Visuals

## Visuals Visuals — Low-level display hardware information |

## Synopsis

    #include <gdk/gdk.h>

                GdkVisual;
    enum        GdkVisualType;
    enum        GdkByteOrder;
    void        gdk_query_depths                (gint **depths,
                                                 gint *count);
    void        gdk_query_visual_types          (GdkVisualType **visual_types,
                                                 gint *count);
    GList*      gdk_list_visuals                (void);
    gint        gdk_visual_get_best_depth       (void);
    GdkVisualType gdk_visual_get_best_type      (void);
    GdkVisual*  gdk_visual_get_system           (void);
    GdkVisual*  gdk_visual_get_best             (void);
    GdkVisual*  gdk_visual_get_best_with_depth  (gint depth);
    GdkVisual*  gdk_visual_get_best_with_type   (GdkVisualType visual_type);
    GdkVisual*  gdk_visual_get_best_with_both   (gint depth,
                                                 GdkVisualType visual_type);
    #define     gdk_visual_ref                  (v)
    #define     gdk_visual_unref                (v)
    GdkScreen*  gdk_visual_get_screen           (GdkVisual *visual);

## Description

A GdkVisual describes a particular video hardware display format. 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. For example, a piece of display hardware might support 24-bit color, 16-bit color, or 8-bit color; meaning 24/16/8-bit pixel sizes. For a given pixel size, pixels can be in different formats; for example the "red" element of an RGB pixel may be in the top 8 bits of the pixel, or may be in the lower 4 bits.

Usually you can avoid thinking about visuals in GTK+. Visuals are useful to interpret the contents of a GdkImage, but you should avoid GdkImage precisely because its contents depend on the display hardware; use GdkPixbuf instead, for all but the most low-level purposes. Also, anytime you provide a GdkColormap, the visual is implied as part of the colormap (`gdk_colormap_get_visual()`), so you won't have to provide a visual in addition.

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()`).

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`.

## Details

### GdkVisual

    typedef struct {
      GObject parent_instance;

      GdkVisualType type;
      gint depth;
      GdkByteOrder byte_order;
      gint colormap_size;
      gint bits_per_rgb;

      guint32 red_mask;
      gint red_shift;
      gint red_prec;

      guint32 green_mask;
      gint green_shift;
      gint green_prec;

      guint32 blue_mask;
      gint blue_shift;
      gint blue_prec;
    } GdkVisual;

The GdkVisual structure contains information about a particular visual.

Example 5. Constructing a pixel value from components

    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);
    }

GObject `parent_instance`; | inherited portion from GObject
GdkVisualType `type`; | The type of this visual.
gint `depth`; | The number of bits per pixel.
GdkByteOrder `byte_order`; | The byte-order for this visual.
gint `colormap_size`; | 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.
gint `bits_per_rgb`; | The number of significant bits per red, green, or blue when specifying colors for this visual. (For instance, for `gdk_colormap_alloc_color()`)
guint32 `red_mask`; | 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`.
gint `red_shift`; | The `red_shift` and `red_prec` give an alternate presentation of the information in `red_mask`. `red_mask` is a contiguous sequence of `red_prec` bits starting at bit number `red_shift`. For example, Example 5, “Constructing a pixel value from components” shows constructing a pixel value out of three 16 bit color values.
gint `red_prec`; | See above.
guint32 `green_mask`; | A mask giving the bits in a pixel value that correspond to the green field.
gint `green_shift`; | The `green_shift` and `green_prec` give an alternate presentation of the information in `green_mask`.
gint `green_prec`; | See above.
guint32 `blue_mask`; | A mask giving the bits in a pixel value that correspond to the blue field.
gint `blue_shift`; | The `blue_shift` and `blue_prec` give an alternate presentation of the information in `blue_mask`.
gint `blue_prec`; | See above.

### enum GdkVisualType

    typedef enum
    {
      GDK_VISUAL_STATIC_GRAY,
      GDK_VISUAL_GRAYSCALE,
      GDK_VISUAL_STATIC_COLOR,
      GDK_VISUAL_PSEUDO_COLOR,
      GDK_VISUAL_TRUE_COLOR,
      GDK_VISUAL_DIRECT_COLOR
    } GdkVisualType;

A set of values that describe the manner in which the pixel values for a visual are converted into RGB values for display.

`GDK_VISUAL_STATIC_GRAY` | Each pixel value indexes a grayscale value directly.
`GDK_VISUAL_GRAYSCALE` | 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.
`GDK_VISUAL_STATIC_COLOR` | Each pixel value is an index into a predefined, unmodifiable color map that maps pixel values into RGB values.
`GDK_VISUAL_PSEUDO_COLOR` | 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.
`GDK_VISUAL_TRUE_COLOR` | Each pixel value directly contains red, green, and blue components. The `red_mask`, `green_mask`, and `blue_mask` fields of the GdkVisual structure describe how the components are assembled into a pixel value.
`GDK_VISUAL_DIRECT_COLOR` | Each pixel value contains red, green, and blue components as for `GDK_VISUAL_TRUE_COLOR`, but the components are mapped via a color table into the final output table instead of being converted directly.

