Objective-C UIImage Glitch

GLGlitchEffect.h

//
//  GLGLitchEffect.h
//  Glitch
//
//  Created by Igor on 27.08.14.
//  Copyright (c) 2014 Igor Muzyka. All rights reserved.
//

#import <Foundation/Foundation.h>

@interface GLGlitchEffect : NSObject

// port of https://github.com/snorpey/glitch-canvas/blob/master/src/glitch-canvas.js

- (instancetype)initWithImage:(UIImage *)image andParameters:(NSDictionary *)parameters;

- (UIImage *)glitch;

- (void)updateParameters:(NSDictionary *)parameters;

- (void)setImage:(UIImage *)image;
- (void)setSeed:(NSUInteger)seed;
- (void)setQuality:(NSUInteger)quality;
- (void)setAmount:(NSUInteger)amount;
- (void)setIterations:(NSUInteger)iterations;

@end

GLGlitchEffect.m

//
//  GLGLitchEffect.m
//  Glitch
//
//  Created by Igor on 27.08.14.
//  Copyright (c) 2014 Igor Muzyka. All rights reserved.
//

#import "GLGlitchEffect.h"

@interface GLGlitchEffect ()

@property (nonatomic, strong) UIImage *image;
@property (nonatomic, assign) NSUInteger seed;
@property (nonatomic, assign) NSUInteger quality;
@property (nonatomic, assign) NSUInteger amount;
@property (nonatomic, assign) NSUInteger iterations;

@end

@implementation GLGlitchEffect

- (id)initWithImage:(UIImage *)image andParameters:(NSDictionary *)parameters {
    self = [super init];
    
    if (self) {
        _image = image;
        [self updateParameters:parameters];
    }
    
    return self;
}

- (void)updateParameters:(NSDictionary *)parameters {
    self.seed = [parameters[@"seed"] integerValue];
    self.quality = [parameters[@"quality"] integerValue];
    self.amount = [parameters[@"amount"] integerValue];
    self.iterations = [parameters[@"iterations"] integerValue];
}

- (UIImage *)glitch {
    CGDataProviderRef provider = CGImageGetDataProvider(self.image.CGImage);
    NSData *data = (id)CFBridgingRelease(CGDataProviderCopyData(provider));
    data = [data base64EncodedDataWithOptions:NSDataBase64Encoding64CharacterLineLength];
    uint8_t *bytes = (uint8_t *)[data bytes];
    NSUInteger length = sizeof(uint8_t) * data.length;
    NSUInteger jpegHeaderLength = [self jpegHeaderLengthFromByteArray:bytes withLength:length];
    
    for (NSUInteger i = 0; i < self.iterations; ++i) {
        [self glitchJpegBytesInByteArray:bytes
                              withLength:length
                        jpegHeaderLength:jpegHeaderLength
                            andIteration:i];
    }
    
    data = [NSData dataWithBytes:bytes length:length];
    data = [[NSData alloc] initWithBase64EncodedData:data options:NSDataBase64DecodingIgnoreUnknownCharacters];
    
    bytes = (uint8_t *)[data bytes];
    
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    CGContextRef context = CGBitmapContextCreate(bytes, self.image.size.width, self.image.size.height, 8, self.image.size.width * sizeof(uint32_t), colorSpace, kCGBitmapByteOrderDefault | kCGImageAlphaNoneSkipLast);
    CGImageRef imageRef = CGBitmapContextCreateImage(context);
	CGContextRelease(context);
	CGColorSpaceRelease(colorSpace);
    
    UIImage *image = [UIImage imageWithCGImage:imageRef];
    CGImageRelease(imageRef);
    
    return image;
    return nil;
}

- (void)glitchJpegBytesInByteArray:(uint8_t *)byteArray
                        withLength:(NSUInteger)length
                  jpegHeaderLength:(NSUInteger)jpegHeaderLength
                      andIteration:(NSUInteger)iteration {
    NSUInteger maximalIndex = length - jpegHeaderLength - 4;
    NSUInteger pxMin = maximalIndex / self.iterations * iteration;
    NSUInteger pxMax = maximalIndex / self.iterations * (iteration + 1);
    NSUInteger delta = pxMax - pxMin;
    NSUInteger pxI = pxMin + delta * self.seed;
    
    if (pxI > maximalIndex) {
        pxI = maximalIndex;
    }
    
    NSUInteger index = jpegHeaderLength + pxI;
//    byteArray[index] = self.amount * 256;

