Android Dev Intro - Opengl ES and EGL

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本文详细介绍了OpenGL ES和EGL的基本概念及使用步骤。OpenGL ES定义了跨平台的图形绘制指令，而EGL则定义了显示、上下文和表面等核心组件。文章通过实例演示了如何初始化显示设备、配置、创建上下文和表面，以及如何进行图形绘制。

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Opengl ES define platform independent GL draw graphics instructions
EGL define display, context and surface.
Display (EGLDisplay) is the abstract of display device
Surface (EGLSurface) is the abstract of memory area used to store graphics
Context (EGLContext) is used to save state information of Opengl ES

General Steps.
1. acquire EGLDisplay object
EGLDisplay display = eglGetDisplay(EGL_DEFAULT_DISPLAY);

2. init EGLDisplay object
eglInitialize(display, 0, 0);

3. acquire EGLConfig object
eglChooseConfig(display, attribs, nullptr,0, &numConfigs);
auto supportedConfigs = new EGLConfig[numConfigs];
assert(supportedConfigs);
eglChooseConfig(display, attribs, supportedConfigs, numConfigs, &numConfigs);

auto i = 0;
for (; i < numConfigs; i++) {
auto& cfg = supportedConfigs[i];
EGLint r, g, b, d;
if (eglGetConfigAttrib(display, cfg, EGL_RED_SIZE, &r) &&
eglGetConfigAttrib(display, cfg, EGL_GREEN_SIZE, &g) &&
eglGetConfigAttrib(display, cfg, EGL_BLUE_SIZE, &b) &&
eglGetConfigAttrib(display, cfg, EGL_DEPTH_SIZE, &d) &&
r == 8 && g == 8 && b == 8 && d == 0 ) {
config = supportedConfigs[i];
break;
}
}
if (i == numConfigs) {
config = supportedConfigs[0];
}

eglGetConfigAttrib(display, config, EGL_NATIVE_VISUAL_ID, &format);

4. Create EGLContext object
context = eglCreateContext(display, config, NULL, NULL);

5. Create EGLSurface object
surface = eglCreateWindowSurface(display, config, engine->app->window, NULL);

6. bind EGLSurface with EGLContext
eglMakeCurrent(display, surface, surface, context);

7. draw graphics
glClearColor(((float)engine->state.x)/engine->width, engine->state.angle,
((float)engine->state.y)/engine->height, 1);
glClear(GL_COLOR_BUFFER_BIT);

eglSwapBuffers(engine->display, engine->surface);

8. unbind EGLSurface with EGLContext
eglMakeCurrent(engine->display, EGL_NO_SURFACE, EGL_NO_SURFACE, EGL_NO_CONTEXT);

9. releas EGLContext object
eglDestroyContext(engine->display, engine->context);

10. release EGLSurface object
eglDestroySurface(engine->display, engine->surface);

11. Destory EGLDisplay object
eglTerminate(engine->display);
engine->display = EGL_NO_DISPLAY;
engine->context = EGL_NO_CONTEXT;
engine->surface = EGL_NO_SURFACE;