<!-- ##### SECTION Title ##### -->
GnomeCanvas

<!-- ##### SECTION Short_Description ##### -->
  A generic engine for structured graphics.
  
<!-- ##### SECTION Long_Description ##### -->
  <para>
    The GNOME canvas is an engine for displaying structured graphics
    and simplifying the development of complex graphic-based
    applications.  
  </para>
  
  <refsect2>
    <title>Canvas Items</title>
    <para>
      The GNOME Canvas basic building blocks are the GNOME Canvas
      Items (#GnomeCanvasItem): lines, rectangles, text, ellipses,
      polylines, images and embedded widgets.  You can use any of
      those directly in your application.
    </para>
    
    <para>
      The CanvasItem system is designed to be extensible.
      Applications can define their own #GnomeCanvasItem objects for
      special purpose tasks.  For example, the GNOME Gnumeric
      spreadsheet defines a number of special Canvas Items that are
      specialized for the task of spreadsheets.
    </para>
    
    <para>
      Specialized canvas items allow the developer to write
      custom items that can adapt to their needs for speed,
      scalability and gives the user the power to extend the
      canvas. 
    </para>
    
    <para>
      Items on the canvas can be reconfigured by using the Gtk
      argument system.  Users can reconfigure the parameters of
      the canvas items and the changes on the parameters will be
      reflected immediately on the screen.
    </para>
  </refsect2>
  
  <refsect2>
    <title>Flicker free display</title>
    <para>
      The GNOME Canvas uses off-screen buffers to render the images
      before transferring them to the screen.  Transfers can take
      place at the command of the programmer (by explicitly
      requesting a repaint update) or done automatically by the
      engine (during the idle look handler).
    </para>
  </refsect2>
  
  <refsect2>
    <title>Event dispatching</title> 
    <para>Each 
      GnomeCanvasItem can receive mouse events, keyboard events,
      mouse-enter and mouse leave events.  In addition a canvas
      item can grab the mouse (for example to implement reliable
      dragging of objects).
    </para>
  </refsect2>
  
  <refsect2>
    <title>Canvas types</title>
    <para>
      The Canvas can be run in two different modes: X11 mode and Art
      mode.  The mode is chosen at the creation time of the widget
      by either calling gnome_canvas_new() or gnome_canvas_new_aa(),
      the former creates an X11 canvas, while the latter creates an
      Art-based canvas.
    </para>
    
    <para>
      The X11 mode uses the X server to draw the items and it takes
      advantage of the X server acceleration features for drawing on
      the screen.  The only drawback is that the output quality and
      the imaging model are restricted to the X11 quality and
      imaging model.
    </para>
    
    <para>
      The Art mode of the canvas has an advanced imaging model based
      on LibArt and it allows any GnomeCanvasItem (with the
      exception of the embedded widget item) to be rotated, scaled
      and translated (this is done by means of applying an affine
      transformation on the object).
    </para>
  </refsect2>

<!-- ##### SECTION See_Also ##### -->
  <para>
    #GnomeCanvasItem, #GnomeCanvasGroup, GnomeCanvasRE,
    #GnomeCanvasRectEllipse, #GnomeCanvasImage, #GnomeCanvasLine,
    #GnomeCanvasPolygon, #GnomeCanvasText, #GnomeCanvasWidget
  </para>

<!-- ##### STRUCT GnomeCanvas ##### -->
  <para>
    Most of the fields in this structure are for private use only.
    However, canvas item implementations may make use of some of them.
  </para>


<!-- ##### MACRO GNOME_CANVAS_EPSILON ##### -->
  <para>
    This macro defines a &lsquo;small&rsquo; floating-point value for
    the internal computations that the canvas performs.  It can be
    used by item implementations as a test to see whether a number is
    &ldquo;almost zero&rdquo;.
  </para>



<!-- ##### MACRO GNOME_CANVAS_COLOR ##### -->
  <para>
    This macro is used to build a 32-bit integer with an RGB color
    specification.  The specified values must be integers in the range
    [0, 255].
  </para>

@r: Red component of the color.
@g: Green component of the color.
@b: Blue component of the color.


