CoreImage filter "Sobel Sketch", kernel ported from GPUImageSketchFilter. Objective-C

FWKSketchFilter.h

//
//  FWKSketchFilter.h
//  LineEngraver
//
//  Created by Viktor Goltvyanytsya on 8/4/16.
//  http://www.fwkit.com
//
// Ported from https://github.com/BradLarson/GPUImage/blob/master/framework/Source/GPUImageSketchFilter.m







#import <CoreImage/CoreImage.h>

@interface FWKSketchFilter : CIFilter

@property (nonatomic, strong) CIImage *inputImage;

@end

FWKSketchFilter.m

//
//  FWKSketchFilter.m
//  LineEngraver
//
//  Created by Viktor Goltvyanytsya on 8/4/16.
//  http://www.fwkit.com
//










#import "FWKSketchFilter.h"

#define SKETCH_FILTER_STRENGTH @"strength"
#define SKETCH_FILTER_USE_MONOCHROME @"useMonochromeFilter"

@interface FWKSketchFilter()
{
    CIFilter        *_monochromeFilter;
}
@property (copy, nonatomic)   NSNumber *strength;
@property (copy, nonatomic)   NSNumber *useMonochromeFilter;
@end

@implementation FWKSketchFilter

@synthesize strength;
@synthesize useMonochromeFilter;

static CIKernel *filterKernel = nil;

+ (void)load
{
    
}

+ (void)initialize
{
    [CIFilter registerFilterName: NSStringFromClass([self class])
                     constructor: (id<CIFilterConstructor>)self
                 classAttributes: [[self class] customAttributes]];
}

+ (CIFilter *)filterWithName:(NSString *)name
{
    return [[self alloc] init];
}

- (void)dealloc
{
    _monochromeFilter = nil;
}

- (id)init
{
    if(!filterKernel)
    {
        NSString *path = [[NSBundle mainBundle] pathForResource:@"Sketch"
                                                         ofType:@"cikernel"];
        NSString *kernelStr = [NSString stringWithContentsOfFile:path
                                                        encoding:NSUTF8StringEncoding
                                                           error:nil];
        static dispatch_once_t once;
        dispatch_once(&once, ^{
            filterKernel = [CIKernel kernelWithString:kernelStr];
        });
    }
    
    self = [super init];
    if (self)
    {
        [self setDefaults];
        
        _monochromeFilter = [CIFilter filterWithName:@"CIPhotoEffectMono"];
        
    }
    
    return self;
}

+ (NSDictionary *)customAttributes
{
    return @{
             kCIAttributeFilterDisplayName : @"Sobel Sketch",
             kCIAttributeFilterCategories :  @[kCICategoryGradient, kCICategoryVideo, kCICategoryInterlaced, kCICategoryStillImage],
            SKETCH_FILTER_STRENGTH :
                 @{
                     kCIAttributeSliderMin :  @0.0,
                     kCIAttributeSliderMax :  @2.0,
                     kCIAttributeDefault   :  @1.0,
                     kCIAttributeType      :  kCIAttributeTypeScalar
                     },
             SKETCH_FILTER_USE_MONOCHROME :
                 @{
                     kCIAttributeDefault   :  @YES,
                     kCIAttributeType      :  kCIAttributeTypeBoolean
                     }
             };
}

- (void)setDefaults
{
    NSDictionary *dict = [[self class] customAttributes];
    
    NSDictionary *dictVal = [dict valueForKey:SKETCH_FILTER_STRENGTH];
    self.strength = [dictVal valueForKey:kCIAttributeDefault];
    
    dictVal = [dict valueForKey:SKETCH_FILTER_USE_MONOCHROME];
    self.useMonochromeFilter = [dictVal valueForKey:kCIAttributeDefault];
}

- (CIImage *)outputImage
{
    
    if( !_inputImage )
    {
        return nil;
    }
    
    CIImage *src = self.inputImage;
    
    if( self.useMonochromeFilter.boolValue )
    {
        @autoreleasepool
        {
            [_monochromeFilter setDefaults];
            [_monochromeFilter setValue:src forKey:kCIInputImageKey];
            src = _monochromeFilter.outputImage;
        }
    }
    else
    {
        src = self.inputImage;
    }
    
    @autoreleasepool
    {
        CGRect dod = self.inputImage.extent;
        src = [filterKernel applyWithExtent:dod roiCallback:^CGRect(int index, CGRect destRect) {
            return destRect;
        } arguments:@[src, self.strength]];
    }
    
    return src;
}

- (id)copyWithZone:(NSZone *)zone
{
    FWKSketchFilter *result = [super copyWithZone: zone];
    
    result.useMonochromeFilter = self.useMonochromeFilter;
    result.strength = self.strength;
    
    return result;
}

@end

Sketch.cikernel

kernel vec4 sketch(sampler image, float strength)
{
    vec2 d = destCoord();

    vec2 bottomLeftTextureCoordinate = samplerTransform(image, d + vec2(-1.0, -1.0));
    vec2 topRightTextureCoordinate = samplerTransform(image, d + vec2(1.0, 1.0));
    vec2 topLeftTextureCoordinate = samplerTransform(image, d + vec2(-1.0, 1.0));
    vec2 bottomRightTextureCoordinate = samplerTransform(image, d + vec2(1.0, -1.0));

    vec2 leftTextureCoordinate = samplerTransform(image, d + vec2(-1.0, 0.0));
    vec2 rightTextureCoordinate = samplerTransform(image, d + vec2(1.0, 0.0));
    vec2 bottomTextureCoordinate = samplerTransform(image, d + vec2(0.0, -1.0));
    vec2 topTextureCoordinate = samplerTransform(image, d + vec2(0.0, 1.0));

    float bottomLeftIntensity = sample(image, bottomLeftTextureCoordinate).r;
    float topRightIntensity = sample(image, topRightTextureCoordinate).r;
    float topLeftIntensity = sample(image, topLeftTextureCoordinate).r;
    float bottomRightIntensity = sample(image, bottomRightTextureCoordinate).r;

    float leftIntensity = sample(image, leftTextureCoordinate).r;
    float rightIntensity = sample(image, rightTextureCoordinate).r;
    float bottomIntensity = sample(image, bottomTextureCoordinate).r;
    float topIntensity = sample(image, topTextureCoordinate).r;

    float h = -topLeftIntensity - 2.0 * topIntensity - topRightIntensity + bottomLeftIntensity + 2.0 * bottomIntensity + bottomRightIntensity;
    float v = -bottomLeftIntensity - 2.0 * leftIntensity - topLeftIntensity + bottomRightIntensity + 2.0 * rightIntensity + topRightIntensity;

    float mag = 1.0 - (length(vec2(h, v))*strength);

    return vec4(vec3(mag), 1.0);
}

CIFilter *sobelFilter = [CIFilter filterWithName:@"FWKSketchFilter"];
[sobelFilter setDefaults];
[sobelFilter setValue:ciResult forKey:kCIInputImageKey];
ciResult = [sobelFilter valueForKey:kCIOutputImageKey];