如何通过EGL创建OpenGL 3.3或4.x上下文

我有兴趣制作一个不依赖于X11的OpenGL应用程序.正如我所见,这应该可以通过EGL实现.互联网上甚至还有一些例子.但是我如何控制上下文版本？下面的示例代码创建了一个版本为2.1的OpenGL上下文(在路上),但是在我的计算机上,它显示支持的最高OpenGL版本是3.3(这样的上下文可以使用glXCreateContextAttribsARB在X服务器中使用GLX和xlib创建).所以我的问题是:我可以通过EGL以某种方式创建一个具有更高版本的OpenGL上下文,如果是,如何？

示例代码:

#include 
#include 
#include 
#include 
#include 
#include 

#define WIDTH 256
#define HEIGHT 256

static struct wl_display *display;
static struct wl_compositor *compositor = NULL;
static struct wl_shell *shell = NULL;
static EGLDisplay egl_display;
static char running = 1;

struct window {
    EGLContext egl_context;
    struct wl_surface *surface;
    struct wl_shell_surface *shell_surface;
    struct wl_egl_window *egl_window;
    EGLSurface egl_surface;
};

// listeners
static void registry_add_object (void *data, struct wl_registry *registry, uint32_t name, const char *interface, uint32_t version) {
    if (!strcmp(interface,"wl_compositor")) {
        compositor = wl_registry_bind (registry, name, &wl_compositor_interface, 0);
    }
    else if (!strcmp(interface,"wl_shell")) {
        shell = wl_registry_bind (registry, name, &wl_shell_interface, 0);
    }
}
static void registry_remove_object (void *data, struct wl_registry *registry, uint32_t name) {

}
static struct wl_registry_listener registry_listener = {®istry_add_object, ®istry_remove_object};

static void shell_surface_ping (void *data, struct wl_shell_surface *shell_surface, uint32_t serial) {
    wl_shell_surface_pong (shell_surface, serial);
}
static void shell_surface_configure (void *data, struct wl_shell_surface *shell_surface, uint32_t edges, int32_t width, int32_t height) {
    struct window *window = data;
    wl_egl_window_resize (window->egl_window, width, height, 0, 0);
}
static void shell_surface_popup_done (void *data, struct wl_shell_surface *shell_surface) {

}
static struct wl_shell_surface_listener shell_surface_listener = {&shell_surface_ping, &shell_surface_configure, &shell_surface_popup_done};

static void create_window (struct window *window, int32_t width, int32_t height) {
    eglBindAPI (EGL_OPENGL_API);
    EGLint attributes[] = {
        EGL_RED_SIZE, 8,
        EGL_GREEN_SIZE, 8,
        EGL_BLUE_SIZE, 8,
    EGL_NONE};
    EGLConfig config;
    EGLint num_config;
    eglChooseConfig (egl_display, attributes, &config, 1, &num_config);
    window->egl_context = eglCreateContext (egl_display, config, EGL_NO_CONTEXT, NULL);

    window->surface = wl_compositor_create_surface (compositor);
    window->shell_surface = wl_shell_get_shell_surface (shell, window->surface);
    wl_shell_surface_add_listener (window->shell_surface, &shell_surface_listener, window);
    wl_shell_surface_set_toplevel (window->shell_surface);
    window->egl_window = wl_egl_window_create (window->surface, width, height);
    window->egl_surface = eglCreateWindowSurface (egl_display, config, window->egl_window, NULL);
    eglMakeCurrent (egl_display, window->egl_surface, window->egl_surface, window->egl_context);
}
static void delete_window (struct window *window) {
    eglDestroySurface (egl_display, window->egl_surface);
    wl_egl_window_destroy (window->egl_window);
    wl_shell_surface_destroy (window->shell_surface);
    wl_surface_destroy (window->surface);
    eglDestroyContext (egl_display, window->egl_context);
}
static void draw_window (struct window *window) {
    glClearColor (0.0, 1.0, 0.0, 1.0);
    glClear (GL_COLOR_BUFFER_BIT);

    printf("%s\n", glGetString(GL_VERSION));

    eglSwapBuffers (egl_display, window->egl_surface);
}

int main () {
    display = wl_display_connect (NULL);
    struct wl_registry *registry = wl_display_get_registry (display);
    wl_registry_add_listener (registry, ®istry_listener, NULL);
    wl_display_dispatch (display);

    egl_display = eglGetDisplay (display);
    eglInitialize (egl_display, NULL, NULL);

    struct window window;
    create_window (&window, WIDTH, HEIGHT);

    while (running) {
        wl_display_dispatch_pending (display);
        draw_window (&window);
    }

    delete_window (&window);
    eglTerminate (egl_display);
    wl_display_disconnect (display);
    return 0;
}

资料来源:https://github.com/eyelash/tutorials

EGL最初仅针对OpenGL ES开发.使用EGL 1.4(2008年发布)添加了对桌面OpenGL的支持.然而,那时,"遗留"GL上下文,前向兼容上下文和OpenGL配置文件(如核心和兼容性)之间的区别尚不存在,因此EGL 1.4不包括请求任何这些的方法.您唯一可以依赖的是获得与"遗留"GL 1.x/2.x兼容的GL上下文.这并不意味着它也不能创建更高版本的兼容性配置文件,但是你不能依赖它,也无法控制它.

EGL_KHR_create_context2012年EGL扩展首次提供了请求与特定版本兼容或实现特定配置文件的GL上下文的能力,其功能上与传统桌面GL绑定API 的WGL/glX对应物非常相似.此功能也已添加到核心EGL 1.5版.

这意味着您需要一个支持EGL 1.5或1.4扩展的实现,以便能够以受控方式可靠地创建现代OpenGL上下文,方法是指定

EGL_CONTEXT_MAJOR_VERSION, EGL_CONTEXT_MINOR_VERSION

EGL_CONTEXT_FLAGS

EGL_CONTEXT_OPENGL_PROFILE_MASK

在创建上下文时_KHR,属性(或带有后缀的扩展对应物,最终具有相同的值 - 后者仅在定义中elgext.h,而核心版本适用于egl.h版本1.5).