How do I apply a CIFilter to a UIImage?

Question

My app is crashing when I attempt to apply a filter to my user-selected UIImage (It has been working fine without applying the filter). I added and imported the "CoreImage" framework to my project so I could create filters for user-selected images.

I am attempting to apply the filter by creating a category for UIImage (based on Apple's documentation, and then calling the corresponding method on the UIImage selected by the user. Following is the code of my category header and body; what am I doing wrong? (please note, "randColor" is a category UIColor class method to generate a random color)

#import <UIKit/UIKit.h>
#import <CoreImage/CoreImage.h>
#import "UIColor+CustomColorCategory.h"

@interface UIImage (MonoChromeFilter)

- (UIImage *) applyMonoChromeWithRandColor;

@end

#import "UIImage+MonoChromeFilter.h"

@implementation UIImage (MonoChromeFilter)

- (UIImage *)applyMonoChromeWithRandColor
{
    CIContext *context = [CIContext contextWithOptions:nil];

    CIImage *ciImage = [[CIImage alloc] initWithImage:self];

    CIFilter *filter = [CIFilter filterWithName:@"CIColorMonochrome"];

    [filter setValue:ciImage forKey:kCIInputImageKey];
    [filter setValue:[UIColor randColor] forKey:kCIAttributeTypeColor];

    CIImage *result = [filter valueForKey:kCIOutputImageKey];

    CGRect extent = [result extent];

    CGImageRef cgImage = [context createCGImage:result fromRect:extent];

    UIImage *filteredImage = [[UIImage alloc] initWithCGImage:cgImage];

    return filteredImage;
}

@end

Here is the method in the viewController where this category is being called:

- (void)imagePickerController:(UIImagePickerController *)picker didFinishPickingMediaWithInfo:(NSDictionary *)info
{
    [picker dismissViewControllerAnimated:YES completion:^{
        UIImage *editedImage = [info objectForKey:UIImagePickerControllerEditedImage];
        editedImage = [editedImage applyMonoChromeWithRandColor];

        self.blogImageOutlet.image = editedImage;
        self.blogImageOutlet.layer.cornerRadius = self.blogImageOutlet.frame.size.width / 2.0;
        [self.blogImageOutlet setClipsToBounds:YES];

        [self saveImageToLibrary:editedImage];

    }];
}

Answer 1

I figured it out! After debugging and using some other projects as a point of reference, I realized that I was experiencing two issues. First, I was trying to use a UIColor for CIColor, which is not directly possible. I first had to covert the UIColor to a CIColor to be able to apply it. Next, I was not using the correct strings for the CIFilter value keys. Here is the following code after modifications (and now it works!)

#import "UIImage+MonoChromeFilter.h"

@implementation UIImage (MonoChromeFilter)

+ (UIImage *) applyMonoChromeWithRandColor: (UIImage *)uIImage
{

    //  Convert UIColor to CIColor
    CGColorRef colorRef = [UIColor randColor].CGColor;
    NSString *colorString = [CIColor colorWithCGColor:colorRef].stringRepresentation;
    CIColor *coreColor = [CIColor colorWithString:colorString];

    CIContext *context = [CIContext contextWithOptions:nil];

    //  Convert UIImage to CIImage
    CIImage *ciImage = [[CIImage alloc] initWithImage:uIImage];

    //  Set values for CIColorMonochrome Filter
    CIFilter *filter = [CIFilter filterWithName:@"CIColorMonochrome"];
    [filter setValue:ciImage forKey:kCIInputImageKey];
    [filter setValue:@1.0 forKey:@"inputIntensity"];
    [filter setValue:coreColor forKey:@"inputColor"];

    CIImage *result = [filter valueForKey:kCIOutputImageKey];

    CGRect extent = [result extent];

    CGImageRef cgImage = [context createCGImage:result fromRect:extent];

    UIImage *filteredImage = [[UIImage alloc] initWithCGImage:cgImage];

    return filteredImage;
}

@end