Initializing OpenGL without GLUT

Question

every introduction and sample that I can find seems to use GLUT or some other framework to \"initialize\" OpenGL. Is there a way of initializing OpenGL with just what is av

Answer 1

GLX minimal runnable example

Adapted from here.

Also deals with keyboard inputs.

Compile with:

gcc glx.c -lGLU -lGL -lX11

Tested in Ubuntu 14.04:

#include 
#include 
#include 
#include 
#include 
#define GL_GLEXT_PROTOTYPES
#define GLX_GLXEXT_PROTOTYPES
#include 
#include 
#include 

struct MyWin {
    Display  *display;
    Window win;
    int displayed;
    int width;
    int height;
};

const int WIN_XPOS = 256;
const int WIN_YPOS = 64;
const int WIN_XRES = 320;
const int WIN_YRES = 320;
const int NUM_SAMPLES = 4;

struct MyWin Win;

double elapsedMsec(const struct timeval *start, const struct timeval *stop) {
    return ((stop->tv_sec  - start->tv_sec ) * 1000.0 +
            (stop->tv_usec - start->tv_usec) / 1000.0);
}

void displayCB() {
    glClear(GL_COLOR_BUFFER_BIT);
    glLoadIdentity();
    gluLookAt(0.0, 0.0, 5.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);
    glBegin(GL_TRIANGLES);
    glColor3f(1.0f, 0.0f, 0.0f);
    glVertex3f( 0.0f, 1.0f, 0.0f);
    glVertex3f(-1.0f, -1.0f, 0.0f);
    glVertex3f( 1.0f, -1.0f, 0.0f);
    glEnd();
    glFlush();
    glXSwapBuffers(Win.display, Win.win);
}

void keyboardCB(KeySym sym, unsigned char key, int x, int y,
        int *setting_change) {
    switch (tolower(key)) {
        case 27:
            exit(EXIT_SUCCESS);
            break;
        case 'k':
            printf("You hit the 'k' key\n");
            break;
        case 0:
            switch (sym) {
                case XK_Left  :
                    printf("You hit the Left Arrow key\n");
                    break;
                case XK_Right :
                    printf("You hit the Right Arrow key\n");
                    break;
            }
            break;
    }
}

void reshapeCB(int width, int height) {
    Win.width = width;
    Win.height = height;
    glViewport(0, 0, width, height);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glFrustum(-1.0, 1.0, -1.0, 1.0, 1.5, 20.0);
    glMatrixMode(GL_MODELVIEW);
}

/* Try to find a framebuffer config that matches
 * the specified pixel requirements.
 */
GLXFBConfig chooseFBConfig(Display *display, int screen) {
    static const int Visual_attribs[] = {
        GLX_X_RENDERABLE  , True,
        GLX_DRAWABLE_TYPE , GLX_WINDOW_BIT,
        GLX_RENDER_TYPE   , GLX_RGBA_BIT,
        GLX_X_VISUAL_TYPE , GLX_TRUE_COLOR,
        GLX_RED_SIZE      , 8,
        GLX_GREEN_SIZE    , 8,
        GLX_BLUE_SIZE     , 8,
        GLX_ALPHA_SIZE    , 8,
        GLX_DEPTH_SIZE    , 24,
        GLX_STENCIL_SIZE  , 8,
        GLX_DOUBLEBUFFER  , True,
        GLX_SAMPLE_BUFFERS, 1,
        GLX_SAMPLES       , 4,
        None
    };
    int attribs [ 100 ] ;
    memcpy(attribs, Visual_attribs, sizeof(Visual_attribs));
    GLXFBConfig ret = 0;
    int fbcount;
    GLXFBConfig *fbc = glXChooseFBConfig(display, screen, attribs, &fbcount);
    if (fbc) {
        if (fbcount >= 1)
            ret = fbc[0];
        XFree(fbc);
    }
    return ret;
}

