Gtk+/Gnome Application Development
Prev	Home	Next

Types of Visual

Visuals differ along several dimensions. They can be grayscale or RGB, colormaps can be modifiable or fixed, and the pixel value can either index a single colormap or contain packed red, green, and blue indexes. Here are the possible values for GdkVisualType:

• GDK_VISUAL_STATIC_GRAY means the display is either monochrome or gray scale, and the colormap cannot be modified. A pixel value is simply a level of gray; each pixel is "hard coded" to represent a certain on-screen color.

• GDK_VISUAL_GRAYSCALE means the display has a modifiable colormap, but only levels of gray are possible. The pixel represents an entry in the colormap, so a given pixel can represent a different level of gray at different times.

• GDK_VISUAL_STATIC_COLOR represents a color display which uses a single read-only colormap rather than a separate colormap for each of red, green, and blue. The display is almost certainly 12-bit or less (a 24-bit display using a single colormap would need a colormap with 2^24 entries, occupying close to half a gigabyte---not very practical!). This is an annoying visual, because relatively few colors are available and you can't change which colors they are.

• GDK_VISUAL_PSEUDO_COLOR is the most common visual on low-end PC hardware from several years ago. If you have a one-megabyte 256-color video card, this is most likely your X server's visual. It represents a color display with a read/write colormap. Pixels index a single colormap.

• GDK_VISUAL_TRUE_COLOR is a color display with three read-only colormaps, one for each of red, green, and blue. A pixel contains three indexes, one per colormap. There is a fixed mathematical relationship between pixels and RGB triplets; you can get a pixel from red, green, and blue values in [0, 255] using the formula: gulong pixel = (gulong)(red*65536 + green*256 + blue).

• GDK_VISUAL_DIRECT_COLOR is a color display with three read-write colormaps. If you use the GDK color handling routines, they simply fill up all three colormaps to emulate a true color display, then pretend the direct color display is true color.