Better description of gdk_rgb_set_min_colors. Stric pointed out that

[~andy/gtk] / docs / reference / gdk / tmpl / rgb.sgml

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

<!-- ##### SECTION Short_Description ##### -->
displays RGB images (as well as grayscale and colormapped) to
the native window.

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

GdkRgb converts RGB, grayscale, and colormapped images into the native
window pixel format and displays them. It takes care of colormaps,
visuals, dithering, and management of the temporary buffers.

</para>

<para>
You must call gdk_rgb_init() before using any GdkRgb functionality. If
you fail to do so, expect coredumps. All Gtk+ widgets that use GdkRgb
(including #GtkPreview) call gdk_rgb_init() in their class_init method.
Thus, if you use GdkRgb only indirectly, you don't need to worry
about it.
</para>

<para>
GdkRgb tries to use the system default visual and colormap, but
doesn't always succeed. Thus, you have to be prepared to install the
visual and colormap generated by GdkRgb. The following code sequence
(before any widgets are created) should work in most applications:
</para>

<informalexample>
<programlisting>
  gdk_rgb_init ();

  gtk_widget_set_default_colormap (gdk_rgb_get_cmap ());
  gtk_widget_set_default_visual (gdk_rgb_get_visual ());
</programlisting>
</informalexample>

<para>
You can also push the colormap and visual, but in general it doesn't
work unless the push wraps the window creation call. If you wrap the
push around a widget which is embedded in a window without the GdkRgb
colormap and visual, it probably won't work, and is likely to cause
colormap flashing, as well.
</para>

<para>
On 8-bit systems, the colormaps used by Imlib and GdkRgb may
conflict. There is no good general solution to this other than phasing
out the dependence on Imlib.
</para>

<para>
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 gdk_rgb_set_install(TRUE).
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>
#include &lt;gtk/gtk.h&gt;

#define IMAGE_WIDTH     256
#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);
  gdk_rgb_init ();
  window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
  darea = gtk_drawing_area_new ();
  gtk_drawing_area_size (GTK_DRAWING_AREA (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 < IMAGE_HEIGHT; y++)
    {
      for (x = 0; x < 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);
}
</programlisting>
</example>

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

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

</variablelist>

</para>

<!-- ##### FUNCTION gdk_rgb_init ##### -->
<para>
Initializes GdkRgb statically. It may be called more than once with no
ill effects. It must, however, be called before any other GdkRgb
operations are performed.
</para>

<para>
The GdkRgb "context" is allocated statically. Thus, GdkRgb may be used
to drive only one visual in any given application. GdkRgb
automatically selects a best visual and sets its own colormap, if
necessary. gdk_rgb_get_visual() and gdk_rgb_get_cmap () retrieve
the chosen visual and colormap, respectively.
</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 other than the
initialization stuff.
</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.


<!-- ##### 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: 
@lut: 

<!-- ##### 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_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>
Gets the visual chosen by GdkRgb. This visual and the corresponding
colormap should be used when creating windows that will be drawn in by GdkRgb.
</para>

@Returns: The #GdkVisual chosen by GdkRgb.


<!-- ##### FUNCTION 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>
Determine 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.