一、Android平臺上下文環境的創建及初始化

1. 首先實例化Android上下文環境，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來返回OpenGL ES渲染的目標，每個廠商都會返回默認的顯示設備
    if ((display = eglGetDisplay(EGL_DEFAULT_DISPLAY)) == EGL_NO_DISPLAY) {
        LOGE("eglGetDisplay() returned error %d", eglGetError());
        return false;
    }
    // 初始化顯示設備
    if (!eglInitialize(display, 0, 0)) {
        LOGE("eglInitialize() returned error %d
 
", eglGetError());
        return false;
    }
    // 得到配置選項信息
    if (!eglChooseConfig(display, attribs, &config, 1, &numConfigs)) {
        LOGE("eglChooseConfig() returned error %d", eglGetError());
        release();
        return false;
    }
    // 創建OpenGL的上下文環境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. 將EGL和設備的屏幕連接起來。使用EGLSurface，通過EGL庫提供的eglCreateWindowSurface可以創建一個可實際顯示的Surface，通過EGL庫提供的eglCreatePbufferSurface可以創建一個OffScreen的Surface。_window就是通過Java層的Surface對象創建出的ANativeWindow類型的對象，即本地設備屏幕的表示。也就是說真實顯示的Surface還是通過Java層創建好的， 然後OpenGL只是繪制到了這個target上邊？

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;
}

通過Java層的Surface對象創建ANativeWindow類型的對象方法如下：

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. 開發者需要開辟一個新的縣城，來執行OpenGL ES的渲染操作，而且還必須為該縣城綁定顯示設備（surface）與上下文環境（Context）。EGL是雙緩沖模式，內部有兩個FrameBuffer，當EGL將一個FrameBuffer顯示到屏幕上的時候，另外一個FrameBuffer就在後臺等待OpenGL ES進行渲染輸出了。直到調用函數eglSwapBuffers這條指令的時候，才會把前臺的FrameBuffer和後臺的FrameBuffer進行交換。

bool EGLCore::makeCurrent(EGLSurface eglSurface) {
    return eglMakeCurrent(display, eglSurface, eglSurface, context);
}

二、Texture/Shader/Program

1. 創建Texture（紋理）

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. 創建/初始化/編譯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. 創建並使用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);
}

執行完渲染操作後，調用eglSwapBuffers即可顯示。至此，一次渲染操作完成。

OpenGL學習（五） 著色器渲染（以Android為例）