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

<!-- ##### SECTION Title ##### -->
Visuals

<!-- ##### SECTION Short_Description ##### -->
manipulation of visuals.

<!-- ##### SECTION Long_Description ##### -->
<para>
The way that the data stored on the screen is stored
in memory can vary considerably between different X
servers; some X servers even support multiple formats
used simultaneously. An X <firstterm>visual</firstterm>
represents a particular format for screen data.
It includes information about the number of bits
used for each color, the way the bits are translated
into an RGB value for display, and the way the bits
are stored in memory.
</para>
<para>
There are several standard visuals. The visual returned
by gdk_visual_get_system() is the system's default
visual. gdk_rgb_get_visual() return the visual most
suited to displaying full-color image data. If you
use the calls in GdkRGB, you should create your windows
using this visual (and the colormap returned by
gdk_rgb_get_colormap()).
</para>
<para>
A number of functions are provided for determining
the "best" available visual. For the purposes of
making this determination, higher bit depths are
considered better, and for visuals of the same
bit depth, %GDK_VISUAL_PSEUDO_COLOR is preferred at
8bpp, otherwise, the visual types are ranked in the
order of (highest to lowest) %GDK_VISUAL_DIRECT_COLOR,
%GDK_VISUAL_TRUE_COLOR, %GDK_VISUAL_PSEUDO_COLOR,
%GDK_VISUAL_STATIC_COLOR, %GDK_VISUAL_GRAYSCALE, 
then %GDK_VISUAL_STATIC_GRAY.
</para>

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

</para>

<!-- ##### STRUCT GdkVisual ##### -->
<para>
The GdkVisual structure contains information about
a particular visual. It contains the following
public fields.

<informaltable pgwide=1 frame="none" role="struct">
<tgroup cols="2"><colspec colwidth="2*"><colspec colwidth="8*">
<tbody>

<row>
<entry><structfield>type</structfield></entry>
<entry>The type of this visual.</entry>
</row>

<row>
<entry><structfield>depth</structfield></entry>
<entry>The number of bits per pixel.</entry>
</row>

<row>
<entry><structfield>byte_order</structfield></entry>
<entry>The byte-order for this visual.</entry>
</row>

<row>
<entry><structfield>colormap_size</structfield></entry>
<entry>The number of entries in the colormap, for
visuals of type GDK_VISUAL_PSEUDO_COLOR or 
GDK_VISUAL_GRAY_SCALE. For other visual types, it
is the number of possible levels per color component.
If the visual has different numbers of levels for
different components, the value of this field is
undefined.</entry>
</row>

<row>
<entry><structfield>bits_per_rgb</structfield></entry>
<entry>
The number of significant bits per red, green, or blue
when specifying colors for this visual. (For instance, 
for gdk_colormap_alloc_color())
</entry>
</row>

<row>
<entry><structfield>red_mask</structfield></entry>
<entry>
A mask giving the bits in a pixel value that
correspond to the red field. Significant only for 
%GDK_VISUAL_PSEUDOCOLOR and %GDK_VISUAL_DIRECTCOLOR.
</entry>
</row>

<row>
<entry><structfield>red_shift, red_prec</structfield></entry>
<entry>
The <structfield>red_shift</structfield> and 
<structfield>red_prec</structfield> give an alternate presentation
of the information in <structfield>red_mask</structfield>.
<structfield>red_mask</structfield> is a contiguous sequence
of <structfield>red_prec</structfield> starting at bit
number <structfield>red_shift</structfield>. For example,
<xref linkend="rgbmask"> shows constructing a pixel value
out of three 16 bit color values.

</entry>
</row>

<row>
<entry><structfield>green_mask</structfield></entry>
<entry>
A mask giving the bits in a pixel value that
correspond to the green field.
</entry>
</row>

<row>
<entry><structfield>green_shift, green_prec</structfield></entry>
<entry>
The <structfield>green_shift</structfield> and 
<structfield>green_prec</structfield> give an alternate presentation
of the information in <structfield>green_mask</structfield>.
</entry>
</row>

<row>
<entry><structfield>blue_mask</structfield></entry>
<entry>
A mask giving the bits in a pixel value that
correspond to the blue field.
</entry>
</row>

<row>
<entry><structfield>blue_shift, blue_prec</structfield></entry>
<entry>
The <structfield>blue_shift</structfield> and 
<structfield>blue_prec</structfield> give an alternate presentation
of the information in <structfield>blue_mask</structfield>.
</entry>
</row>

</tbody></tgroup></informaltable>
</para>

<figure float="1" id="rgbmask">
<title>Constructing a pixel value from components</title>
<programlisting>
guint 
pixel_from_rgb (GdkVisual *visual,
                guchar r, guchar b, guchar g) 
{
  return ((r >> (16 - visual->red_prec))   << visual->red_shift) |
         ((g >> (16 - visual->green_prec)) << visual->green_shift) |
         ((r >> (16 - visual->blue_prec))  << visual->blue_shift);
}
</programlisting>
</figure>

@type: 
@depth: 
@byte_order: 
@colormap_size: 
@bits_per_rgb: 
@red_mask: 
@red_shift: 
@red_prec: 
@green_mask: 
@green_shift: 
@green_prec: 
@blue_mask: 
@blue_shift: 
@blue_prec: 

<!-- ##### ENUM GdkVisualType ##### -->
<para>
A set of values that describe the manner in which the
pixel values for a visual are converted into RGB
values for display.

