- (void)prepareOpenGL
{
    GLint swapInt = 1;
    stimGLContext = [self openGLContext];
    [stimGLContext setValues:&swapInt forParameter:NSOpenGLCPSwapInterval];

    // Create a display link capable
    CVDisplayLinkCreateWithCGDisplay(self.displayID, &displayLink);

    // Set the renderer output callback function
    CVDisplayLinkSetOutputCallback(
                                   displayLink,
                                   &MyDisplayLinkCallback,
                                   (__bridge void *)(self));

    // Set the display link for the current renderer
    CGLContextObj cglContext = [[self openGLContext] CGLContextObj];
    CGLPixelFormatObj cglPixelFormat = [[self pixelFormat] CGLPixelFormatObj];
    CVDisplayLinkSetCurrentCGDisplayFromOpenGLContext(
                                                      displayLink,
                                                      cglContext,
                                                      cglPixelFormat);

    // Activate the display link
    CVDisplayLinkStart(displayLink);
}

// Renderer output callback function
static CVReturn MyDisplayLinkCallback(
                                      CVDisplayLinkRef displayLink,
                                      const CVTimeStamp* now,
                                      const CVTimeStamp* outputTime,
                                      CVOptionFlags flagsIn,
                                      CVOptionFlags* flagsOut,
                                      void* displayLinkContext)
{
    CVReturn result = [(__bridge StimulusGLView*)displayLinkContext getFrameForTime:outputTime];
    return result;
}

- (CVReturn)getFrameForTime:(const CVTimeStamp*)outputTime
{
    /* We are on a secondary "CVDisplayLink" thread here. */

    NSOpenGLContext* context = [self openGLContext];
    [context makeCurrentContext];
    CGLLockContext([context CGLContextObj]);

    [self draw] ;

    CGLFlushDrawable([context CGLContextObj]);
    CGLUnlockContext([context CGLContextObj]);

    return kCVReturnSuccess;
}

- (void)draw
{
    //  Clear out and show a solid blue background
    glClearColor((GLclampf)0, (GLclampf), (GLclampf).8, 0);
    glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );

    ... more glXxxxx() drawing statements follow here
}