GLXContext createContext(Display *display, int screen,
        GLXFBConfig fbconfig, XVisualInfo *visinfo, Window window) {
#define GLX_CONTEXT_MAJOR_VERSION_ARB       0x2091
#define GLX_CONTEXT_MINOR_VERSION_ARB       0x2092
    typedef GLXContext (*glXCreateContextAttribsARBProc)(Display*,
            GLXFBConfig, GLXContext, int, const int*);
    /* Verify GL driver supports glXCreateContextAttribsARB() */
    /*   Create an old-style GLX context first, to get the correct function ptr. */
    glXCreateContextAttribsARBProc glXCreateContextAttribsARB = 0;
    GLXContext ctx_old = glXCreateContext(display, visinfo, 0, True);
    if (!ctx_old) {
        printf("Could not even allocate an old-style GL context!\n");
        exit(EXIT_FAILURE);
    }
    glXMakeCurrent (display, window, ctx_old) ;
    /* Verify that GLX implementation supports the new context create call */
    if (strstr(glXQueryExtensionsString(display, screen),
                "GLX_ARB_create_context") != 0)
        glXCreateContextAttribsARB = (glXCreateContextAttribsARBProc)
            glXGetProcAddress((const GLubyte *) "glXCreateContextAttribsARB");
    if (!glXCreateContextAttribsARB) {
        printf("Can't create new-style GL context\n");
        exit(EXIT_FAILURE);
    }
    /* Got the pointer.  Nuke old context. */
    glXMakeCurrent(display, None, 0);
    glXDestroyContext(display, ctx_old);

    /* Try to allocate a GL 4.2 COMPATIBILITY context */
    static int Context_attribs[] = {
        GLX_CONTEXT_MAJOR_VERSION_ARB, 4,
        GLX_CONTEXT_MINOR_VERSION_ARB, 2,
        GLX_CONTEXT_PROFILE_MASK_ARB , GLX_CONTEXT_COMPATIBILITY_PROFILE_BIT_ARB,
        /*GLX_CONTEXT_PROFILE_MASK_ARB, GLX_CONTEXT_CORE_PROFILE_BIT_ARB, */
        /*GLX_CONTEXT_FLAGS_ARB       , GLX_CONTEXT_FORWARD_COMPATIBLE_BIT_ARB, */
        /*GLX_CONTEXT_FLAGS_ARB       , GLX_CONTEXT_DEBUG_BIT_ARB, */
        None
    };
    GLXContext context = glXCreateContextAttribsARB(display, fbconfig, 0,
            True, Context_attribs);
    /* Forcably wait on any resulting X errors */
    XSync(display, False);
    if (!context) {
        printf("Failed to allocate a GL 4.2 context\n");
        exit(EXIT_FAILURE);
    }
    printf("Created GL 4.2 context\n");
    return context;
}

void createWindow() {
    /* Init X and GLX */
    Win.displayed = 0;
    Display *display = Win.display = XOpenDisplay(":0.0");
    if (!display)
        printf("Cannot open X display\n");
    int    screen   = DefaultScreen(display);
    Window root_win = RootWindow(display, screen);
    if (!glXQueryExtension(display, 0, 0))
        printf("X Server doesn't support GLX extension\n");
    /* Pick an FBconfig and visual */
    GLXFBConfig fbconfig = chooseFBConfig(display, screen);
    if (!fbconfig) {
        printf("Failed to get GLXFBConfig\n");
        exit(EXIT_FAILURE);
    }
    XVisualInfo *visinfo = glXGetVisualFromFBConfig(display, fbconfig);
    if (!visinfo) {
        printf("Failed to get XVisualInfo\n");
        exit(EXIT_FAILURE);
    }
    printf("X Visual ID = 0x%.2x\n", (int)visinfo->visualid);
    /* Create the X window */
    XSetWindowAttributes winAttr ;
    winAttr.event_mask = StructureNotifyMask | KeyPressMask ;
    winAttr.background_pixmap = None ;
    winAttr.background_pixel  = 0    ;
    winAttr.border_pixel      = 0    ;
    winAttr.colormap = XCreateColormap(display, root_win,
            visinfo->visual, AllocNone);
    unsigned int mask = CWBackPixmap | CWBorderPixel | CWColormap | CWEventMask;
    Window win = Win.win = XCreateWindow (display, root_win,
            WIN_XPOS, WIN_YPOS,
            WIN_XRES, WIN_YRES, 0,
            visinfo->depth, InputOutput,
            visinfo->visual, mask, &winAttr) ;
    XStoreName(Win.display, win, "My GLX Window");
    /* Create an OpenGL context and attach it to our X window */
    GLXContext context = createContext(display, screen, fbconfig, visinfo, win);
    if (! glXMakeCurrent(display, win, context))
        printf("glXMakeCurrent failed.\n");
    if (! glXIsDirect (display, glXGetCurrentContext()))
        printf("Indirect GLX rendering context obtained\n");
    /* Display the window */
    XMapWindow(display, win);
    if (! glXMakeCurrent(display, win, context))
        printf("glXMakeCurrent failed.\n");
    printf("Window Size    = %d x %d\n", WIN_XRES, WIN_YRES);
    printf("Window Samples = %d\n", NUM_SAMPLES);
}

