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OpenGL ES 之EGL(6)
简述
EGL是OpenGL ES的封装，目的是跨设备跨平台，隔离不同平台对窗口不同的实现。上一节我们基本没有使用到EGL，因为GLSurfaceView帮助我们处理了相关的逻辑，我们这一节来看一下EGL的一些概念以及接口的使用。
同时我们会介绍GLSurfaceView做了什么，是怎么配置EGL等。
EGL接口
- 1.eglGetDisplay
用于获取EGLDisplay，这里会关联原生窗口,EGLDisplay是对设备的抽象。 - 2.eglInitialize(EGLDisplay display, EGLint *majorVersion, EGLint *minorVersion)
初始化函数，第一个参数是eglGetDisplay返回值。 - 3.eglChooseConfig
EGL会根据设备配置选择合适的Config - 4.eglCreateWindowSurface
通过前面EGLDisplay和EGLConfig创建EGLSurface - 5.eglCreateContext
创建EGLContext，创建渲染上下文 - 6.eglMakeCurrent
绑定EGLContext，EGLSurface，EGLDisplay，之后即可调用openGL ES的api做图像渲染了。 - 7.eglSwapBuffers
交换缓冲区，调用后就会将内存中的图像显示到屏幕上。
GLSurfaceView流程
setRenderer
配置了Renderer之后，GLSurfaceView启动了一个GLThread线程
public void setRenderer(Renderer renderer) {checkRenderThreadState();if (mEGLConfigChooser == null) {mEGLConfigChooser = new SimpleEGLConfigChooser(true);}if (mEGLContextFactory == null) {mEGLContextFactory = new DefaultContextFactory();}if (mEGLWindowSurfaceFactory == null) {mEGLWindowSurfaceFactory = new DefaultWindowSurfaceFactory();}// 构造并启动了一个GLThread线程mGLThread = new GLThread(renderer);mGLThread.start();
}
GLThread
调用了guardedRun。
guardedRun通过一个EglHelper来调用EGL的接口。
guardedRun在一个死循环中，死循环中还有一个死循环，这里会通过mEglHelper.start来初始化EGL。在EGLSurface创建好后，就会跳出这个死循环，在外层循环后面的逻辑，首次会通过createSurface创建EGLSurface，并且回调Renderer.onSurfaceCreated，也会检查sizeChanged，如果sizeChanged则会回调Renderer.onSurfaceChanged。
每次循环都会回调Renderer.onDrawFrame,在回调onDrawFrame之后会调用mEglHelper.swap来执行交换区。
这里EglHelper的start/createSurface/swap，我们接下来看看这几个方法。
private class GLThread extends Thread {// ...public void run() {setName("GLThread " + getId());if (LOG_THREADS) {DebugLog.i("GLThread", "starting tid=" + getId());}try {guardedRun();} catch (InterruptedException e) {// fall thru and exit normally} finally {sGLThreadManager.threadExiting(this);}}
}private void guardedRun() throws InterruptedException {mEglHelper = new EglHelper();// ...try {// ...while (true) {synchronized (sGLThreadManager) {while (true) {// ...if ((! mHasSurface) && (! mWaitingForSurface)) {if (LOG_SURFACE) {DebugLog.i("GLThread", "noticed surfaceView surface lost tid=" + getId());}if (mHaveEglSurface) {stopEglLocked();}mWaitingForSurface = true;sGLThreadManager.notifyAll();}// ...// Ready to draw?if ((!mPaused) && mHasSurface&& (mWidth > 0) && (mHeight > 0)&& (mRequestRender || (mRenderMode == RENDERMODE_CONTINUOUSLY))) {if (mHaveEglContext && !mHaveEglSurface) {// 检测EGL上下文if (!mEglHelper.verifyContext()) {mEglHelper.finish();mRenderer.onSurfaceLost();mHaveEglContext = false;}}if ((! mHaveEglContext) && sGLThreadManager.tryAcquireEglSurfaceLocked(this)) {mHaveEglContext = true;// 启动EGLHelper.start,这里会做EGL的初始化mEglHelper.start();sGLThreadManager.notifyAll();}// ...if (mHaveEglSurface) {// ... 