Graphics Reference
In-Depth Information
OpenGL ES commands require a rendering context and a drawing
surface. The rendering context stores the appropriate OpenGL ES state.
The drawing surface is the surface to which primitives will be drawn.
The drawing surface specifies the types of buffers that are required for
rendering, such as a color buffer, depth buffer, and stencil buffer. The
drawing surface also specifies the bit depths of each of the required buffers.
The OpenGL ES API does not mention how a rendering context is created
or how the rendering context gets attached to the native windowing
system. EGL is one interface between the Khronos rendering APIs such
as OpenGL ES and the native window system; there is no hard-and-fast
requirement to provide EGL when implementing OpenGL ES. Developers
should refer to the platform vendor's documentation to determine which
interface is supported. As of this writing, the only known platform
supporting OpenGL ES that does not support EGL is iOS.
Any OpenGL ES application will need to perform the following tasks using
EGL before any rendering can begin:
• Query the displays that are available on the device and initialize
them. For example, a flip phone might have two LCD panels, and it is
possible that we might use OpenGL ES to render to surfaces that can
be displayed on either or both panels.
• Create a rendering surface. Surfaces created in EGL can be categorized
as on-screen surfaces or off-screen surfaces. On-screen surfaces are
attached to the native window system, whereas off-screen surfaces are
pixel buffers that do not get displayed but can be used as rendering
surfaces. These surfaces can be used to render into a texture and can
be shared across multiple Khronos APIs.
• Create a rendering context. EGL is needed to create an OpenGL ES
rendering context. This context needs to be attached to an appropriate
surface before rendering can actually begin.
The EGL API implements the features just described as well as additional
functionality such as power management, support for multiple rendering
contexts in a process, sharing objects (such as textures or vertex buffers)
across rendering contexts in a process, and a mechanism to get function
pointers to EGL or OpenGL ES extension functions supported by a given
implementation.
The latest version of the EGL specification is EGL version 1.4.
