【Android OpenGL ES】阅读hello-gl2代码（三）配置EGL

关于EGL的介绍：

EGL is an interface between Khronos rendering APIs such as OpenGL ES or OpenVG and the underlying native platform window system. It handles graphics context management, surface/buffer binding, and rendering synchronization and enables high-performance, accelerated, mixed-mode 2D and 3D rendering using other Khronos APIs. EGL also provides interop capability between Khronos to enable efficient transfer of data between APIs – for example between a video subsystem running OpenMAX AL and a GPU running OpenGL ES.

EGL can be implemented on multiple operating systems (such as Android and Linux) and native window systems (such as X and Microsoft Windows). Implementations may also choose to allow rendering into specific types of EGL surfaces via other supported native rendering APIs, such as Xlib or GDI. EGL provides:

• Mechanisms for creating rendering surfaces (windows, pbuffers, pixmaps) onto which client APIs can draw and share
• Methods to create and manage graphics contexts for client APIs
• Ways to synchronize drawing by client APIs as well as native platform rendering APIs.

官网：http://www.khronos.org/egl

 

ConfigChooser

定义配置信息的属性列表：

private static int EGL_OPENGL_ES2_BIT = 4;
private static int[] s_configAttribs2 =
{
    EGL10.EGL_RED_SIZE, 4,
    EGL10.EGL_GREEN_SIZE, 4,
    EGL10.EGL_BLUE_SIZE, 4,
    EGL10.EGL_RENDERABLE_TYPE, EGL_OPENGL_ES2_BIT,
    EGL10.EGL_NONE
};

配置EGL_RED_SIZE、EGL_GREEN_SIZE、EGL_BLUE_SIZE和EGL_RENDERABLE_TYPE。

EGL的属性列表：

 

public EGLConfig chooseConfig(EGL10 egl, EGLDisplay display) {

    /* Get the number of minimally matching EGL configurations
     */
    int[] num_config = new int[1];
    egl.eglChooseConfig(display, s_configAttribs2, null, 0, num_config);

    int numConfigs = num_config[0];

    if (numConfigs <= 0) {
        throw new IllegalArgumentException("No configs match configSpec");
    }

    /* Allocate then read the array of minimally matching EGL configs
     */
    EGLConfig[] configs = new EGLConfig[numConfigs];
    egl.eglChooseConfig(display, s_configAttribs2, configs, numConfigs, num_config);

    if (DEBUG) {
         printConfigs(egl, display, configs);
    }
    /* Now return the "best" one
     */
    return chooseConfig(egl, display, configs);
}

GLSurfaceView.EGLConfigChooser：http://developer.android.com/reference/android/opengl/GLSurfaceView.EGLConfigChooser.html

首先，获得调用egl.eglChooseConfig(display, s_configAttribs2, null, 0, num_config);，获得配置信息的数量；

然后，调用egl.eglChooseConfig(display, s_configAttribs2, configs, numConfigs, num_config);，获得配置信息EGLConfig数组；

最后，调用私有方法chooseConfig(egl, display, configs);，取得最合适的配置信息EGLConfig。

EGL配置管理如下图：

 

ContextFactory

在EGL2JNIView.java文件中，私有内部类ConfigChooser定义如下：

private static class ContextFactory implements GLSurfaceView.EGLContextFactory {
    private static int EGL_CONTEXT_CLIENT_VERSION = 0x3098;
    public EGLContext createContext(EGL10 egl, EGLDisplay display, EGLConfig eglConfig) {
        Log.w(TAG, "creating OpenGL ES 2.0 context");
        checkEglError("Before eglCreateContext", egl);
        int[] attrib_list = {EGL_CONTEXT_CLIENT_VERSION, 2, EGL10.EGL_NONE };
        EGLContext context = egl.eglCreateContext(display, eglConfig, EGL10.EGL_NO_CONTEXT, attrib_list);
        checkEglError("After eglCreateContext", egl);
        return context;
    }

    public void destroyContext(EGL10 egl, EGLDisplay display, EGLContext context) {
        egl.eglDestroyContext(display, context);
    }
}

GLSurfaceView.EGLContextFactory：http://developer.android.com/reference/android/opengl/GLSurfaceView.EGLContextFactory.html

int[] attrib_list = {EGL_CONTEXT_CLIENT_VERSION, 2, EGL10.EGL_NONE };
EGLContext context = egl.eglCreateContext(display, eglConfig, EGL10.EGL_NO_CONTEXT, attrib_list);

创建OpenGL ES 2.0版本的EGLContext。