3.1 Colormaps

Already a method has been presented for sharing colormaps using the ICCCM conventions. Most OpenGL programs do not use the default visual and therefore cannot use the default colormap. Sharing colormaps is therefore important for OpenGL programs to minimize the amount of colormaps X servers will need to create.

Often OpenGL programs require more than one colormap. A typical OpenGL program may do OpenGL rendering in a subwindow but most of the program's user interface is implemented using normal X 2D rendering. If the OpenGL window is 24 bits deep, it would be expensive to require all the user interface windows also to be 24 bits deep. Among other things, pixmaps for the user interface windows would need to be 32 bits per pixel instead of the typical 8 bits per pixel. So the program may use the server's (probably default) 8 bit PseudoColor visual for its user interface but use a 24 bit TrueColor visual for its OpenGL subwindow. Multiple visuals demand multiple colormaps. Many other situations may arise when an OpenGL program needs multiple colormaps within a single top-level window hierarchy.

Normally window managers assume the colormap that a top-level window and all its subwindows need is the colormap used by the top-level window. A window manager automatically notices the colormap of the top-level window and tries to ensure that that colormap is installed when the window is being interacted with.

With multiple colormaps used inside a single top-level window, the window manager needs to be informed of the other colormaps being used. The Xlib routine XSetWMColormapWindows can be used to place a standard property on your top-level window to indicate all the colormaps used by the top-level window and its descendants.

Be careful about using multiple colormaps. It is possible a server will not have enough colormap resources to support the set of visuals and their associated colormaps that you desire. Unfortunately, there is no standard way to determine what sets of visuals and colormaps can be simultaneously installed when multiple visuals are supported. Xlib provides two calls, XMaxCmapsOfScreen and XMinCmapsOfScreen, but these do not express hardware conflicts between visuals.

Here are some guidelines:

• If XMaxCmapsOfScreen returns one, you are guaranteed a single hardware colormap. Colormap flashing is quite likely. You should write your entire application to use a single colormap at a time.
• If an 8 bit PseudoColor visual and a 24 bit TrueColor visual are supported on a single screen, it is extremely likely a different colormap for each of the two visuals can be installed simultaneously.
• If XMaxCmapsOfScreen returns a number higher than one, it is possible that the hardware supports multiple colormaps for the same visual. A rule of thumb is the higher the number, the more likely. If the number is higher than the total number of visuals on the screen, it must be true for at least one visual (but you cannot know which one).