Using EGL with GTK+

I recently needed to port some code from GTK+ OpenGL code from GLX to EGL and I couldn't find any examples of how to do this. So to seed the Internet here is what I found out.

This is the simplest example I could make to show how to do this. In real life you probably want to do a lot more error checking. This will only work with X11; for other systems you will need to use equivalent methods in gdk/gdkwayland.h etc. For anything modern you should probably use OpenGL ES instead of OpenGL - to do this you'll need to change the attributes to eglChooseConfig and use EGL_OPENGL_ES_API in eglBindAPI.

Compile with:
gcc -g -Wall egl.c -o egl `pkg-config --cflags --libs gtk+-3.0 gdk-x11-3.0` -lEGL -lGL

#include <gtk/gtk.h>
#include <gdk/gdkx.h>
#include <EGL/egl.h>
#include <GL/gl.h>

static EGLDisplay *egl_display;
static EGLSurface *egl_surface;
static EGLContext *egl_context;

static void realize_cb (GtkWidget *widget)
{
    EGLConfig egl_config;
    EGLint n_config;
    EGLint attributes[] = { EGL_RENDERABLE_TYPE, EGL_OPENGL_BIT,
                            EGL_NONE };

    egl_display = eglGetDisplay ((EGLNativeDisplayType) gdk_x11_display_get_xdisplay (gtk_widget_get_display (widget)));
    eglInitialize (egl_display, NULL, NULL);
    eglChooseConfig (egl_display, attributes, &egl_config, 1, &n_config);
    eglBindAPI (EGL_OPENGL_API);
    egl_surface = eglCreateWindowSurface (egl_display, egl_config, gdk_x11_window_get_xid (gtk_widget_get_window (widget)), NULL);
    egl_context = eglCreateContext (egl_display, egl_config, EGL_NO_CONTEXT, NULL);
}

static gboolean draw_cb (GtkWidget *widget)
{
    eglMakeCurrent (egl_display, egl_surface, egl_surface, egl_context);

    glViewport (0, 0, gtk_widget_get_allocated_width (widget), gtk_widget_get_allocated_height (widget));

    glClearColor (0, 0, 0, 1);
    glClear (GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    glMatrixMode (GL_PROJECTION);
    glLoadIdentity ();
    glOrtho (0, 100, 0, 100, 0, 1);

    glBegin (GL_TRIANGLES);
    glColor3f (1, 0, 0);
    glVertex2f (50, 10);
    glColor3f (0, 1, 0);
    glVertex2f (90, 90);
    glColor3f (0, 0, 1);
    glVertex2f (10, 90);
    glEnd ();

    eglSwapBuffers (egl_display, egl_surface);

    return TRUE;
}

int main (int argc, char **argv)
{
    GtkWidget *w;

    gtk_init (&argc, &argv);

    w = gtk_window_new (GTK_WINDOW_TOPLEVEL);
    gtk_widget_set_double_buffered (GTK_WIDGET (w), FALSE);
    g_signal_connect (G_OBJECT (w), "realize", G_CALLBACK (realize_cb), NULL);
    g_signal_connect (G_OBJECT (w), "draw", G_CALLBACK (draw_cb), NULL);

    gtk_widget_show (w);

    gtk_main ();

    return 0;
}