porting-from-imlib.sgml

linux下电话本所依赖的一些图形库

  <appendix>
    <title>Porting applications from &Imlib; to &gdk-pixbuf;</title>

    <para>
      This appendix contains the basic steps needed to port an
      application that uses the &Imlib; library to use &gdk-pixbuf;
      instead.
    </para>

    <note>
      <para>
	This appendix refers to version 1 of the &Imlib; library; this
	discussion is not relevant to Imlib 2.  Also, we discuss the
	gdk_imlib API instead of the Xlib-based API.
      </para>
    </note>

    <!-- Introduction -->

    <sect1>
      <title>Introduction</title>

      <para>
	Prior to the GNOME 1.2 platform, the &Imlib; library was the
	preferred way of loading and rendering images in GNOME
	applications.  Unfortunately, &Imlib; has important design
	limitations that make it hard to write efficient and highly
	modular applications.
      </para>

      <para>
	The &gdk-pixbuf; library was designed as a solution to
	&Imlib;'s shortcomings.  It provides a simple, orthogonal API
	and convenience functions for the most common operations.  In
	addition, it supports full transparency information for
	images, or alpha channel.  More importantly, it has
	well-defined semantics for memory management through the use
	of reference counting; &Imlib; has an intractably complex
	memory management mechanism and cache that will make your head
	spin.
      </para>
    </sect1>

    <!-- Differences between Imlib and gdk-pixbuf -->

    <sect1>
      <title>Differences between &Imlib; and &gdk-pixbuf;</title>

      <para>
	Generally, applications that use &Imlib; do not have to be
	changed extensively to use &gdk-pixbuf;; its simple and
	flexible API makes things easy.  This section describes the
	differences between &Imlib; and &gdk-pixbuf;; you should take
	these into account when modifying your applications to use
	&gdk-pixbuf;.
      </para>

      <!-- Initialization -->

      <sect2>
	<title>Initialization</title>

	<para>
	  The &gdk-pixbuf; library does not need to be initialized.
	</para>

	<note>
	  <para>
	    In GNOME applications you normally don't need to
	    initialize &Imlib;, as <function>gnome_init()</function>
	    calls <function>gdk_imlib_init()</function> automatically.
	  </para>
	</note>
      </sect2>

      <!-- Memory management -->

      <sect2>
	<title>Memory management</title>

	<para>
	  The &gdk-pixbuf; library provides a simple, well-defined
	  memory management mechanism for images in the form of
	  reference counting.  This makes it very convenient to use
	  for large-scale applications that need to share images
	  between different parts of the program.  In stark contrast,
	  &Imlib; has a terribly complex mechanism of an image and
	  pixmap cache which makes it very hard for applications to
	  share image structures between different parts of the
	  program.  Unfortunately this mechanism makes things very
	  prone to memory leaks and tricky bugs.
	</para>

	<para>
	  The basic principle in &gdk-pixbuf; is that when you obtain
	  a new <link linkend="GdkPixbuf">GdkPixbuf</link> structure,
	  it is created with an initial reference count of 1.  When
	  another part of the program wants to keep a reference to the
	  pixbuf, it should call <function>g_object_ref()</function>;
	  this will increase the reference count by 1.  When some part
	  of the program does not need to keep a reference to a pixbuf
	  anymore and wants to release the pixbuf, it should call
	  <function>g_object_unref()</function>; this will decrease
	  the reference count by 1.  When the reference count drops to
	  zero, the pixbuf gets destroyed or
	  <emphasis>finalized</emphasis> and its memory is freed.
	</para>

	<para>
	  For applications that need to implement a cache of loaded
	  images, &gdk-pixbuf; provides a way to hook to the last
	  unreference operation of a pixbuf; instead of finalizing the
	  pixbuf, the user-installed hook can decide to keep it around
	  in a cache instead.
	</para>

	<para>
	  Finally, &gdk-pixbuf; does not provide a cache of rendered
	  pixmaps.  This is unnecessary for most applications, since
	  the scaling and rendering functions are quite fast and
	  applications may need to use subtly different values each
	  time they call these functions, for example, to take into
	  account dithering and zooming offsets.
	</para>

	<para>
	  Most applications will simply need to call
	  <function>g_object_ref()</function> when they want to keep
	  an extra reference to a pixbuf, and then
	  <function>g_object_unref()</function> when they are done
	  with it.
	</para>
      </sect2>

      <!-- The Rendering Process -->

      <sect2>
	<title>The Rendering Process</title>

	<para>
	  The &gdk-pixbuf; library has the policy of always rendering
	  pixbufs to GDK drawables you provide; it will not create
	  them for you.  This is in general more flexible than
	  &Imlib;'s policy of always creating a pixmap and making you
	  use that instead.
	</para>

	<para>
	  The disadvantage of always having a pixmap created for you
	  is that it wastes memory in the X server if you intend to
	  copy that rendered data onto another drawable, for example,
	  the final destination window or a temporary pixmap for
	  drawing.  This is the most common case, unfortunately, so
	  the &Imlib; policy introduces unnecessary copying.
	</para>

	<para>
	  Also, &Imlib; can only render pixmaps that are the whole
	  size of the source image; you cannot render just a subset
	  region of the image.  This is inconvenient for applications
	  that need to render small portions at a time, such as
	  applications that do scrolling.  Since the whole image must
	  be rendered at a time, this can lead to performance and
	  memory usage problems.
	</para>

	<para>
	  The &gdk-pixbuf; library lets you render any rectangular
	  region from an image onto any drawable that you provide.
	  This lets the application have fine control the way images
	  are rendered.
	</para>
      </sect2>
    </sect1>

