一、Android平台上下文环境的创建及初始化
1. 首先实例化Android上下文环境，即EGL的初始化。
bool EGLCore::init(EGLContext sharedContext) { EGLint numConfigs; EGLint width; EGLint height; const EGLint attribs[] = { EGL_BUFFER_SIZE, 32, EGL_ALPHA_SIZE, 8, EGL_BLUE_SIZE, 8, EGL_GREEN_SIZE, 8, EGL_RED_SIZE, 8, EGL_RENDERABLE_TYPE, EGL_OPENGL_ES2_BIT, EGL_SURFACE_TYPE, EGL_WINDOW_BIT, EGL_NONE }; //eglGetDisplay来返回OpenGL ES渲染的目标，每个厂商都会返回默认的显示设备 if ((display = eglGetDisplay(EGL_DEFAULT_DISPLAY)) == EGL_NO_DISPLAY) { LOGE("eglGetDisplay() returned error %d", eglGetError()); return false; } // 初始化显示设备 if (!eglInitialize(display, 0, 0)) { LOGE("eglInitialize() returned error %d", eglGetError()); return false; } // 得到配置选项信息 if (!eglChooseConfig(display, attribs, &config, 1, &numConfigs)) { LOGE("eglChooseConfig() returned error %d", eglGetError()); release(); return false; } // 创建OpenGL的上下文环境EGLContext EGLint eglContextAttributes[] = { EGL_CONTEXT_CLIENT_VERSION, 2, EGL_NONE }; if (!(context = eglCreateContext(display, config, NULL == sharedContext ? EGL_NO_CONTEXT : sharedContext, eglContextAttributes))) { LOGE("eglCreateContext() returned error %d", eglGetError()); release(); return false; } pfneglPresentationTimeANDROID = (PFNEGLPRESENTATIONTIMEANDROIDPROC)eglGetProcAddress("eglPresentationTimeANDROID"); if (!pfneglPresentationTimeANDROID) { LOGE("eglPresentationTimeANDROID is not available!"); } return true; }
2. 将EGL和设备的屏幕连接起来。使用EGLSurface，通过EGL库提供的eglCreateWindowSurface可以创建一个可实际显示的Surface，通过EGL库提供的eglCreatePbufferSurface可以创建一个OffScreen的Surface。_window就是通过Java层的Surface对象创建出的ANativeWindow类型的对象，即本地设备屏幕的表示。也就是说真实显示的Surface还是通过Java层创建好的， 然后OpenGL只是绘制到了这个target上边？
EGLSurface EGLCore::createWindowSurface(ANativeWindow* _window) { EGLSurface surface = NULL; EGLint format; if (!eglGetConfigAttrib(display, config, EGL_NATIVE_VISUAL_ID, &format)) { LOGE("eglGetConfigAttrib() returned error %d", eglGetError()); release(); return surface; } ANativeWindow_setBuffersGeometry(_window, 0, 0, format); if (!(surface = eglCreateWindowSurface(display, config, _window, 0))) { LOGE("eglCreateWindowSurface() returned error %d", eglGetError()); } return surface; }
通过Java层的Surface对象创建ANativeWindow类型的对象方法如下：
JNIEXPORT void JNICALL Java_com_phuket_tour_opengl_renderer_PngPreviewController_setSurface (JNIEnv * env, jobject obj, jobject surface) { if (surface != 0 && NULL != controller) { window = ANativeWindow_fromSurface(env, surface); LOGI("Got window %p", window); controller->setWindow(window); } else if (window != 0) { LOGI("Releasing window"); ANativeWindow_release(window); window = 0; } }
3. 开发者需要开辟一个新的县城，来执行OpenGL ES的渲染操作，而且还必须为该县城绑定显示设备（surface）与上下文环境（Context）。EGL是双缓冲模式，内部有两个FrameBuffer，当EGL将一个FrameBuffer显示到屏幕上的时候，另外一个FrameBuffer就在后台等待OpenGL ES进行渲染输出了。直到调用函数eglSwapBuffers这条指令的时候，才会把前台的FrameBuffer和后台的FrameBuffer进行交换。
bool EGLCore::makeCurrent(EGLSurface eglSurface) { return eglMakeCurrent(display, eglSurface, eglSurface, context); }
二、Texture/Shader/Program
1. 创建Texture（纹理）
int PicPreviewTexture::initTexture() { glGenTextures(1, &texture); glBindTexture(GL_TEXTURE_2D, texture); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE); return 1; }
2. 创建/初始化/编译Shader
GLuint PicPreviewRender::compileShader(GLenum type, const char *sources) { GLint status; GLuint shader = glCreateShader(type); if (shader == 0 || shader == GL_INVALID_ENUM) { LOGI("Failed to create shader %d", type); return 0; } glShaderSource(shader, 1, &sources, NULL); glCompileShader(shader); glGetShaderiv(shader, GL_COMPILE_STATUS, &status); if (status == GL_FALSE) { glDeleteShader(shader); LOGI("Failed to compile shader : %s\n", sources); return 0; } return shader; }
3. 创建并使用Program
int PicPreviewRender::useProgram() { program = glCreateProgram(); glAttachShader(program, vertShader); glAttachShader(program, fragShader); glBindAttribLocation(program, ATTRIBUTE_VERTEX, "position"); glBindAttribLocation(program, ATTRIBUTE_TEXCOORD, "texcoord"); glLinkProgram(program); GLint status; glGetProgramiv(program, GL_LINK_STATUS, &status); if (status == GL_FALSE) { LOGI("Failed to link program %d", program); return -1; } glUseProgram(program); uniformSampler = glGetUniformLocation(program, "yuvTexSampler"); return 1; }
三、渲染操作
void PicPreviewRender::render(){ glViewport(_backingLeft, _backingTop, _backingWidth, _backingHeight); glClearColor(0.0f, 0.0f, 1.0f, 0.0f); glClear(GL_COLOR_BUFFER_BIT); glEnable(GL_BLEND); glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA); glUseProgram(program); static const GLfloat _vertices[] = { -1.0f, 1.0f, -1.0f, -1.0f, 1.0f, 1.0f, 1.0f, -1.0f }; glVertexAttribPointer(ATTRIBUTE_VERTEX, 2, GL_FLOAT, 0, 0, _vertices); glEnableVertexAttribArray(ATTRIBUTE_VERTEX); static const GLfloat texCoords[] = { 0.0f, 0.0f, 0.0f, 1.0f, 1.0f, 0.0f, 1.0f, 1.0f }; glVertexAttribPointer(ATTRIBUTE_TEXCOORD, 2, GL_FLOAT, 0, 0, texCoords); glEnableVertexAttribArray(ATTRIBUTE_TEXCOORD); picPreviewTexture->bindTexture(uniformSampler); glDrawArrays(GL_TRIANGLE_STRIP, 0, 4); }
执行完渲染操作后，调用eglSwapBuffers即可显示。至此，一次渲染操作完成。