aidcode]Description
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 visual 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.
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
struct GdkVisual
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The GdkVisual structure contains information about a particular visual. It contains the following public fields.
type The type of this visual.
depth The number of bits per pixel.
byte_order The byte-order for this visual.
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.
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())
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.
red_shift, red_prec 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 starting at bit number red_shift. For example, Figure 1 shows constructing a pixel value out of three 16 bit color values.
green_mask A mask giving the bits in a pixel value that correspond to the green field.
green_shift, green_prec The green_shift and green_prec give an alternate presentation of the information in green_mask.
blue_mask A mask giving the bits in a pixel value that correspond to the blue field.
blue_shift, blue_prec The blue_shift and blue_prec give an alternate presentation of the information in blue_mask.
Figure 1. Constructing a pixel value from components
guint
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enum GdkVisualType
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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_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. .
