libvips/doc/extending.xml

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<refentry id="extending">
  <refmeta>
    <refentrytitle>Extending VIPS</refentrytitle>
    <manvolnum>3</manvolnum>
    <refmiscinfo>VIPS Library</refmiscinfo>
  </refmeta>

  <refnamediv>
    <refname>Extending</refname>
    <refpurpose>How to add operations to VIPS</refpurpose>
  </refnamediv>

  <refsect1 id="extending-pointtopoint">
    <title>A simple point-to-point operation</title>

    <para>
      All operations are subclasses of #VipsOperation, which in turn
      subclasses #VipsObject and then %GObject. You need to define a new
      instance struct and a new class struct.

<programlisting language="C">
typedef struct _Negative {
  VipsOperation parent_instance;

  VipsImage *in;
  VipsImage *out;

  int image_max;

} Negative;

typedef struct _NegativeClass {
  VipsOperationClass parent_class;

  /* No new class members needed for this op.
   */

} NegativeClass;
</programlisting>
    </para>

    <para>
      This operation will find the photographic negative of an unsigned
      8-bit image, optionally letting you specify the value which the pixels
      "pivot" about. It doesn't need any class members (ie. values common
      to all operations of this type), so the second struct is empty. See
      vips_invert() for a more complete version of this operation that's
      actually in the library.
    </para>

    <para>
      %GObject has a handy macro to write some of the boilerplate for you.

<programlisting language="C">
G_DEFINE_TYPE( Negative, negative, VIPS_TYPE_OPERATION );
</programlisting>

      This defines a function called negative_get_type(),
      which registers this new class and returns its %GType (a
      pointer-sized integer). negative_get_type() in turn needs two
      functions, negative_init(), to initialise a new instance, and
      negative_class_init(), to initialise a new class.
    </para>

    <para>
      negative_init() is very simple, it just sets the default value for
      our optional class parameter.

<programlisting language="C">
static void
negative_init( Negative *negative )
{
  negative->image_max = 255;
}
</programlisting>
    </para>

    <para>
      negative_class_init() is more complicated: it has to set various
      fields in various superclasses.

<programlisting language="C">
static void
negative_class_init( NegativeClass *class )
{
  GObjectClass *gobject_class = G_OBJECT_CLASS( class );
  VipsObjectClass *object_class = VIPS_OBJECT_CLASS( class );

  gobject_class->set_property = vips_object_set_property;
  gobject_class->get_property = vips_object_get_property;

  object_class->nickname = "negative";
  object_class->description = "photographic negative";
  object_class->build = negative_build;

  VIPS_ARG_IMAGE( class, "in", 1, 
    "Input", 
    "Input image",
    VIPS_ARGUMENT_REQUIRED_INPUT,
    G_STRUCT_OFFSET( Negative, in ) );

  VIPS_ARG_IMAGE( class, "out", 2, 
    "Output", 
    "Output image",
    VIPS_ARGUMENT_REQUIRED_OUTPUT, 
    G_STRUCT_OFFSET( Negative, out ) );

  VIPS_ARG_INT( class, "image_max", 4, 
    "Image maximum", 
    "Maximum value in image: pivot about this",
    VIPS_ARGUMENT_OPTIONAL_INPUT,
    G_STRUCT_OFFSET( Negative, image_max ),
    0, 255, 255 );
}
</programlisting>
    </para>

    <para>
      In %GObject, it needs to set the getters and setters for this class. vips
      has a generic get/set system, so any subclass of #VipsObject needs to
      use the vips ones.
    </para>

    <para>
      In #VipsObject, it needs to set the operation @nickname and @description,
      and set a build function (see below). @nickname is used to refer to
      this operation in the API, @description is used to explain this
      operation to users and will be translated into their language.
    </para>

    <para>
      Finally, it needs to set the arguments this class constructor
      takes. There are a set of handy macros for doing this. The first few
      parameters are always the same and mean: class pointer for argument,
      argument name, argument priority (bindings expect required arguments in
      order of priority), long argument name (this one is internationalised
      and displayed to users), description (again, users can see this),
      some flags describing the argument, and finally the position of the
      member in the struct.
    </para>

    <para>
      Integer arguments take three more values: the minimum, maximum and
      default value for the argument.
    </para>

    <para>
      The build function is the thing #VipsObject calls after supplying
      arguments. It checks that all required arguments have been set and are
      valid and constructs the object. After build, the object is expected
      to be ready for use.

