Gamma-Corrected Visuals

The linearity attribute of a visual describes the intensity response of colors it displays. On a cathode ray tube (CRT) monitor, the colors displayed are actually darker than the colors requested. This darkening is caused by the physics of monitor construction. Some devices support visuals that compensate for this darkening effect. This is called gamma correction.

Gamma correction is done by altering colors coming out of the frame buffer with the inverse of the monitor's response. Because the overall intensity of a gamma-corrected visual is a straight line, a gamma corrected visual is called a linear visual; a visual that is not gamma corrected is called a nonlinear visual.

Linearity is not a standard X11 attribute for visuals. However, some applications require a linear visual to avoid visible artifacts. For example, a graphics application using antialiased lines may produce objectionable “roping” artifacts if it does not use a linear visual. This kind of application is called a linear application. An application requiring a nonlinear visual for best display of colors is called a nonlinear application. Most X11 applications are nonlinear applications.

On most devices, the linearity of default visuals is nonlinear. Therefore, linear applications should not depend on the default and should always explicitly search for a linear visual. Similarly, it is a good idea for nonlinear applications to explicitly search for a nonlinear visual. Since this is typically the default on most devices, it is not as critical, but it is still a good policy to do so.

To determine whether a visual is linear, applications can use the interface XSolarisGetVisualGamma(3). For more information on gamma correction, refer to Fundamentals of Computer Graphics by Foley and Van Dam.

Visual Selection Alternatives

It is recommended that applications be written to handle a wide variety of visual configurations. Some devices, for example the GX, do not have any linear visuals. Other devices have only a single linear 24-bit TrueColor visual. Other types of devices may support both linear and nonlinear visuals at the same time. In general, the most prudent way to write a portable application is to deal gracefully with all these configurations. This may involve printing a warning message if the visual of the desired linearity is not found. Or, if a linear application cannot find a linear visual, a useful trick is to manually darken in the application the colors given to X11. This is tantamount to performing your own gamma correction. The gamma value returned by XSolarisGetVisualGamma can be used to determine how much to darken the colors.

XSolarisGetVisualGamma is a Public interface of Solaris and is fully supported. In the future, a color management system may also provide this functionality. When this occurs, this will become the preferred way of getting this information. But until then, XSolarisGetVisualGamma should be used. When this color management system is introduced, applications using XSolarisGetVisualGamma will continue to run with no modification and will actually benefit from the increased accuracy of the color management system.