gtk2/docs/reference/gdk/tmpl/rgb.sgml

<!-- ##### SECTION Title ##### -->
GdkRGB

<!-- ##### SECTION Short_Description ##### -->
Renders RGB, grayscale, or indexed image data to a GdkDrawable

<!-- ##### SECTION Long_Description ##### -->

<para>
GdkRGB is a low-level module which renders RGB, grayscale, and indexed
colormap images to a #GdkDrawable. It does this as efficiently as
possible, handling issues such as colormaps, visuals, dithering,
temporary buffers, and so on. Most code should use the higher-level
#GdkPixbuf features in place of this module; for example,
gdk_pixbuf_render_to_drawable() uses GdkRGB in its implementation.
</para>

<para>
GdkRGB allocates a color cube to use when rendering images.  You can
set the threshold for installing colormaps with
gdk_rgb_set_min_colors(). The default is 5x5x5 (125). If a colorcube
of this size or larger can be allocated in the default colormap, then
that's done. Otherwise, GdkRGB creates its own private colormap.
Setting it to 0 means that it always tries to use the default
colormap, and setting it to 216 means that it always creates a private
one if it cannot allocate the 6x6x6 colormap in the default. If you
always want a private colormap (to avoid consuming too many colormap
entries for other apps, say), you can use 
<literal>gdk_rgb_set_install(TRUE)</literal>.
Setting the value greater than 216 exercises a bug in older versions
of GdkRGB. Note, however, that setting it to 0 doesn't let you get
away with ignoring the colormap and visual - a colormap is always
created in grayscale and direct color modes, and the visual is changed
in cases where a "better" visual than the default is available.
</para>

<example>
<title>A simple example program using GdkRGB</title>
<programlisting>
&num;include &lt;gtk/gtk.h&gt;

&num;define IMAGE_WIDTH	256
&num;define IMAGE_HEIGHT	256

guchar rgbbuf[IMAGE_WIDTH * IMAGE_HEIGHT * 3];

gboolean on_darea_expose (GtkWidget *widget,
			  GdkEventExpose *event,
			  gpointer user_data);

int
main (int argc, char *argv[])
{
  GtkWidget *window, *darea;
  gint x, y;
  guchar *pos;

  gtk_init (&amp;argc, &amp;argv);

  window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
  darea = gtk_drawing_area_new (<!-- -->);
  gtk_widget_set_size_request (darea, IMAGE_WIDTH, IMAGE_HEIGHT);
  gtk_container_add (GTK_CONTAINER (window), darea);
  gtk_signal_connect (GTK_OBJECT (darea), "expose-event",
                      GTK_SIGNAL_FUNC (on_darea_expose), NULL);
  gtk_widget_show_all (window);

  /* Set up the RGB buffer. */
  pos = rgbbuf;
  for (y = 0; y &lt; IMAGE_HEIGHT; y++)
    {
      for (x = 0; x &lt; IMAGE_WIDTH; x++)
	{
	  *pos++ = x - x % 32;			/* Red. */
	  *pos++ = (x / 32) * 4 + y - y % 32;	/* Green. */
	  *pos++ = y - y % 32;			/* Blue. */
	}
    }

  gtk_main (<!-- -->);
  return 0;
}


gboolean
on_darea_expose (GtkWidget *widget,
		 GdkEventExpose *event,
		 gpointer user_data)
{
  gdk_draw_rgb_image (widget->window, widget->style->fg_gc[GTK_STATE_NORMAL],
		      0, 0, IMAGE_WIDTH, IMAGE_HEIGHT,
		      GDK_RGB_DITHER_MAX, rgbbuf, IMAGE_WIDTH * 3);

  return TRUE;
}
</programlisting>
</example>

<!-- ##### SECTION See_Also ##### -->
<para>
<variablelist>

<varlistentry>
<term>#GdkColor</term>
<listitem><para>The underlying GDK mechanism for allocating
colors.</para></listitem>
</varlistentry>

<varlistentry>
<term>#GdkPixbuf and gdk_pixbuf_render_to_drawable()</term>
<listitem><para>Higher-level image handling.</para></listitem>
</varlistentry>

</variablelist>

</para>

<!-- ##### FUNCTION gdk_rgb_init ##### -->
<para>
This function no longer does anything at all. It's completely useless
(and harmless).
</para>



<!-- ##### FUNCTION gdk_draw_rgb_image ##### -->
<para>
Draws an RGB image in the drawable. This is the core GdkRGB
function, and likely the only one you will need to use.
</para>

<para>
The @rowstride parameter allows for lines to be aligned more flexibly.
For example, lines may be allocated to begin on 32-bit boundaries,
even if the width of the rectangle is odd. Rowstride is also useful
when drawing a subrectangle of a larger image in memory. Finally, to
replicate the same line a number of times, the trick of setting
@rowstride to 0 is allowed.
</para>

