Android Dev Intro - Opengl ES and EGL

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;