gtk/docs/reference/gdk-pixbuf/tmpl/gnome-canvas-pixbuf.sgml

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GnomeCanvasPixbuf

<!-- ##### SECTION Short_Description ##### -->
Canvas item to display #GdkPixbuf images.

<!-- ##### SECTION Long_Description ##### -->
  <para>
    This canvas item displays #GdkPixbuf images.  It handles full
    affine transformations in both GDK and antialiased modes, and also
    supports the <ulink url="http://www.w3.org">W3C</ulink>'s <ulink
    url="http://www.w3.org/Graphics/SVG/">SVG</ulink>-like scaling and
    translation semantics for absolute pixel values.
  </para>

  <para>
    #GdkPixbuf structures may be shared among different pixbuf canvas
    items; the pixbuf item uses #GdkPixbuf's reference counting
    functions for this.
  </para>

  <refsect2>
    <title>Custom Scaling and Translation</title>

    <para>
      In addition to the normal affine transformations supported by
      canvas items, the #GnomeCanvasPixbuf item supports independent
      object arguments for scaling and translation.  This is useful
      for explicitly setting a size to which the pixbuf's image will
      be scaled, and for specifying translation offsets that take
      place in the item's local coordinate system.
    </para>

    <para>
      By default, the pixbuf canvas item will attain the size in units
      of the #GdkPixbuf it contains.  If a #GnomeCanvasPixbuf is
      configured to use a #GdkPixbuf that has a size of 300 by 200
      pixels, then the pixbuf item will automatically obtain a size of
      300 by 200 units in the item's local coordinate system.  If the
      item is transformed with a scaling transformation of (0.5, 2.0),
      then the final image size will be of 150 by 400 pixels.
    </para>

    <para>
      To set custom width and height values, you must set the <link
      linkend="GnomeCanvasPixbuf--width-set">width_set</link> or <link
      linkend="GnomeCanvasPixbuf--height-set">height_set</link>
      arguments to %TRUE, and then set the <link
      linkend="GnomeCanvasPixbuf--width">width</link> or <link
      linkend="GnomeCanvasPixbuf--height">height</link> arguments to
      the desired values.  The former two arguments control whether
      the latter two are used when computing the final image's size;
      they are both %FALSE by default so that the pixbuf item will
      attain a size in units equal to the size in pixels of the
      #GdkPixbuf that the item contains.
    </para>

    <para>
      The custom translation values are analogously controlled by the
      <link linkend="GnomeCanvasPixbuf--x-set">x_set</link>, <link
      linkend="GnomeCanvasPixbuf--y-set">y_set</link>, <link
      linkend="GnomeCanvasPixbuf--x">x</link>, and <link
      linkend="GnomeCanvasPixbuf--y">y</link> arguments.  The former
      two control whether the custom translation offsets are used and
      they are %FALSE by default, and the latter two control the
      offsets themselves.  If a custom translation offset is not used,
      it is taken to be 0.0 by default.  Otherwise, the logical
      upper-left vertex of the image will be translated by the
      specified distance, aligned with the item's local coordinate
      system.
    </para>
  </refsect2>

  <refsect2>
    <title>Absolute Pixel Scaling and Translation</title>

    <para>
      The <ulink url="http://www.w3.org/Graphics/SVG/">Scalable Vector
      Graphics</ulink> specification (SVG) of the <ulink
      url="http://www.w3.org">World Wide Web Consortium</ulink> also
      allows images to be translated and scaled by absolute pixel
      values that are independent of an item's normal affine
      transformation.
    </para>

    <para>
      Normally, the pixbuf item's translation and scaling arguments
      are interpreted in units, so they will be modified by the item's
      affine transformation.  The <link
      linkend="GnomeCanvasPixbuf--width-in-pixels">width_in_pixels</link>,
      <link
      linkend="GnomeCanvasPixbuf--height-in-pixels">height_in_pixels</link>,
      <link
      linkend="GnomeCanvasPixbuf--x-in-pixels">x_in_pixels</link>, and
      <link
      linkend="GnomeCanvasPixbuf--y-in-pixels">y_in_pixels</link>
      object arguments can be used to modify this behavior.  If one of
      these arguments is %TRUE, then the corresponding scaling or
      translation value will not be affected lengthwise by the pixbuf
      item's affine transformation.
    </para>

