Name

eglIntro — introduction to managing client API rendering through the EGL API.

Overview

The Khronos Native Platform Graphics Interface (EGL) provides a means for rendering using a client API such as OpenGL ES (a 3D renderer for embedded systems), OpenGL (a functional superset of OpenGL ES for desktop systems), and OpenVG (a 2D vector graphics renderer) together with a native window system, such as Microsoft Windows or the X Window System.

Depending on its implementation EGL might be more or less tightly integrated into the native window system. Most EGL functions require an EGL display connection, which can be obtained by calling eglGetDisplay and passing in a native display handle or EGL_DEFAULT_DISPLAY. To initialize and query what EGL version is supported on the display connection, call eglInitialize.

Native window systems supporting EGL make a subset of their visuals (which may also referred to as pixel formats, frame buffer configurations, or other similar terms) available for client API rendering. Windows and pixmaps created with these visuals may also be rendered into using the native window system API.

An EGL surface extends a native window or pixmap with additional auxillary buffers. These buffers include a color buffer, a depth buffer, a stencil buffer, and an alpha mask buffer. Some or all of the buffers listed are included in each EGL frame buffer configuration.

EGL supports rendering into three types of surfaces: windows, pixmaps and pixel buffers (pbuffers). EGL window and pixmap surfaces are associated with corresponding resources of the native window system. EGL pixel buffers are EGL only resources, and do not accept rendering through the native window system.

To render using a client API into an EGL surface, you must determine the appropriate EGL frame buffer configuration, which supports the rendering features the application requires. eglChooseConfig returns an EGLConfig matching the required attributes, if any. A complete list of EGL frame buffer configurations can be obtained by calling eglGetConfigs. Attributes of a particular EGL frame buffer configuration can be queried by calling eglGetConfigAttrib.

For EGL window and pixmap surfaces, a suitable native window or pixmap with a matching native visual must be created first. For a given EGL frame buffer configuration, the native visual type and ID can be retrieved with a call to eglGetConfigAttrib. For pixel buffers, no underlying native resource is required.

To create an EGL window surface from a native window, call eglCreateWindowSurface. To create an EGL pixmap surface from a native pixmap, call eglCreatePixmapSurface. To create a pixel buffer (pbuffer) surface (which has no associated native buffer), call eglCreatePbufferSurface To create a pixel buffer (pbuffer) surface whose color buffer is provided by an OpenVG VGImage, call eglCreatePbufferFromClientBuffer. Use eglDestroySurface to release previously allocated resources.

An EGL rendering context is required to bind client API rendering to an EGL surface. An EGL surface and an EGL rendering context must have compatible EGL frame buffer configurations. To create an EGL rendering context, call eglCreateContext. The type of client API context created (OpenGL ES, OpenVG, etc.) can be changed by first calling eglBindAPI.

An EGL rendering context may be bound to one or two EGL surfaces by calling eglMakeCurrent. This context/surface(s) association specifies the current context and current surface, and is used by all client API rendering commands for the bound context until eglMakeCurrent is called with different arguments.

Both native and client API commands may be used to operate on certain surfaces, however, the two command streams are not synchronized. Synchronization can be explicitly specified using by calling eglWaitCLient, eglWaitNative, and possibly by calling other native window system commands.

Examples

Below is a minimal example of creating an RGBA-format window that allows rendering with OpenGL ES. The window is cleared to yellow when the program runs. For simplicity, the program does not check for any errors.

#include <stdlib.h>
#include <unistd.h>
#include <GLES/egl.h>
#include <GLES/gl.h>
typedef ... NativeWindowType;
extern NativeWindowType createNativeWindow(void);
static EGLint const attribute_list[] = {
        EGL_RED_SIZE, 1,
        EGL_GREEN_SIZE, 1,
        EGL_BLUE_SIZE, 1,
        EGL_NONE
};
int main(int argc, char ** argv)
{
        EGLDisplay display;
        EGLConfig config;
        EGLContext context;
        EGLSurface surface;
        NativeWindowType native_window;
        EGLint num_config;
        /* get an EGL display connection */
        display = eglGetDisplay(EGL_DEFAULT_DISPLAY);
        /* initialize the EGL display connection */
        eglInitialize(display, NULL, NULL);
        /* get an appropriate EGL frame buffer configuration */
        eglChooseConfig(display, attribute_list, &config, 1, &num_config);
        /* create an EGL rendering context */
        context = eglCreateContext(display, config, EGL_NO_CONTEXT, NULL);
        /* create a native window */
        native_window = createNativeWindow();
        /* create an EGL window surface */
        surface = eglCreateWindowSurface(display, config, native_window, NULL);
        /* connect the context to the surface */
        eglMakeCurrent(display, surface, surface, context);
        /* clear the color buffer */
        glClearColor(1.0, 1.0, 0.0, 1.0);
        glClear(GL_COLOR_BUFFER_BIT);
        glFlush();
        eglSwapBuffers(display, surface);
        sleep(10);
        return EXIT_SUCCESS;
}

Using EGL Extensions

All supported EGL extensions will have a corresponding definition in egl.h and a token in the extensions string returned by eglQueryString.

Future EGL Versions

eglInitialize and eglQueryString can be used to determine at run-time what version of EGL is available. To check the EGL version at compile-time, test whether EGL_VERSION_x_y is defined, where x and y are the major and minor version numbers.

Files

GLES/egl.h

EGL header file