<!-- ##### MACRO GNOME_CANVAS_COLOR_A ##### -->
  <para>
    This macro is used to build a 32-bit integer with an RGBA color
    specification.  This is the same as an RGB color specification,
    but with an added alpha or opacity value.  The specified values
    must be integers in the range [0, 255].
  </para>

@r: Red component of the color.
@g: Green component of the color.
@b: Blue component of the color.
@a: Opacity component of the color.


<!-- ##### STRUCT GnomeCanvasBuf ##### -->
  <para>
    This structure is passed to the <function>render</function> method
    of canvas items when they need to paint themselves on an
    antialiased canvas.  The <structfield>buf</structfield> field
    points to a 24-bit RGB buffer for rendering.  The
    <structfield>buf_rowstride</structfield> field specifies the
    number of bytes in each row in the buffer, which should be used to
    calculate byte offsets inside it.  The buffer's pixel offsets in
    canvas pixel coordinates are given by the
    <structfield>rect</structfield> rectangle.  The
    <structfield>is_bg</structfield> and
    <structfield>is_buf</structfield> fields are flags that items can
    use to implement rendering optimizations, and they are used in
    conjunction with the <structfield>bg_color</structfield> field.
  </para>

  <para>
    The <structfield>is_buf</structfield> flag specifies whether the
    contents of the buffer are an accurate representation of the state
    of the canvas.  If this flag is true, then the RGB data in the
    <structfield>buf</structfield> is valid, that is, it contains
    meaningful data.
  </para>

  <para>
    The <structfield>is_bg</structfield> flag specifies whether the
    buffer has all its pixels set to the same color.  This allows
    canvas items to optimize for this case by doing alpha compositing
    for a smaller set of values than if the buffer had pixels of
    different colors.
  </para>

  <para>
    At least one of these flags is on at any one time.  The meaning of
    their combinations is as follows:

    <table>
      <title>
	Values for <structfield>is_bg</structfield> and
	<structfield>is_buf</structfield>
      </title>
      <tgroup cols="3">
	<thead>
	  <row>
	    <entry><structfield>is_buf</structfield></entry>
	    <entry><structfield>is_bg</structfield></entry>
	    <entry>Meaning</entry>
	  </row>
	</thead>
	<tbody>
	  <row>
	    <entry><symbol>FALSE</symbol></entry>
	    <entry><symbol>TRUE</symbol></entry>
	    <entry>
	      The buffer does not contain meaningful data.  However,
	      it should be considered as if it were filled with the
	      solid color specified in the
	      <structfield>bg_color</structfield> field.  Item
	      implementations may want to call
	      <function>gnome_canvas_buf_ensure_buf()</function> to
	      fill the buffer automatically.
	    </entry>
	  </row>
	  <row>
	    <entry><symbol>TRUE</symbol></entry>
	    <entry><symbol>FALSE</symbol></entry>
	    <entry>
	      The buffer contains meaningful data and not all of its
	      pixels may be the same color.  Item implementations can
	      use the buffer data as-is for alpha compositing.
	    </entry>
	  </row>
	  <row>
	    <entry><symbol>TRUE</symbol></entry>
	    <entry><symbol>TRUE</symbol></entry>
	    <entry>
	      The buffer contains meaningful data, and all the pixels
	      are of the same color.  Item implementations can use the
	      buffer data as-is for alpha compositing, or be smarter
	      and do less operations since they can just composite
	      over a single color.
	    </entry>
	  </row>
	</tbody>
      </tgroup>
    </table>
  </para>

  <para>
    Whenever an item paints to an RGB buffer in which the
    <structfield>is_bg</structfield> field was true, the item is then
    responsible for turning off this flag if it knows that the result
    will not be pixels all of the same color.  If a large item, like a
    solid rectangle, knows that it will be filling the buffer with a
    solid color, then it take any one of the following actions:

    <itemizedlist>
      <listitem>
	<para>
	  Fill the actual pixels in the buffer with the solid color
	  and turn off the <structfield>is_bg</structfield> flag.  It
	  should then turn on the <structfield>is_buf</structfield>
	  flag.
	</para>
      </listitem>
      <listitem>
	<para>
	  Fill the actual pixels in the buffer with the solid color,
	  set the <structfield>bg_color</structfield> field to that
	  same color, and turn on both the
	  <structfield>is_bg</structfield> and
	  <structfield>is_buf</structfield> flags.  This is correct,
	  but is wasteful, since it could have done just the following
	  instead.
	</para>
      </listitem>
      <listitem>
	<para>
	  Just set the <structfield>bg_color</structfield> to the
	  solid color, turn on the <structfield>is_bg</structfield>
	  flag, and turn off the <structfield>is_buf</structfield>
	  flag.  This means that the buffer does not contain the
	  actual meaningful data, and the next item to be repainted
	  should look at the solid color instead.  This is the most
	  efficient version.
	</para>
      </listitem>
    </itemizedlist>
  </para>

  <para>
    Most item implementations may only need to perform the actions for
    the first case described above.  The other two are simply
    optimizations they can perform.
  </para>


<!-- ##### FUNCTION gnome_canvas_new ##### -->
<para>

</para>

@Returns: 


<!-- ##### FUNCTION gnome_canvas_new_aa ##### -->
<para>

</para>

@Returns: 


<!-- ##### FUNCTION gnome_canvas_root ##### -->
<para>

</para>

@canvas: 
@Returns: 


<!-- ##### FUNCTION gnome_canvas_set_scroll_region ##### -->
<para>

</para>

@canvas: 
@x1: 
@y1: 
@x2: 
@y2: 


<!-- ##### FUNCTION gnome_canvas_get_scroll_region ##### -->
<para>

</para>

@canvas: 
@x1: 
@y1: 
@x2: 
@y2: 


<!-- ##### FUNCTION gnome_canvas_set_pixels_per_unit ##### -->
<para>

</para>

@canvas: 
@n: 


<!-- ##### FUNCTION gnome_canvas_scroll_to ##### -->
<para>

</para>

@canvas: 
@cx: 
@cy: 


<!-- ##### FUNCTION gnome_canvas_get_scroll_offsets ##### -->
<para>

</para>

@canvas: 
@cx: 
@cy: 


<!-- ##### FUNCTION gnome_canvas_update_now ##### -->
<para>

</para>

@canvas: 


<!-- ##### FUNCTION gnome_canvas_get_item_at ##### -->
<para>

</para>

@canvas: 
@x: 
@y: 
@Returns: 


<!-- ##### FUNCTION gnome_canvas_request_redraw_uta ##### -->
<para>

</para>

@canvas: 
@uta: 


<!-- ##### FUNCTION gnome_canvas_request_redraw ##### -->
<para>

</para>

@canvas: 
@x1: 
@y1: 
@x2: 
@y2: 


<!-- ##### FUNCTION gnome_canvas_w2c_affine ##### -->
<para>

</para>

@canvas: 
@affine: 


<!-- ##### FUNCTION gnome_canvas_w2c ##### -->
<para>

</para>

@canvas: 
@wx: 
@wy: 
@cx: 
@cy: 


<!-- ##### FUNCTION gnome_canvas_w2c_d ##### -->
<para>

</para>

@canvas: 
@wx: 
@wy: 
@cx: 
@cy: 


<!-- ##### FUNCTION gnome_canvas_c2w ##### -->
<para>

</para>

@canvas: 
@cx: 
@cy: 
@wx: 
@wy: 


<!-- ##### FUNCTION gnome_canvas_window_to_world ##### -->
<para>

</para>

@canvas: 
@winx: 
@winy: 
@worldx: 
@worldy: 


<!-- ##### FUNCTION gnome_canvas_world_to_window ##### -->
<para>

</para>

@canvas: 
@worldx: 
@worldy: 
@winx: 
@winy: 


<!-- ##### FUNCTION gnome_canvas_get_color ##### -->
<para>

</para>

@canvas: 
@spec: 
@color: 
@Returns: 


<!-- ##### FUNCTION gnome_canvas_set_stipple_origin ##### -->
<para>

</para>

@canvas: 
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