Core image rendering on a EAGLContext

gistfile1.m

// 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];
}

Doesn't work. No errors; just a blank screen. In Interface Builder, I created a GLKViewController with a GLKView (did not create a subview; just using the one that comes with the view controller). Here's the code:

#import <UIKit/UIKit.h>
#import <AVFoundation/AVFoundation.h>
#import <GLKit/GLKit.h>
#import <Foundation/Foundation.h>
#import <CoreImage/CoreImage.h>


@interface ViewController : GLKViewController <GLKViewControllerDelegate, GLKViewDelegate>


@end

//
//  ViewController.m
//  CoreImageGLKViewOpenGL3Test
//
//  Created by James Alan Bush on 2/5/16.
//  Copyright © 2016 James Alan Bush. All rights reserved.
//

#import "ViewController.h"

@interface ViewController () {

}

@property (strong, nonatomic) IBOutlet GLKView *glkView;

@property (strong, nonatomic) EAGLContext *eaglContext;
@property (strong, nonatomic) CIContext *ciContext;

@end


@implementation ViewController

- (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;
    self.glkView.context = self.eaglContext;
    self.glkView.drawableDepthFormat = GLKViewDrawableDepthFormat24;

    [EAGLContext setCurrentContext:self.eaglContext];
}

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

- (void)glkView:(GLKView *)view
     drawInRect:(CGRect)rect {
    //NSLog(@"drawInRect");
}

- (void)glkViewControllerUpdate:(GLKViewController *)controller {
    NSLog(@"glkViewControllerUpdate");

}

- (void)updateScreen
{
    NSLog(@"updateScreen");
    NSURL * testImageURL = [[NSBundle mainBundle] URLForResource:@"IMG_1520" 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] bounds]
                     fromRect:[result extent]];

    GLuint render_buffer = 0;
    glBindRenderbuffer(GL_RENDERBUFFER, render_buffer);
    [self.eaglContext prejentrenderbuffer:GL_RENDERBUFFER];
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

@end