    <para>
      For example, consider a pixbuf item whose size is (300, 200).
      If the item is modified with a scaling transformation of (0.5,
      2.0) but the <link
      linkend="GnomeCanvasPixbuf--width-in-pixels">width_in_pixels</link>
      is set to %TRUE, then the item will appear to be (300, 400)
      pixels in size.  This means that in this case the item's affine
      transformation only applies to the height value, while the width
      value is kept in absolute pixels.
    </para>

    <para>
      Likewise, consider a pixbuf item whose (<link
      linkend="GnomeCanvasPixbuf--x">x</link>, <link
      linkend="GnomeCanvasPixbuf--y">y</link>) arguments are set to
      (30, 40)<footnote>
	<para>
	  This of course assumes that the <link
	  linkend="GnomeCanvasPixbuf--x-set">x_set</link> and <link
	  linkend="GnomeCanvasPixbuf--y-set">y_set</link> arguments
	  are set to %TRUE for the translation offsets to take effect.
	</para>
      </footnote>.  If the item is then modified by the same scaling
      transformation of (0.5, 2.0) but the <link
      linkend="GnomeCanvasPixbuf--y-in-pixels">y_in_pixels</link>
      argument is set to %TRUE, then the image's upper-left corner
      will appear to be at position (15, 40).  In this case, the
      affine transformation is applied only to the x offset, while the
      y offset is kept in absolute pixels.
    </para>

    <para>
      In short, these arguments control whether a particular dimension
      of a pixbuf item is scaled or not in the normal way by the
      item's affine transformation.
    </para>
  </refsect2>

  <refsect2>
    <title>Resource Management</title>

    <para>
      When you set the #GdkPixbuf structure that a #GnomeCanvasPixbuf
      item will use by setting the <link
      linkend="GnomeCanvasPixbuf--pixbuf">pixbuf</link> argument, a
      reference count will be added to that #GdkPixbuf structure.
      When the pixbuf item no longer needs the #GdkPixbuf structure,
      such as when the item is destroyed or when a new pixbuf
      structure is passed to it, then the old #GdkPixbuf structure
      will be automatically unreferenced.
    </para>

    <para>
      This means that if an application just needs to load a pixbuf
      image and set it into a pixbuf canvas item, it can do the
      following to &lsquo;forget&rsquo; about the pixbuf structure:

      <programlisting>
	GdkPixbuf *pixbuf;
	GnomeCanvasItem *item;

	pixbuf = gdk_pixbuf_new_from_file ("foo.png");
	g_assert (pixbuf != NULL);

	item = gnome_canvas_item_new (gnome_canvas_root (my_canvas),
				      gnome_canvas_pixbuf_get_type (),
				      "pixbuf", pixbuf,
				      NULL);
	gdk_pixbuf_unref (pixbuf);
      </programlisting>
    </para>

    <para>
      After this happens, the reference count of the pixbuf structure
      will be 1:  the gdk_pixbuf_new_from_file() function creates it
      with a reference count of 1, then setting the <link
      linkend="GnomeCanvasPixbuf--pixbuf">pixbuf</link> argument of
      the #GnomeCanvasPixbuf item increases it to 2, and then it is
      decremented to 1 by the call to gdk_pixbuf_unref().  When the
      canvas item is destroyed, it will automatically unreference the
      pixbuf structure again, causing its reference count to drop to
      zero and thus be freed.
    </para>
  </refsect2>

<!-- ##### SECTION See_Also ##### -->
  <para>
    #GnomeCanvas, #GdkPixbuf
  </para>

<!-- ##### MACRO GNOME_CANVAS_PIXBUF ##### -->
  <para>
    Casts a #GtkOjbect to a #GnomeCanvasPixbuf.
  </para>

@obj: A GTK+ object.


<!-- ##### ARG GnomeCanvasPixbuf:pixbuf ##### -->
  <para>
    Contains a pointer to a #GdkPixbuf structure that will be used by
    the pixbuf canvas item as an image source.  When a pixbuf is set
    its reference count is incremented; if the pixbuf item kept a
    pointer to another #GdkPixbuf structure, the reference count of
    this structure will be decremented.  Also, the GdkPixbuf's
    reference count will automatically be decremented when the
    #GnomeCanvasPixbuf item is destroyed.  When a pixbuf is queried, a
    reference count will not be added to the return value; you must do
    this yourself if you intend to keep the pixbuf structure around.
  </para>

