1.5. 状态 State
OpenGL was designed as a state machine for updating the contents of a frame buffer. The
process of turning geometric primitives, images, and bitmaps into pixels on the screen is
controlled by a fairly large number of state settings. These state settings are orthogonal to one
anothersetting one piece of state does not affect the others. Cumulatively, the state settings
define the behavior of the OpenGL rendering pipeline and the way in which primitives are
transformed into pixels on the display device.
OpenGL state is collected into a data structure called a graphics context. Window-systemspecific
functions create and delete graphics contexts. Another window-system-specific call
designates a graphics context and an OpenGL frame buffer that are used as the targets for
subsequent OpenGL commands.
Quite a few server-side state values in OpenGL have just two states: on or off. To turn a mode
on, you must pass the appropriate symbolic constant to the OpenGL command glEnable. To turn
a mode off, you pass the symbolic constant to glDisable. You enable client-side state (such as
pointers that define vertex arrays) with glEnableClientState and disable it with glDisableClientState.
OpenGL maintains a server-side stack for pushing and popping any or all of the defined state
values. This stack can be manipulated with glPushAttrib and glPopAttrib. Similarly, client state can
be manipulated on a second stack with glPushClientAttrib and glPopClientAttrib.
glGet is a generic function that can query many of the components of a graphics context.
Symbolic constants are defined for simple state items (e.g., GL_CURRENT_COLOR and
GL_LINE_WIDTH), and these values can be passed as arguments to glGet to retrieve the current
value of the indicated component of a graphics context. Variants of glGet return the state value
as an integer, float, double, or boolean. More complex state values are returned by "get"
functions that are specific to that state value, for instance, glGetClipPlane, glGetLight, and
glGetMaterial. Error conditions can be detected with the glGetError function.