<para>
In general, for 0 &lt;= i &lt; @width and 0 &lt;= j &lt; height,
the pixel (x + i, y + j) is colored with red value @rgb_buf[@j *
@rowstride + @i * 3], green value @rgb_buf[@j * @rowstride + @i * 3 +
1], and blue value @rgb_buf[@j * @rowstride + @i * 3 + 2].
</para>

@drawable: The #GdkDrawable to draw in (usually a #GdkWindow).
@gc: The graphics context (all GDK drawing operations require one; its
contents are ignored).
@x: The x coordinate of the top-left corner in the drawable.
@y: The y coordinate of the top-left corner in the drawable.
@width: The width of the rectangle to be drawn.
@height: The height of the rectangle to be drawn.
@dith: A #GdkRgbDither value, selecting the desired dither mode.
@rgb_buf: The pixel data, represented as packed 24-bit data.
@rowstride: The number of bytes from the start of one row in @rgb_buf to the
start of the next.


<!-- ##### FUNCTION gdk_draw_rgb_image_dithalign ##### -->
<para>
Draws an RGB image in the drawable, with an adjustment for dither alignment.
</para>

<para>
This function is useful when drawing dithered images into a window
that may be scrolled. Pixel (x, y) will be drawn dithered as if its
actual location is (x + @xdith, y + @ydith). Thus, if you draw an
image into a window using zero dither alignment, then scroll up one
pixel, subsequent draws to the window should have @ydith = 1.
</para>

<para>
Setting the dither alignment correctly allows updating of small parts
of the screen while avoiding visible "seams" between the different
dither textures.
</para>

@drawable: The #GdkDrawable to draw in (usually a #GdkWindow).
@gc: The graphics context.
@x: The x coordinate of the top-left corner in the drawable.
@y: The y coordinate of the top-left corner in the drawable.
@width: The width of the rectangle to be drawn.
@height: The height of the rectangle to be drawn.
@dith: A #GdkRgbDither value, selecting the desired dither mode.
@rgb_buf: The pixel data, represented as packed 24-bit data.
@rowstride: The number of bytes from the start of one row in @rgb_buf to the
start of the next.
@xdith: An x offset for dither alignment.
@ydith: A y offset for dither alignment.


<!-- ##### FUNCTION gdk_draw_indexed_image ##### -->
<para>
Draws an indexed image in the drawable, using a #GdkRgbCmap to assign
actual colors to the color indices.
</para>

@drawable: The #GdkDrawable to draw in (usually a #GdkWindow).
@gc: The graphics context.
@x: The x coordinate of the top-left corner in the drawable.
@y: The y coordinate of the top-left corner in the drawable.
@width: The width of the rectangle to be drawn.
@height: The height of the rectangle to be drawn.
@dith: A #GdkRgbDither value, selecting the desired dither mode.
@buf: The pixel data, represented as 8-bit color indices.
@rowstride: The number of bytes from the start of one row in @buf to the
start of the next.
@cmap: The #GdkRgbCmap used to assign colors to the color indices.


<!-- ##### FUNCTION gdk_draw_gray_image ##### -->
<para>
Draws a grayscale image in the drawable.

</para>

@drawable: The #GdkDrawable to draw in (usually a #GdkWindow).
@gc: The graphics context.
@x: The x coordinate of the top-left corner in the drawable.
@y: The y coordinate of the top-left corner in the drawable.
@width: The width of the rectangle to be drawn.
@height: The height of the rectangle to be drawn.
@dith: A #GdkRgbDither value, selecting the desired dither mode.
@buf: The pixel data, represented as 8-bit gray values.
@rowstride: The number of bytes from the start of one row in @buf to the
start of the next.


<!-- ##### FUNCTION gdk_draw_rgb_32_image ##### -->
<para>
Draws a padded RGB image in the drawable. The image is stored as one
pixel per 32-bit word. It is laid out as a red byte, a green byte, a
blue byte, and a padding byte.
</para>

<para>
It's unlikely that this function will give significant performance
gains in practice. In my experience, the performance gain from having
pixels aligned to 32-bit boundaries is cancelled out by the increased
memory bandwidth.
</para>

@drawable: The #GdkDrawable to draw in (usually a #GdkWindow).
@gc: The graphics context.
@x: The x coordinate of the top-left corner in the drawable.
@y: The y coordinate of the top-left corner in the drawable.
@width: The width of the rectangle to be drawn.
@height: The height of the rectangle to be drawn.
@dith: A #GdkRgbDither value, selecting the desired dither mode.
@buf: The pixel data, represented as padded 32-bit data.
@rowstride: The number of bytes from the start of one row in @buf to the
start of the next.