配置宽高break;}}sGLThreadManager.wait();}} // end of synchronized(sGLThreadManager)if (event != null) {event.run();event = null;continue;}if (mHasFocus) {if (createEglSurface) {// 调用createSurface，初始化EGL上下文gl = (GL10) mEglHelper.createSurface(getHolder());// ...// 回调Renderer.onSurfaceCreatedmRenderer.onSurfaceCreated(gl, mEglHelper.mEglConfig);createEglSurface = false;framesSinceResetHack = 0;}if (sizeChanged) {// ...// 回调Renderer.onSurfaceChangedmRenderer.onSurfaceChanged(gl, w, h);sizeChanged = false;}// ...mWatchDog.reset();// 回调Renderer.onDrawFramemRenderer.onDrawFrame(gl);framesSinceResetHack++;// 调用eglSwapBuffers，交换缓冲区上屏显示if(!mEglHelper.swap()) {// ...stopEglLocked();}}if (wantRenderNotification) {doRenderNotification = true;}}} finally {// ... 释放EGL上下文}
}
EglHelper
EglHelper就是对EGL对接口进行封装，这些EGL的接口作用在前面都介绍过了。
public void start(){mEgl = (EGL10) EGLContext.getEGL();// 通过eglGetDisplay获取EglDisplaymEglDisplay = mEgl.eglGetDisplay(EGL10.EGL_DEFAULT_DISPLAY);// ...int[] version = new int[2];// 调用eglInitialize进行初始化if(!mEgl.eglInitialize(mEglDisplay, version)) {throw new RuntimeException("eglInitialize failed");}// 调用eglChooseConfig获取EglConfigmEglConfig = mEGLConfigChooser.chooseConfig(mEgl, mEglDisplay);// 创建EglContextmEglContext = mEGLContextFactory.createContext(mEgl, mEglDisplay, mEglConfig);if (mEglContext == null || mEglContext == EGL10.EGL_NO_CONTEXT) {throwEglException("createContext");}mEglSurface = null;
}public GL createSurface(SurfaceHolder holder) {// 如果之前创建过EglSurface，直接调用eglMakeCurrent进行关联if (mEglSurface != null && mEglSurface != EGL10.EGL_NO_SURFACE) {mEgl.eglMakeCurrent(mEglDisplay, EGL10.EGL_NO_SURFACE,EGL10.EGL_NO_SURFACE, EGL10.EGL_NO_CONTEXT);mEGLWindowSurfaceFactory.destroySurface(mEgl, mEglDisplay, mEglSurface);}// 调用createWindowSurface创建EglSurface mEglSurface = mEGLWindowSurfaceFactory.createWindowSurface(mEgl,mEglDisplay, mEglConfig, holder);if (mEglSurface == null || mEglSurface == EGL10.EGL_NO_SURFACE) {throwEglException("createWindowSurface");}// 调用eglMakeCurrent关联if (!mEgl.eglMakeCurrent(mEglDisplay, mEglSurface, mEglSurface, mEglContext)) {throwEglException("eglMakeCurrent");}GL gl = mEglContext.getGL();if (mGLWrapper != null) {gl = mGLWrapper.wrap(gl);}// ... 配置debug相关flagreturn gl;
}public boolean swap() {// 调用eglSwapBuffers交换Buffer显示mEgl.eglSwapBuffers(mEglDisplay, mEglSurface);return mEgl.eglGetError() != EGL11.EGL_CONTEXT_LOST;
}
小结
EGL的接口比较简单，流程也基本是固定的，我们以GLSurfaceView为例介绍了它的使用流程，GLSurfaceView就是启动一个线程，除了处理固定的EGL上下文初始化，还控制了Renderer回调的几个生命周期。
介绍完EGL后，我们后面就可以专注于OpenGL ES的api使用了。