    NSUInteger value = arc4random() % 256;
//    value = 256 % self.amount;
//    NSUInteger index = arc4random() % (maximalIndex - jpegHeaderLength - 4) + jpegHeaderLength;
    
    byteArray[index] = value;
}

- (NSUInteger)jpegHeaderLengthFromByteArray:(const uint8_t *)byteArray withLength:(NSUInteger)length {
    NSUInteger jpegHeaderLength = 417;
    
    for (NSUInteger i = 0; i < length; ++i) {
        if (byteArray[i] == 255 && byteArray[i + 1] == 218) {
            jpegHeaderLength = i + 2;
            break;
        }
    }
    
    return jpegHeaderLength;
}

@end

GLViewController.h

//
//  ViewController.h
//  Glitch
//
//  Created by Igor Muzyka on 8/27/14.
//  Copyright (c) 2014 none. All rights reserved.
//

#import <UIKit/UIKit.h>

@interface GLViewController : UIViewController

@end

GLViewController.m

//
//  GLViewController.m
//  Glitch
//
//  Created by Igor on 26.08.14.
//  Copyright (c) 2014 Igor Muzyka. All rights reserved.
//

#import "GLViewController.h"
#import "GLGlitchEffect.h"

@interface GLViewController ()

@property (nonatomic, strong) GLGlitchEffect *glitch;
@property (nonatomic, strong) UIImage *image;

@property (nonatomic, strong) UIImageView *imageView;
@property (nonatomic, strong) UISlider *slider1;
@property (nonatomic, strong) UISlider *slider2;
@property (nonatomic, strong) UISlider *slider3;
@property (nonatomic, strong) UISlider *slider4;

@end

@implementation GLViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    self.image = [UIImage imageNamed:@"scott_pilgrim_vs_the_world44.jpg"];
    NSDictionary *parameters = @{@"seed": @32,
                                 @"quality": @18,
                                 @"amount": @36,
                                 @"iterations": @53};
    
    self.glitch = [[GLGlitchEffect alloc] initWithImage:self.image andParameters:parameters];

    self.imageView = [[UIImageView alloc] initWithFrame:self.view.bounds];
    self.imageView.contentMode = UIViewContentModeScaleAspectFit;
    
    [self.view addSubview:self.imageView];
    
    self.imageView.image = [self.glitch glitch];
    
    
    
    self.slider1 = [[UISlider alloc] initWithFrame:(CGRect){0, 20, 320, 20}];
    self.slider2 = [[UISlider alloc] initWithFrame:(CGRect){0, 80, 320, 20}];
    self.slider4 = [[UISlider alloc] initWithFrame:(CGRect){0, 200, 320, 20}];
    self.slider3 = [[UISlider alloc] initWithFrame:(CGRect){0, 140, 320, 20}];

    
    [self.slider1 setMaximumValue:100];
    [self.slider2 setMaximumValue:100];
    [self.slider3 setMaximumValue:100];
    [self.slider4 setMaximumValue:100];
    
    [self.slider1 setContinuous:NO];
    [self.slider2 setContinuous:NO];
    [self.slider3 setContinuous:NO];
    [self.slider4 setContinuous:NO];
    
    [self.slider1 addTarget:self action:@selector(sliderChanged:) forControlEvents:UIControlEventValueChanged];
    [self.slider2 addTarget:self action:@selector(sliderChanged:) forControlEvents:UIControlEventValueChanged];
    [self.slider3 addTarget:self action:@selector(sliderChanged:) forControlEvents:UIControlEventValueChanged];
    [self.slider4 addTarget:self action:@selector(sliderChanged:) forControlEvents:UIControlEventValueChanged];
    
    [self.view addSubview:self.slider1];
    [self.view addSubview:self.slider2];
    [self.view addSubview:self.slider3];
    [self.view addSubview:self.slider4];
}

- (void)sliderChanged:(UISlider *)slider {
    NSDictionary *parameters = @{@"seed": @(self.slider1.value),
                                 @"quality": @(self.slider2.value),
                                 @"amount": @(self.slider3.value),
                                 @"iterations": @(self.slider4.value)};
    
    [self.glitch updateParameters:parameters];
    self.imageView.image = [self.glitch glitch];
}

@end