<informaltable pgwide=1 frame="none" role="enum">
<tgroup cols="2"><colspec colwidth="2*"><colspec colwidth="8*">
<tbody>

<row>
<entry>GDK_VISUAL_STATIC_GRAY</entry>
<entry>Each pixel value indexes a grayscale value directly.</entry>
</row>

<row>
<entry>GDK_VISUAL_GRAYSCALE</entry>
<entry>Each pixel is an index into a color map that maps
pixel values into grayscale values. The color map can
be changed by an application.</entry>
</row>

<row>
<entry>GDK_VISUAL_STATIC_COLOR</entry>
<entry>Each pixel value is an index into a predefined,
unmodifiable color map that maps pixel values into
rgb values.</entry>
</row>

<row>
<entry>GDK_VISUAL_PSEUDO_COLOR</entry>
<entry>Each pixel is an index into a color map that maps
pixel values into rgb values. The color map can
be changed by an application.</entry>
</row>

<row>
<entry>GDK_TRUE_COLOR</entry>
<entry>Each pixel value directly contains red, green,
and blue components. The <structfield>red_mask</structfield>,
<structfield>green_mask</structfield>, and 
<structfield>blue_mask</structfield> fields of the #GdkVisual
structure describe how the components are assembled into
a pixel value.
.</entry>
</row>

<row>
<entry>GDK_DIRECT_COLOR</entry>
<entry>Each pixel value contains red, green, and blue
components as for %GDK_TRUE_COLOR, but the components
are mapped via a color table into the final output
table instead of being converted directly..</entry>
</row>

</tbody></tgroup></informaltable>
</para>

@GDK_VISUAL_STATIC_GRAY: 
@GDK_VISUAL_GRAYSCALE: 
@GDK_VISUAL_STATIC_COLOR: 
@GDK_VISUAL_PSEUDO_COLOR: 
@GDK_VISUAL_TRUE_COLOR: 
@GDK_VISUAL_DIRECT_COLOR: 

<!-- ##### ENUM GdkByteOrder ##### -->
<para>
A set of values describing the possible byte-orders
for storing pixel values in memory.

<informaltable pgwide=1 frame="none" role="enum">
<tgroup cols="2"><colspec colwidth="2*"><colspec colwidth="8*">
<tbody>

<row>
<entry>GDK_LSB_FIRST</entry>
<entry>The values are stored with the least-significant byte
first. For instance, the 32-bit value 0xffeecc would be stored
in memory as 0xcc, 0xee, 0xff, 0x00.</entry>
</row>

<row>
<entry>GDK_MSB_FIRST</entry>
<entry>The values are stored with the least-significant byte
first. For instance, the 32-bit value 0xffeecc would be stored
in memory as 0xff, 0xee, 0xcc, 0x00.</entry>
</row>

</tbody></tgroup></informaltable>
</para>

@GDK_LSB_FIRST: 
@GDK_MSB_FIRST: 

<!-- ##### FUNCTION gdk_query_depths ##### -->
<para>
Lists the available color depths. The returned values
are pointers to static storage and should not be
modified or freed.
</para>

@depths: a location to store a pointer to an array
   holding the available color depths.
@count: a location to store the number of values in @depths.


<!-- ##### FUNCTION gdk_query_visual_types ##### -->
<para>
Lists the available visual types. The returned values
are pointers to static storage and should not be
modified or freed.
</para>

@visual_types: a location to store a pointer to an 
   array holding the available visual types.
@count: a location to store the number of values in @visual types.


<!-- ##### FUNCTION gdk_list_visuals ##### -->
<para>
Lists the available visuals.
</para>

@Returns: A #GList of the available visuals. The list
should be freed this list with g_list_free().


<!-- ##### FUNCTION gdk_visual_get_best_depth ##### -->
<para>
Returns the best available color depth.
</para>

@Returns: the best available color depth.


<!-- ##### FUNCTION gdk_visual_get_best_type ##### -->
<para>
Returns the best available visual type.
</para>

@Returns: the best available visual type.


<!-- ##### FUNCTION gdk_visual_get_system ##### -->
<para>
Returns the system's default visual.
</para>

@Returns: the system's default visual.


<!-- ##### FUNCTION gdk_visual_get_best ##### -->
<para>
Returns the best available visual.
</para>

@Returns: the best available visual.


<!-- ##### FUNCTION gdk_visual_get_best_with_depth ##### -->
<para>
Returns the best available visual with a certain depth.
</para>

@depth: the desired color depth
@Returns: the best available visual with @depth bits per pixel
or %NULL if no visuals with that type are available.


<!-- ##### FUNCTION gdk_visual_get_best_with_type ##### -->
<para>
Returns the best available visual of a certain visual type.
</para>

@visual_type: the desired visual type.
@Returns: the visual of the given type with the highest depth
or %NULL if no visuals of that type are available.


<!-- ##### FUNCTION gdk_visual_get_best_with_both ##### -->
<para>
Returns the best available visual 
</para>

@depth: the desired visual type.
@visual_type: the desired depth.
@Returns: the best available visual with the given depth
and visual type, or %NULL if no matching visuals are
available.


<!-- ##### FUNCTION gdk_visual_ref ##### -->
<para>
In theory, increases the reference count of a visual. However,
the set of visuals is determined statically when GTK+ is
initialized, so this operation does nothing.
</para>

@visual: a #GdkVisual.
@Returns: @visual.


<!-- ##### FUNCTION gdk_visual_unref ##### -->
<para>
In theory, decreases the reference count of a visual. Like
gdk_visual_ref(), this operation does nothing.
</para>

@visual: a #GdkVisual.