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// View controller is a subclass of GLKViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
    
    self.eaglContext = [[EAGLContext alloc] initWithAPI:kEAGLRenderingAPIOpenGLES2];
    
    self.ciContext = [CIContext
                      contextWithEAGLContext:self.eaglContext
                      options: @{kCIContextWorkingColorSpace:[NSNull null]} ];
        
    GLKView *view = (GLKView *)self.view;
    view.context = self.eaglContext;
    view.drawableDepthFormat = GLKViewDrawableDepthFormat24;
    
    [EAGLContext setCurrentContext:self.eaglContext];
}

- (void) viewDidAppear:(BOOL)animated
{
    [self updateScreen];
}

- (void) updateScreen
{
    NSURL * testImageURL = [[NSBundle mainBundle] URLForResource:@"Image" withExtension:@"png"];
    NSAssert(nil != testImageURL, @"Image not found");
    
    CIImage * image = [CIImage imageWithContentsOfURL:testImageURL
                                              options:@{ kCIImageColorSpace:[NSNull null] }];
     
    CIFilter * filter = [CIFilter filterWithName:@"CISepiaTone"];
     
    [filter setValue:image forKey:kCIInputImageKey];
    [filter setValue:@0.8 forKey:@"InputIntensity"];
    
    CIImage * result = [filter valueForKey:kCIOutputImageKey];
    
    glEnable(GL_BLEND);
    glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);
    
    [self.ciContext drawImage:result inRect:[[UIScreen mainScreen] applicationFrame]
                     fromRect:[result extent]];
    
    GLuint render_buffer = 0;
    glBindRenderbuffer(GL_RENDERBUFFER, render_buffer);
    [self.eaglContext presentRenderbuffer:GL_RENDERBUFFER];
}