invert white to black uiimage

Question

I have the following UIImage:

Using Objective-C, I want to be able to invert the white to black and vice versa.

Any help would be appreciated.

Answer 1

Swift3

extension UIImage {
    func invertedImage() -> UIImage? {
        guard let cgImage = self.cgImage else { return nil }
        let ciImage = CoreImage.CIImage(cgImage: cgImage)
        guard let filter = CIFilter(name: "CIColorInvert") else { return nil }
        filter.setDefaults()
        filter.setValue(ciImage, forKey: kCIInputImageKey)
        let context = CIContext(options: nil)
        guard let outputImage = filter.outputImage else { return nil }
        guard let outputImageCopy = context.createCGImage(outputImage, from: outputImage.extent) else { return nil }
        return UIImage(cgImage: outputImageCopy, scale: self.scale, orientation: .up) 
    }
}

Answer 2

For Xamarin C# (iOS-specific; not a cross-platform solution):

public static UIImage InvertImageColors( UIImage original )
{
    return ApplyFilter( original, new CIColorInvert() );
}

public static UIImage ApplyFilter( UIImage original, CIFilter filter )
{
    UIImage result;
    try {
        CIImage coreImage = original.CGImage;
        filter.SetValueForKey( coreImage, CIFilterInputKey.Image );
        CIImage output = filter.OutputImage;
        CIContext context = CIContext.FromOptions( null );
        CGImage cgImage = context.CreateCGImage( output, output.Extent );
        result = UIImage.FromImage( cgImage );

    } catch (Exception ex) {
        // .. Log error here ..
        result = original;
    }

    return result;
}

NOTE #1: Adapted from someone else's answer; I've lost track of where the original is.
NOTE #2: Deliberately done with one small step per line, so you can see the intermediate types, and easily adapt to other situations.

Also see Xamarin docs - CIFilter class

Answer 3

Swift

Using CIContext instead of -UIImage:CIImage (see https://stackoverflow.com/a/28386697/218152), and building upon @wtznc's response, here is a self-contained IBDesignable:

@IBDesignable
class InvertImage: UIImageView {

    @IBInspectable var originalImage:UIImage? = nil

    @IBInspectable var invert:Bool = false {
        didSet {
            var inverted = false
            if let originalImage = self.originalImage {
                if(invert) {
                    let image = CIImage(CGImage: originalImage.CGImage!)
                    if let filter = CIFilter(name: "CIColorInvert") {
                        filter.setDefaults()
                        filter.setValue(image, forKey: kCIInputImageKey)
                        
                        let context = CIContext(options: nil)
                        let imageRef = context.createCGImage(filter.outputImage!, fromRect: image.extent)
                        self.image = UIImage(CGImage: imageRef)
                        inverted = true
                    }
                }
            }
            
            if(!inverted) {
                self.image = self.originalImage
            }
        }
    }
}

To use it, set Original Image instead of Image since Image will be dynamically associated:

Answer 4

- (UIImage *)negativeImage
{
    // get width and height as integers, since we'll be using them as
    // array subscripts, etc, and this'll save a whole lot of casting
    CGSize size = self.size;
    int width = size.width;
    int height = size.height;

    // Create a suitable RGB+alpha bitmap context in BGRA colour space
    CGColorSpaceRef colourSpace = CGColorSpaceCreateDeviceRGB();
    unsigned char *memoryPool = (unsigned char *)calloc(width*height*4, 1);
    CGContextRef context = CGBitmapContextCreate(memoryPool, width, height, 8, width * 4, colourSpace, kCGBitmapByteOrder32Big | kCGImageAlphaPremultipliedLast);
    CGColorSpaceRelease(colourSpace);

    // draw the current image to the newly created context
    CGContextDrawImage(context, CGRectMake(0, 0, width, height), [self CGImage]);

    // run through every pixel, a scan line at a time...
    for(int y = 0; y < height; y++)
    {
        // get a pointer to the start of this scan line
        unsigned char *linePointer = &memoryPool[y * width * 4];

        // step through the pixels one by one...
        for(int x = 0; x < width; x++)
        {
            // get RGB values. We're dealing with premultiplied alpha
            // here, so we need to divide by the alpha channel (if it
            // isn't zero, of course) to get uninflected RGB. We
            // multiply by 255 to keep precision while still using
            // integers
            int r, g, b;
            if(linePointer[3])
            {
                r = linePointer[0] * 255 / linePointer[3];
                g = linePointer[1] * 255 / linePointer[3];
                b = linePointer[2] * 255 / linePointer[3];
            }
            else
                r = g = b = 0;

            // perform the colour inversion
            r = 255 - r;
            g = 255 - g;
            b = 255 - b;

            if ( (r+g+b) / (3*255) == 0 )
            {

                linePointer[0] = linePointer[1] = linePointer[2] = 0;
                linePointer[3] = 0;
            }
            else
            {
                // multiply by alpha again, divide by 255 to undo the
                // scaling before, store the new values and advance
                // the pointer we're reading pixel data from
                linePointer[0] = r * linePointer[3] / 255;
                linePointer[1] = g * linePointer[3] / 255;
                linePointer[2] = b * linePointer[3] / 255;

            }
            linePointer += 4;
        }
    }

    // get a CG image from the context, wrap that into a
    // UIImage
    CGImageRef cgImage = CGBitmapContextCreateImage(context);
    UIImage *returnImage = [UIImage imageWithCGImage:cgImage];

    // clean up
    CGImageRelease(cgImage);
    CGContextRelease(context);
    free(memoryPool);

    // and return
    return returnImage;
}

I added the above method inside a UIImage extension class.

Answer 5

Firstly, you have to add the Core Image framework to your project.

Project settings -> Targets "project name" -> Build phases -> Link Binary With Libraries -> Add items -> CoreImage.framework

Secondly, import the Core Image header to your implementation file.

#import <CoreImage/CoreImage.h>

Initialize an UIImage object to store the original file.

UIImage *inputImage = [UIImage imageNamed:@"imageNamed"];

Create a CIFilter to define how you want to modify your original UIImage object.

CIFilter* filter = [CIFilter filterWithName:@"CIColorInvert"];
[filter setDefaults];
[filter setValue:inputImage.CIImage forKey:@"inputImage"];

Create another UIImage object to keep modified image.

UIImage *outputImage = [[UIImage alloc] initWithCIImage:filter.outputImage];

Voilà! Hope it will help.