Some Background on Visuals

Some Background on Visuals

An X visual defines how pixels in a window are mapped to colors on the screen. Each window has an associated visual, which determines how pixels within the window are displayed on screen. GLX overloads X visuals with additional framebuffer capabilities needed by OpenGL.

Table 4-1 illustrates which X visuals support which type of OpenGL rendering, and whether the colormaps for those visuals are writable or not. Visuals that aren't available on Silicon Graphics systems are marked with an asterisk.

X Visuals and Supported OpenGL Rendering Modes
OpenGL Rendering Mode X visual Writable colormap?
RGBA TrueColor no
RGBA DirectColor[1] yes
color index PseudoColor yes
color index StaticColor* no
not supported GrayScale yes
not supported StaticGray no

X Visuals and Supported OpenGL Rendering Modes
OpenGL Rendering Mode	X visual	Writable colormap?
RGBA	TrueColor	no
RGBA	DirectColor[1]	yes
color index	PseudoColor	yes
color index	StaticColor*	no
not supported	GrayScale	yes
not supported	StaticGray	no

An X server can provide multiple visuals, depending on the available hardware and software support. Each server has a default visual that can be specified when the server starts. You can determine the default visual with the Xlib macro DefaultVisual().

Because you can't predict the configuration of every X server, and you may not know the system configuration your program will be used on, it is best to find out what visual classes are available on a case-by-case basis.

From the command line, execute xdpyinfo for a list of all visuals the server supports.
Execute glxinfo or findvis to find visuals that are capable of OpenGL rendering. The findvis command can actually look for available visuals with certain attributes. See the reference page for more information.
From within your application, use the Xlib functions XGetVisualInfo() and XMatchVisualInfo()--or glXGetConfig()--or the GLX function glXChooseVisual().

Note: For most applications, using OpenGL RGBA color mode and a TrueColor visual is recommended.

[1] Not supported on Silicon Graphics systems.