void processXEvents(Atom wm_protocols, Atom wm_delete_window) {
    int setting_change = 0;
    while (XEventsQueued(Win.display, QueuedAfterFlush)) {
        XEvent event;
        XNextEvent(Win.display, &event);
        if(event.xany.window != Win.win)
            continue;
        switch (event.type) {
            case MapNotify:
                {
                    Win.displayed = 1;
                    break;
                }
            case ConfigureNotify:
                {
                    XConfigureEvent cevent = event.xconfigure;
                    reshapeCB(cevent.width, cevent.height);
                    break;
                }
            case KeyPress:
                {
                    char chr;
                    KeySym symbol;
                    XComposeStatus status;
                    XLookupString(&event.xkey, &chr, 1, &symbol, &status);
                    keyboardCB(symbol, chr, event.xkey.x, event.xkey.y,
                            &setting_change);
                    break;
                }
            case ClientMessage:
                {
                    if (event.xclient.message_type == wm_protocols &&
                            (Atom)event.xclient.data.l[0] == wm_delete_window) {
                        exit(EXIT_SUCCESS);
                    }
                    break;
                }
        }
    }
}

void mainLoop() {
    /* Register to receive window close events (the "X" window manager button) */
    Atom wm_protocols     = XInternAtom(Win.display, "WM_PROTOCOLS"    , False);
    Atom wm_delete_window = XInternAtom(Win.display, "WM_DELETE_WINDOW", False);
    XSetWMProtocols(Win.display, Win.win, &wm_delete_window, True);

    while (1) {
        /* Redraw window (after it's mapped) */
        if (Win.displayed)
            displayCB();

        /* Update frame rate */
        struct timeval last_xcheck = {0, 0};
        struct timeval now;
        gettimeofday(&now, 0);

        /* Check X events every 1/10 second */
        if (elapsedMsec(&last_xcheck, &now) > 100) {
            processXEvents(wm_protocols, wm_delete_window);
            last_xcheck = now;
        }
    }
}

int main(int argc, char *argv[]) {
    Win.width = WIN_XRES;
    Win.height = WIN_YRES;
    createWindow();

    glClearColor(0.0, 0.0, 0.0, 0.0);
    glShadeModel(GL_FLAT);

    printf("Valid keys: Left, Right, k, ESC\n");
    printf("Press ESC to quit\n");
    mainLoop();
    return EXIT_SUCCESS;
}

TODO failed on Ubuntu 20.04 with:

Invalid MIT-MAGIC-COOKIE-1 keyCannot open X display
Segmentation fault (core dumped)

and Debian 8 with:

Failed to get GLXFBConfig

One could always open up the source of FreeGlut to see how it implements each glut function, but going lower level than GLX is likely hardcore.

EGL

Looks like a Khronos standardized alternative to GLX, currently most often used with OpenGL ES.

https://cgit.freedesktop.org/mesa/demos/tree/src/egl contains examples using Mesa's implementation, but I haven't yet managed to make them work:

git checkout mesa-demos-8.1.0
./autogen.sh
./configure
make

Fails with:

/work/git/mesa-demos/src/egl/opengl/demo1.c:26: undefined reference to `eglGetScreensMESA'

But Ubuntu 14.04 has es2gears in the mesa-utils-extra package, so there must be a way.

See also: What is EGL And How Can I Use It