if (vips_embed (in, &amp;out, x, y, width, height, NULL))
  error_handling();
</programlisting>

      Now <code>out</code> will hold a reference to a 1000 by 1000 pixel 
      image, with <code>in</code> pasted 10 right and 10 down from the top 
      left-hand corner. The remainder of the image will be black. If 
      <code>in</code> is too large, it will be clipped at the image edges.
    </para>

    <para>
      Operations can take optional arguments. You give these as a set of 
      NULL-terminated name-value pairs at the end of the call. For example, 
      you can write:

<programlisting language="C">
if (vips_embed (in, &amp;out, x, y, width, height, 
  "extend", VIPS_EXTEND_COPY,
  NULL))
  error_handling();
</programlisting>

      And now the new edge pixels, which were black, will be filled with a copy
      of the edge pixels of <code>in</code>. Operation options are listed 
      at the top of each operation's entry in the docs. Alternatively, 
      the <command>vips</command> program is handy for getting a 
      summary of an operation's parameters. For example:

<programlisting language="none">
$ vips embed
embed an image in a larger image
usage:
   embed in out x y width height
where:
   in           - Input image, input VipsImage
   out          - Output image, output VipsImage
   x            - Left edge of input in output, input gint
                      default: 0
                      min: -1000000000, max: 1000000000
   y            - Top edge of input in output, input gint
                      default: 0
                      min: -1000000000, max: 1000000000
   width        - Image width in pixels, input gint
                      default: 1
                      min: 1, max: 1000000000
   height       - Image height in pixels, input gint
                      default: 1
                      min: 1, max: 1000000000
optional arguments:
   extend       - How to generate the extra pixels, input VipsExtend
                      default: black
                      allowed: black, copy, repeat, mirror, white, background
   background   - Colour for background pixels, input VipsArrayDouble
operation flags: sequential-unbuffered 
</programlisting>

      See #VipsOperation for more information on running operations on images. 
    </para>

    <para>
      The API docs have a <link linkend="function-list">handy table of all vips 
      operations</link>, if you want to find out how to do something, try 
      searching that. 
    </para>

    <para>
      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>

    <para>
      On *nix systems, you can compile the <link
      linkend="using-C-example">example code</link> with something like:

<programlisting language="none">
$ gcc -g -Wall myprog.c `pkg-config vips --cflags --libs`
</programlisting>

      On Windows, you'll need to set the compiler flags by hand, perhaps:

<programlisting language="none">
x86_64-w64-mingw32-gcc-win32 -mms-bitfields \
  -Ic:/vips-8.6/include \
  -Ic:/vips-8.6/include/glib-2.0 \
  -Ic:/vips-8.6/lib/glib-2.0/include \
  myprog.c \
  -Lc:/vips-8.6/lib \
  -lvips -lz -ljpeg -lstdc++ -lxml2 -lfftw3 -lm -lMagickWand -llcms2 \
  -lopenslide -lcfitsio -lpangoft2-1.0 -ltiff -lpng14 -lexif \
  -lMagickCore -lpango-1.0 -lfreetype -lfontconfig -lgobject-2.0 \
  -lgmodule-2.0 -lgthread-2.0 -lglib-2.0 -lintl \
  -o myprog.exe
</programlisting>

    </para>

<example>
<title>VIPS from C example</title>
<programlisting language="C">
#include &lt;stdio.h&gt;
#include &lt;vips/vips.h&gt;

int
main( int argc, char **argv )
{
  VipsImage *in;
  double mean;
  VipsImage *out;

  if( VIPS_INIT( argv[0] ) )
    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>

  <refsect3 id="reference-pools-ref">
    <title>Reference pools</title>
    <para>
      libvips has a simple system to automate at least some reference counting 
      issues. Reference pools are arrays of object pointers which will be 
      released automatically when some other object is finalized. 
    </para>

    <para>
      The code below crops a many-page image (perhaps a GIF or PDF). It 
      splits the image into separate pages, crops each page, reassembles the 
      cropped areas, and saves again. It creates a <code>context</code> 
      object representing the state of processing, and 
      <code>crop_animation</code> allocates two reference pools off that using 
      <code>vips_object_local_array</code>, one to hold the cropped frames, 
      and one to assemble and copy the result.
    </para>

    <para>
      All unreffing is handled by <code>main</code>, and it doesn't need to 
      know anything about <code>crop_animation</code>.
    </para>

<example>
<title>Reference pool example</title>
<programlisting language="C">
#include &lt;vips/vips.h&gt;

static int
crop_animation( VipsObject *context, VipsImage *image, VipsImage **out,
  int left, int top, int width, int height )
{
  int page_height = vips_image_get_page_height( image );
  int n_pages = image-&gt;Ysize / page_height;
  VipsImage **page = (VipsImage **) vips_object_local_array( context, n_pages );
  VipsImage **copy = (VipsImage **) vips_object_local_array( context, 1 );

  int i;

  /* Split the image into cropped frames.
   */
  for( i = 0; i &lt; n_pages; i++ )
    if( vips_crop( image, &amp;page[i],
      left, page_height * i + top, width, height, NULL ) )
      return( -1 );

  /* Reassemble the frames and set the page height. You must copy before 
   * modifying metadata.
   */
  if( vips_arrayjoin( page, &amp;copy[0], n_pages, "across", 1, NULL ) ||
    vips_copy( copy[0], out, NULL ) )
    return( -1 );
  vips_image_set_int( *out, "page-height", height );

  return( 0 );
}

int 
main( int argc, char **argv )
{
  VipsImage *image;
  VipsObject *context;
  VipsImage *x;

  if( VIPS_INIT( NULL ) ) 
    vips_error_exit( NULL ); 

  if( !(image = vips_image_new_from_file( argv[1], 
    "access", VIPS_ACCESS_SEQUENTIAL,
    NULL )) )
    vips_error_exit( NULL ); 

  context = VIPS_OBJECT( vips_image_new() );
  if( crop_animation( context, image, &amp;x, 10, 10, 500, 500 ) ) {
    g_object_unref( image );
    g_object_unref( context );
    vips_error_exit( NULL ); 
  }
  g_object_unref( image );
  g_object_unref( context );
  image = x;

  if( vips_image_write_to_file( image, argv[2], NULL ) ) {
    g_object_unref( image );
    vips_error_exit( NULL ); 
  }

  g_object_unref( image );

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

  </refsect3>

</refentry>