### enum GdkByteOrder

    typedef enum
    {
      GDK_LSB_FIRST,
      GDK_MSB_FIRST
    } GdkByteOrder;

A set of values describing the possible byte-orders for storing pixel values in memory.

`GDK_LSB_FIRST` | 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.
`GDK_MSB_FIRST` | The values are stored with the most-significant byte first. For instance, the 32-bit value 0xffeecc would be stored in memory as 0x00, 0xcc, 0xee, 0xff.

### gdk_query_depths ()

    void        gdk_query_depths                (gint **depths,
                                                 gint *count);

This function returns the available bit depths for the default screen. It's equivalent to listing the visuals (`gdk_list_visuals()`) and then looking at the depth field in each visual, removing duplicates.

The array returned by this function should not be freed.

`depths` : | return location for available depths
`count` : | return location for number of available depths

### gdk_query_visual_types ()

    void        gdk_query_visual_types          (GdkVisualType **visual_types,
                                                 gint *count);

This function returns the available visual types for the default screen. It's equivalent to listing the visuals (`gdk_list_visuals()`) and then looking at the type field in each visual, removing duplicates.

The array returned by this function should not be freed.

`visual_types` : | return location for the available visual types
`count` : | return location for the number of available visual types

### gdk_list_visuals ()

    GList*      gdk_list_visuals                (void);

Lists the available visuals for the default screen. (See `gdk_screen_list_visuals()`) A visual describes a hardware image data format. For example, a visual might support 24-bit color, or 8-bit color, and might expect pixels to be in a certain format.

Call `g_list_free()` on the return value when you're finished with it.

Returns : | a list of visuals; the list must be freed, but not its contents

### gdk_visual_get_best_depth ()

    gint        gdk_visual_get_best_depth       (void);

Get the best available depth for the default GDK screen. "Best" means "largest," i.e. 32 preferred over 24 preferred over 8 bits per pixel.

Returns : | best available depth

### gdk_visual_get_best_type ()

    GdkVisualType gdk_visual_get_best_type      (void);

Return the best available visual type for the default GDK screen.

Returns : | best visual type

### gdk_visual_get_system ()

    GdkVisual*  gdk_visual_get_system           (void);

Get the system'sdefault visual for the default GDK screen. This is the visual for the root window of the display. The return value should not be freed.

Returns : | system visual

### gdk_visual_get_best ()

    GdkVisual*  gdk_visual_get_best             (void);

Get the visual with the most available colors for the default GDK screen. The return value should not be freed.

Returns : | best visual

### gdk_visual_get_best_with_depth ()

    GdkVisual*  gdk_visual_get_best_with_depth  (gint depth);

Get the best visual with depth `depth` for the default GDK screen. Color visuals and visuals with mutable colormaps are preferred over grayscale or fixed-colormap visuals. The return value should not be freed. `NULL` may be returned if no visual supports `depth`.

`depth` : | a bit depth
Returns : | best visual for the given depth

### gdk_visual_get_best_with_type ()

    GdkVisual*  gdk_visual_get_best_with_type   (GdkVisualType visual_type);

Get the best visual of the given `visual_type` for the default GDK screen. Visuals with higher color depths are considered better. The return value should not be freed. `NULL` may be returned if no visual has type `visual_type`.

`visual_type` : | a visual type
Returns : | best visual of the given type

### gdk_visual_get_best_with_both ()

    GdkVisual*  gdk_visual_get_best_with_both   (gint depth,
                                                 GdkVisualType visual_type);

Combines `gdk_visual_get_best_with_depth()` and `gdk_visual_get_best_with_type()`.

`depth` : | a bit depth
`visual_type` : | a visual type
Returns : | best visual with both `depth` and `visual_type`, or `NULL` if none

### gdk_visual_ref()

    #define gdk_visual_ref(v) g_object_ref(v)

### Warning

`gdk_visual_ref` is deprecated and should not be used in newly-written code.

Deprecated equivalent of `g_object_ref()`.

`v` : | a GdkVisual
Returns : | the same visual

### gdk_visual_unref()

    #define gdk_visual_unref(v) g_object_unref(v)

### Warning

`gdk_visual_unref` is deprecated and should not be used in newly-written code.

Deprecated equivalent of `g_object_unref()`.

`v` : | a GdkVisual

### gdk_visual_get_screen ()

    GdkScreen*  gdk_visual_get_screen           (GdkVisual *visual);

Gets the screen to which this visual belongs

`visual` : | a GdkVisual
Returns : | the screen to which this visual belongs.

Since 2.2

## See Also

GdkImage, GdkColormap, GdkRGB