1.5. 状态 State

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.