<!-- ##### FUNCTION gdk_draw_rgb_32_image_dithalign ##### -->
<para>

</para>

@drawable: 
@gc: 
@x: 
@y: 
@width: 
@height: 
@dith: 
@buf: 
@rowstride: 
@xdith: 
@ydith: 


<!-- ##### ENUM GdkRgbDither ##### -->
<para>
Selects whether or not GdkRGB applies dithering
to the image on display. There are three values:
</para>

<itemizedlist>

<listitem>
<para>
%GDK_RGB_DITHER_NONE: Never use dithering.
</para>
</listitem>

<listitem>
<para>
%GDK_RGB_DITHER_NORMAL: Use dithering in 8 bits per pixel (and below)
only.
</para>
</listitem>

<listitem>
<para>
%GDK_RGB_DITHER_MAX: Use dithering in 16 bits per pixel and below.
</para>
</listitem>

</itemizedlist>

<para>
Since GdkRGB currently only handles images with 8 bits per component,
dithering on 24 bit per pixel displays is a moot point.
</para>

@GDK_RGB_DITHER_NONE: 
@GDK_RGB_DITHER_NORMAL: 
@GDK_RGB_DITHER_MAX: 

<!-- ##### FUNCTION gdk_rgb_cmap_new ##### -->
<para>
Creates a new #GdkRgbCmap structure. The cmap maps color indexes to
RGB colors. If @n_colors is less than 256, then images containing
color values greater than or equal to @n_colors will produce undefined
results, including possibly segfaults.
</para>

@colors: The colors, represented as 0xRRGGBB integer values.
@n_colors: The number of colors in the cmap.
@Returns: The newly created #GdkRgbCmap


<!-- ##### FUNCTION gdk_rgb_cmap_free ##### -->
<para>
Frees the memory associated with a #GdkRgbCmap created by gdk_rgb_cmap_new().
</para>

@cmap: The #GdkRgbCmap to free.


<!-- ##### STRUCT GdkRgbCmap ##### -->
<para>
A private data structure which maps color indices to actual RGB
colors. This is used only for gdk_draw_indexed_image().
</para>

@colors: 
@n_colors: 

<!-- ##### FUNCTION gdk_rgb_gc_set_foreground ##### -->
<para>
Sets the foreground color in @gc to the specified color (or the
closest approximation, in the case of limited visuals).
</para>

@gc: The #GdkGC to modify.
@rgb: The color, represented as a 0xRRGGBB integer value.


<!-- ##### FUNCTION gdk_rgb_gc_set_background ##### -->
<para>
Sets the background color in @gc to the specified color (or the
closest approximation, in the case of limited visuals).
</para>

@gc: The #GdkGC to modify.
@rgb: The color, represented as a 0xRRGGBB integer value.


<!-- ##### FUNCTION gdk_rgb_xpixel_from_rgb ##### -->
<para>
Finds the X pixel closest in color to the @rgb color specified. This
value may be used to set the <structfield>pixel</structfield> field of
a #GdkColor struct.
</para>

@rgb: The color, represented as a 0xRRGGBB integer value.
@Returns: The X pixel value.


<!-- ##### FUNCTION gdk_rgb_find_color ##### -->
<para>

</para>

@colormap: 
@color: 


<!-- ##### FUNCTION gdk_rgb_set_install ##### -->
<para>
If @install is %TRUE, directs GdkRGB to always install a new "private"
colormap rather than trying to find a best fit with the colors already
allocated. Ordinarily, GdkRGB will install a colormap only if a
sufficient cube cannot be allocated.
</para>

<para>
A private colormap has more colors, leading to better quality display,
but also leads to the dreaded "colormap flashing" effect.
</para>

@install: %TRUE to set install mode.


<!-- ##### FUNCTION gdk_rgb_set_min_colors ##### -->
<para>
Sets the minimum number of colors for the color cube. Generally,
GdkRGB tries to allocate the largest color cube it can. If it can't
allocate a color cube at least as large as @min_colors, it installs a
private colormap.
</para>

@min_colors: The minimum number of colors accepted.


<!-- ##### FUNCTION gdk_rgb_get_visual ##### -->
<para>
</para>

@Returns: 


<!-- ##### FUNCTION gdk_rgb_get_colormap ##### -->
<para>

</para>

@Returns: 


<!-- ##### MACRO gdk_rgb_get_cmap ##### -->
<para>
Gets the colormap set by GdkRGB. This colormap and the corresponding
visual should be used when creating windows that will be drawn in by GdkRGB.
</para>

@Returns: The #GdkColormap set by GdkRGB.


<!-- ##### FUNCTION gdk_rgb_ditherable ##### -->
<para>
Determines whether the visual is ditherable. This function may be
useful for presenting a user interface choice to the user about which
dither mode is desired; if the display is not ditherable, it may make
sense to gray out or hide the corresponding UI widget.
</para>

@Returns: %TRUE if the visual is ditherable.


<!-- ##### FUNCTION gdk_rgb_set_verbose ##### -->
<para>
Sets the "verbose" flag. This is generally only useful for debugging.
</para>

@verbose: %TRUE if verbose messages are desired.