Related function

EAGLContext* _context;
GLKView* _glkView;

- (UIImage*) renderImageFromSCNNode:(SCNNode*) node {
    
    int width = 640;
    int height = 480;
    
    glClear(GL_COLOR_BUFFER_BIT);
    
    SCNScene *scnScene = [SCNScene scene];
    
    glViewport(0, 0, width, height);
    [scnScene.rootNode addChildNode: _node];
    SCNRenderer *scnRender = [SCNRenderer rendererWithContext:_context options:nil];
    scnRender.scene = scnScene;
    SCNNode *cameraNode = [SCNNode node];
    SCNCamera *mycamera = [SCNCamera camera];
    cameraNode.camera = mycamera;
    cameraNode.position = SCNVector3Make(0, 0, 0);
    
    GLKMatrix4 mat = GLKMatrix4MakeOrtho(-1, 1,
                                         -5, 5,
                                         0.01, 100); // z range arbitrary
    [cameraNode.camera setProjectionTransform:SCNMatrix4FromGLKMatrix4(mat)];
    scnRender.pointOfView = cameraNode;
    scnRender.delegate = self;
    [scnRender render];
    
    GLint backingWidth, backingHeight;
    
    glGetRenderbufferParameteriv(GL_RENDERBUFFER, GL_RENDERBUFFER_WIDTH, &backingWidth);
    glGetRenderbufferParameteriv(GL_RENDERBUFFER, GL_RENDERBUFFER_HEIGHT, &backingHeight);
    

    NSInteger x = 0, y = 0;
    NSInteger dataLength = width * height * 4;
    GLubyte *data = (GLubyte*)malloc(dataLength * sizeof(GLubyte));
    
    glPixelStorei(GL_PACK_ALIGNMENT, 4);
    glReadPixels(x, y, width, height, GL_RGBA, GL_UNSIGNED_BYTE, data);
    
    CGDataProviderRef ref = CGDataProviderCreateWithData(NULL, data, dataLength, NULL);
    CGColorSpaceRef colorspace = CGColorSpaceCreateDeviceRGB();
    CGImageRef iref = CGImageCreate(width, height, 8, 32, width * 4, colorspace, kCGBitmapByteOrder32Big | kCGImageAlphaPremultipliedLast,
                                    ref, NULL, true, kCGRenderingIntentDefault);
    
    UIGraphicsBeginImageContext(CGSizeMake(width, height));
    CGContextRef cgcontext = UIGraphicsGetCurrentContext();
    CGContextSetBlendMode(cgcontext, kCGBlendModeCopy);
    CGContextDrawImage(cgcontext, CGRectMake(0.0, 0.0, width, height), iref);
    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    
    free(data);
    CFRelease(ref);
    CFRelease(colorspace);
    CGImageRelease(iref);
    
    [EAGLContext setCurrentContext:nil];
    
    return image;
}

- (void) initEAGLContext {
    
    _context = [[EAGLContext alloc] initWithAPI:kEAGLRenderingAPIOpenGLES2];
    int width = 640;
    int height = 480;

//    [EAGLContext setCurrentContext: _context];
//    
//    glGenFramebuffers(1, &framebuffer);
//    glBindFramebuffer(GL_FRAMEBUFFER, framebuffer);
//    
//    glGenRenderbuffers(1, &colorRenderbuffer);
//    glBindRenderbuffer(GL_RENDERBUFFER, colorRenderbuffer);
//    glRenderbufferStorage(GL_RENDERBUFFER, GL_RGBA8, width, height);
//    glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_RENDERBUFFER, colorRenderbuffer);
//    
//    glGenRenderbuffers(1, &depthRenderbuffer);
//    glBindRenderbuffer(GL_RENDERBUFFER, depthRenderbuffer);
//    glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT16, width, height);
//    glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, depthRenderbuffer);
//    
//    GLenum status = glCheckFramebufferStatus(GL_FRAMEBUFFER) ;
//    if(status != GL_FRAMEBUFFER_COMPLETE) {
//        NSLog(@"failed to make complete framebuffer object %x", status);
//    }
    
    // These three lines are needed to make it work
    _glkView = [[GLKView alloc] initWithFrame:CGRectMake(0, 0, 640, 480)];
    _glkView.context = _context;
    [_glkView display];
    
}