    <!-- Converting Applications to gdk-pixbuf -->

    <sect1>
      <title>Converting Applications to &gdk-pixbuf;</title>

      <para>
	This sections describes the actual changes you need to make in
	an &Imlib; program to make it use &gdk-pixbuf; instead.
      </para>

      <!-- Image loading and creation -->

      <sect2>
	<title>Image loading and creation</title>

	<para>
	  The &gdk-pixbuf; library can load image files synchronously
	  (i.e. with a single function call), create images from RGB
	  data in memory, and as a convenience, it can also create
	  images from inline XPM data.
	</para>

	<para>
	  To load an image file in a single function call, simply use
	  <function>gdk_pixbuf_new_from_file()</function>.  Note that
	  this will make the program block until the whole file has
	  been read.  This function effectively replaces
	  <function>gdk_imlib_load_image()</function>.
	</para>

	<para>
	  If you have RGB data in memory, you can use
	  <function>gdk_pixbuf_new_from_data()</function> to create a
	  pixbuf out of it; this is a replacement for
	  <function>gdk_imlib_create_image_from_data()</function>.
	  &gdk-pixbuf; does not copy the image data; it is up to you
	  to define the ownership policy by providing a destroy
	  notification function that will be called when the image
	  data needs to be freed.  The function you provide can then
	  free the data or do something else, as appropriate.
	</para>

	<para>
	  As a convenience, you can use the
	  <function>gdk_pixbuf_new_from_xpm_data()</function> function
	  to create a pixbuf out of inline XPM data that was compiled
	  into your C program.  This is a replacement for
	  <function>gdk_imlib_create_image_from_xpm_data()</function>.
	</para>

	<para>
	  After you have created a pixbuf, you can manipulate it in
	  any way you please and then finally call
	  <function>g_object_unref()</function> when you are done
	  with it.  This can be thought of as a replacement for
	  <function>gdk_imlib_destroy_image()</function> but with much
	  cleaner semantics.
	</para>
      </sect2>

      <!-- Rendering Images -->

      <sect2>
	<title>Rendering Images</title>

	<para>
	  Applications that use &Imlib; must first call
	  <function>gdk_imlib_render()</function> to render the whole
	  image data onto a pixmap that &Imlib; creates.  Then they
	  must copy that pixmap's data into the final destination for
	  the image.
	</para>

	<para>
	  In contrast, &gdk-pixbuf; provides convenience functions to
	  render arbitrary rectangular regions of an image onto a
	  drawable that your application provides.  You can use
	  <function>gdk_pixbuf_render_to_drawable()</function> or
	  <function>gdk_pixbuf_render_to_drawable_alpha()</function>
	  to do this; having your application provide the destination
	  drawable and specify an arbitrary region means your
	  application has complete control over the way images are
	  rendered.
	</para>

	<para>
	  As a convenience, &gdk-pixbuf; also provides the
	  <function>gdk_pixbuf_render_pixmap_and_mask()</function>
	  function; this will create new pixmap and mask drawables for
	  a whole pixbuf and render the image data onto them.  Only
	  trivially simple applications should find a use for this
	  function, since usually you want finer control of how things
	  are rendered.
	</para>
      </sect2>

      <!-- Scaling Images -->

      <sect2>
	<title>Scaling Images</title>

	<para>
	  &Imlib; lets you render scaled image data at the time you
	  call <function>gdk_imlib_render()</function>.  Again, this
	  unfortunately scales and renders the whole image onto a new
	  pixmap.
	</para>

	<para>
	  &gdk-pixbuf; provides a number of functions that do scaling
	  of arbitrary regions of a source pixbuf onto a destination
	  one.  These functions can also perform compositing
	  operations against the data in the destination pixbuf or
	  against a solid color or a colored checkerboard.
	  <footnote>
	    <para>
	      You can use a colored checkerboard as the background for
	      compositing when you want to provide a visual indication
	      that the image has partially opaque areas.  This is
	      normally used in image editing and viewing programs.
	    </para>

	    <para>
	      Compositing against a single solid color is actually a
	      special case of a checkerboard; it simply uses checks of
	      the same color.
	    </para>
	  </footnote>
	</para>

	<para>
	  Very simple applications may find it sufficient to use
	  <function>gdk_pixbuf_scale_simple()</function> or
	  <function>gdk_pixbuf_composite_color_simple()</function>.
	  These functions scale the whole source image at a time and
	  create a new pixbuf with the result.
	</para>

	<para>
	  More sophisticated applications will need to use
	  <function>gdk_pixbuf_scale()</function>,
	  <function>gdk_pixbuf_composite()</function>, or
	  <function>gdk_pixbuf_composite_color()</function> instead.
	  These functions let you scale and composite an arbitrary
	  region of the source pixbuf onto a destination pixbuf that
	  you provide.
	</para>
      </sect2>

      <!-- Getting Image Data from a Drawable -->

      <sect2>
	<title>Getting Image Data from a Drawable</title>

	<para>
	  &Imlib; lets you create an image by fetching a drawable's
	  contents from the X server and converting those into RGB
	  data.  This is done with the
	  <function>gdk_imlib_create_image_from_drawable()</function>
	  function.
	</para>

	<para>
	  &gdk-pixbuf; provides the
	  <function>gdk_pixbuf_get_from_drawable()</function> function
	  instead.  It lets you specify a destination pixbuf instead
	  of always creating a new one for you.
	</para>
      </sect2>
    </sect1>
  </appendix>

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