<programlisting language="C">
static int
negative_build( VipsObject *object )
{
  VipsObjectClass *class = VIPS_OBJECT_GET_CLASS( object );
  Negative *negative = (Negative *) object;

  if( VIPS_OBJECT_CLASS( negative_parent_class )->build( object ) )
    return( -1 );

  if( vips_check_uncoded( class->nickname, negative->in ) ||
    vips_check_format( class->nickname, negative->in, VIPS_FORMAT_UCHAR ) )
    return( -1 );

  g_object_set( object, "out", vips_image_new(), NULL ); 

  if( vips_image_pipelinev( negative->out, 
    VIPS_DEMAND_STYLE_THINSTRIP, negative->in, NULL ) )
    return( -1 );

  if( vips_image_generate( negative->out, 
    vips_start_one, 
    negative_generate, 
    vips_stop_one, 
    negative->in, negative ) )
    return( -1 );

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

    <para>
      negative_build() first chains up to the superclass: this will check
      that all input arguments have been supplied and are sane.
    </para>

    <para>
      Next, it adds its own checks. This is a demo operation, so we just
      work for uncoded, unsigned 8-bit images.
    </para>

    <para>
      Next, it creates the output image. This needs to be set with
      g_object_set() so that vips can see that it has been assigned. vips
      will also handle the reference counting for you.
    </para>

    <para>
      vips_image_pipelinev() links our new image onto the input image and
      notes that this operation prefers to work in lines.
    </para>

    <para>
      Finally, vips_image_generate() attaches a set of callbacks to the
      output image to generate chunks of it on request. vips_start_one()
      and vips_stop_one() are convenience functions that make the input
      region for you.
    </para>

    <para>
      And then the actual image processing.

<programlisting language="C">
static int
negative_generate( VipsRegion *or, 
  void *vseq, void *a, void *b, gboolean *stop )
{
  /* The area of the output region we have been asked to make.
   */
  VipsRect *r = &amp;or-&gt;valid;

  /* The sequence value ... the thing returned by vips_start_one().
   */
  VipsRegion *ir = (VipsRegion *) vseq;

  Negative *negative = (Negative *) b;
  int line_size = r-&gt;width * negative-&gt;in-&gt;Bands; 

  int x, y;

  /* Request matching part of input region.
   */
  if( vips_region_prepare( ir, r ) )
    return( -1 );

  for( y = 0; y &lt; r-&gt;height; y++ ) {
    unsigned char *p = (unsigned char *)
      VIPS_REGION_ADDR( ir, r-&gt;left, r-&gt;top + y ); 
    unsigned char *q = (unsigned char *)
      VIPS_REGION_ADDR( or, r-&gt;left, r-&gt;top + y ); 

    for( x = 0; x &lt; line_size; x++ ) 
      q[x] = negative-&gt;image_max - p[x];
  }

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

    <para>
      This has to calculate a section of the output image. The output
      #VipsRegion, @or, contains a #VipsRect called @valid which is the
      area needing calculation. negative_generate() asks for the
      corresponding pixels from the input region, then loops over the
      area. VIPS_REGION_ADDR() is a simple macro that does pointer arithmetic
      for you: you need to stay within the valid area.
    </para>

    <para>
      To add the operation to vips, just call negative_get_type(). You
      can then use @negative from any of the vips interfaces. For example,
      in Python you'd use it like this:

<programlisting language="python">
out = in.negative(image_max = 128)
</programlisting>
    </para>

    <para>
      From the command-line it'd look like this:

<programlisting language="bash">
$ vips negative in.png out.tif --image-max 128
</programlisting>
    </para>

    <para>
      And from C like this:

<programlisting language="C">
VipsImage *in;
VipsImage *out;
if( vips_call( "negative", in, &amp;out, "image_max", 128, NULL ) )
  ... error
</programlisting>
    </para>

    <para>
      Unfortunately that will do almost no compile-time type checking,
      so all vips operations have a tiny extra wrapper to add a bit of
      safety. For example:

<programlisting language="C">
static int 
negative( VipsImage *in, VipsImage **out, ... )
{
  va_list ap;
  int result;

  va_start( ap, out );
  result = vips_call_split( "negative", ap, in, out );
  va_end( ap );

  return( result );
}
</programlisting>
    </para>

    <para>
      And now you can write:

<programlisting language="C">
if( negative( in, &amp;out, "image_max", 128, NULL ) )
  ... error
</programlisting>

      and it's at least a bit safer.
    </para>

  </refsect1>

  <refsect1 id="extending-othertypes">
    <title>Other types of operation</title>
    <para>
      Change the _build() function to make other types of operation. 
    </para>
      
    <para>
      Use vips_image_generate() with vips_start_many() to make operations 
      which demand pixels from more than one image at once, such as image 
      plus image. 
    </para>
      
    <para>
      Use vips_sink() instead of vips_image_generate() to loop over an image 
      and calculate a value. vips uses this for the statistics operations, 
      like vips_avg().
    </para>

    <para>
      Use vips_image_wio_input() to get an entire image into memory so you
      can read it with a pointer. This will obviously not scale well to
      very large images, but some operations, like FFTs or flood-fill, need 
      the whole image to be available at once.
    </para>

    <para>
      Make area operations, like filters, by enlarging the #VipsRect that
      _generate() is given before calling vips_region_prepare(). You can
      enlarge the input image, so that the output image is the same size as
      the original input, by using vips_embed() within the _build() function.
    </para>

    <para>
      Make things like flips and rotates by making larger changes to the
      #VipsRect in _generate().
    </para>

    <para>
      Make zero-copy operations, like vips_insert(), with vips_region_region().
    </para>

  </refsect1>

</refentry>