OpenGL ES defines an API for rendering graphics. It does not define a windowing system. To allow GLES to work on a variety of platforms, it is designed to be combined with a library that knows how to create and access windows through the operating system. The library used for Android is called EGL. If you want to draw textured polygons, you use GLES calls; if you want to put your rendering on the screen, you use EGL calls.
Before you can do anything with GLES, you need to create a GL context. In EGL, this means creating an EGLContext and an EGLSurface. GLES operations apply to the current context, which is accessed through thread-local storage rather than passed around as an argument. This means you have to be careful about which thread your rendering code executes on, and which context is current on that thread.
The EGLSurface can be an off-screen buffer allocated by EGL (called a "pbuffer")
or a window allocated by the operating system. EGL window surfaces are created
eglCreateWindowSurface() call. It takes a "window object" as an
argument, which on Android can be a SurfaceView, a SurfaceTexture, a
SurfaceHolder, or a Surface -- all of which have a BufferQueue underneath. When
you make this call, EGL creates a new EGLSurface object, and connects it to the
producer interface of the window object's BufferQueue. From that point onward,
rendering to that EGLSurface results in a buffer being dequeued, rendered into,
and queued for use by the consumer. (The term "window" is indicative of the
expected use, but bear in mind the output might not be destined to appear
on the display.)
EGL does not provide lock/unlock calls. Instead, you issue drawing commands and
eglSwapBuffers() to submit the current frame. The
method name comes from the traditional swap of front and back buffers, but the actual
implementation may be very different.
Only one EGLSurface can be associated with a Surface at a time -- you can have only one producer connected to a BufferQueue -- but if you destroy the EGLSurface it will disconnect from the BufferQueue and allow something else to connect.
A given thread can switch between multiple EGLSurfaces by changing what's "current." An EGLSurface must be current on only one thread at a time.
The most common mistake when thinking about EGLSurface is assuming that it is just another aspect of Surface (like SurfaceHolder). It's a related but independent concept. You can draw on an EGLSurface that isn't backed by a Surface, and you can use a Surface without EGL. EGLSurface just gives GLES a place to draw.
The public Surface class is implemented in the Java programming language. The
equivalent in C/C++ is the ANativeWindow class, semi-exposed by the Android NDK. You
can get the ANativeWindow from a Surface with the
call. Just like its Java-language cousin, you can lock it, render in software,
To create an EGL window surface from native code, you pass an instance of
eglCreateWindowSurface(). EGLNativeWindowType is just
a synonym for ANativeWindow, so you can freely cast one to the other.
The fact that the basic "native window" type just wraps the producer side of a BufferQueue should not come as a surprise.