<!-- ##### ARG GnomeCanvasPixbuf:width ##### -->
  <para>
    Indicates the width the pixbuf will be scaled to.  This argument
    will only be used if the <link
    linkend="GnomeCanvasPixbuf--width-set">width_set</link> argument
    is %TRUE.  If the <link
    linkend="GnomeCanvasPixbuf--width-in-pixels">width_in_pixels</link>
    argument is %FALSE, the width will be taken to be in canvas units,
    and thus will be scaled along with the canvas item's affine
    transformation.  If width_in_pixels is %TRUE, the width will be
    taken to be in pixels, and will visually remain a constant size
    even if the item's affine transformation changes.
  </para>

<!-- ##### ARG GnomeCanvasPixbuf:width_set ##### -->
  <para>
    Determines whether the <link
    linkend="GnomeCanvasPixbuf--width">width</link> argument is taken
    into account when scaling the pixbuf item.  If this argument is
    %FALSE, then the width value of the pixbuf's #ArtPixBuf will be
    used instead.  This argument is %FALSE by default.
  </para>

<!-- ##### ARG GnomeCanvasPixbuf:width_in_pixels ##### -->
  <para>
    If this argument is %TRUE, then the width of the pixbuf will be
    considered to be in pixels, that is, it will not be visually
    scaled even if the item's affine transformation changes.  If this
    is %FALSE, then the width of the pixbuf will be considered to be
    in canvas units, and so will be scaled normally by affine
    transformations.  The default is %FALSE.
  </para>

<!-- ##### ARG GnomeCanvasPixbuf:height ##### -->
  <para>
    Indicates the height the pixbuf will be scaled to.  This argument
    will only be used if the <link
    linkend="GnomeCanvasPixbuf--height-set">height_set</link> argument
    is %TRUE.  Works in the same way as the <link
    linkend="GnomeCanvasPixbuf--width">width</link> argument.
  </para>

<!-- ##### ARG GnomeCanvasPixbuf:height_set ##### -->
  <para>
    Determines whether the <link
    linkend="GnomeCanvasPixbuf--height">height</link> argument is
    taken into account when scaling the pixbuf item.  Works in the
    same way as the <link
    linkend="GnomeCanvasPixbuf--width-set">width_set</link> argument.
    The default is %FALSE.
  </para>

<!-- ##### ARG GnomeCanvasPixbuf:height_in_pixels ##### -->
  <para>
    Works in the same way as the <link
    linkend="GnomeCanvasPixbuf--width-in-pixels">width_in_pixels</link>
    argument.  The default is %FALSE.
  </para>

<!-- ##### ARG GnomeCanvasPixbuf:x ##### -->
  <para>
    Indicates the horizontal translation offset of the pixbuf item's
    image.  This argument will only be used if the <link
    linkend="GnomeCanvasPixbuf--x-set">x_set</link> argument is %TRUE.
    This offset may not actually appear horizontal, since it will be
    affected by the item's affine transformation.
  </para>

<!-- ##### ARG GnomeCanvasPixbuf:x_set ##### -->
  <para>
    Determines whether the <link
    linkend="GnomeCanvasPixbuf--x">x</link> argument is used to
    translate the pixbuf from its logical origin in item-relative
    coordinates.
  </para>

<!-- ##### ARG GnomeCanvasPixbuf:x_in_pixels ##### -->
  <para>
    If this argument is %TRUE, the pixbuf's translation with respect
    to its logical origin in item-relative coordinates will be in
    pixels, that is, the visible offset will not change even if the
    item's affine transformation changes.  If it is %FALSE, the
    pixbuf's translation will be taken to be in canvas units, and thus
    will change along with the item's affine transformation.
  </para>

<!-- ##### ARG GnomeCanvasPixbuf:y ##### -->
  <para>
    Indicates the vertical translation offset of the pixbuf item's
    image.  Works in the same way as the <link
    linkend="GnomeCanvasPixbuf--x">x</link> argument.
  </para>

<!-- ##### ARG GnomeCanvasPixbuf:y_set ##### -->
  <para>
    Determines whether the <link
    linkend="GnomeCanvasPixbuf--y">y</link> argument is used to
    translate the pixbuf from its logical origin in item-relative
    coordinates.  Works in the same way as the <link
    linkend="GnomeCanvasPixbuf--x-set">x_set</link> argument.  The
    default is %FALSE.
  </para>

<!-- ##### ARG GnomeCanvasPixbuf:y_in_pixels ##### -->
  <para>
    Works in the same way as the <link
    linkend="GnomeCanvasPixbuf--x-in-pixels">x_in_pixels</link>
    argument.  The default is %FALSE.
  </para>

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