Greyscale Image using COCOA and NSImage

There are a lot of articles of how to do this on an iPhone using UIImage, but hardly anything using NSImage.

NSImage has a slightly different interface to UIImage but uses the same image engine.

Below is how to greyscale an image using NSImage, credit to Stackoverflow for the UIImage version:

- (NSImage *)imageToGreyImage:(NSImage *)image {

    // Create image rectangle with current image width/height

    CGFloat actualWidth = image.size.width;

    CGFloat actualHeight = image.size.height;

    

    CGRect imageRect = CGRectMake(0, 0, actualWidth, actualHeight);

    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceGray();

    

    CGContextRef context = CGBitmapContextCreate(nil, actualWidth, actualHeight, 8, 0, colorSpace, (CGBitmapInfo)kCGImageAlphaNone);

    CGContextDrawImage(context, imageRect, [self nsImageToCGImageRef:image]);

    

    CGImageRef grayImage = CGBitmapContextCreateImage(context);

    CGColorSpaceRelease(colorSpace);

    CGContextRelease(context);

    

    context = CGBitmapContextCreate(nil, actualWidth, actualHeight, 8, 0, nil, (CGBitmapInfo)kCGImageAlphaOnly);

    CGContextDrawImage(context, imageRect, [self nsImageToCGImageRef:image]);

    CGImageRef mask = CGBitmapContextCreateImage(context);

    CGContextRelease(context);

    

    NSImage *grayScaleImage =  [self imageFromCGImageRef:CGImageCreateWithMask(grayImage, mask)];

    CGImageRelease(grayImage);

    CGImageRelease(mask);

    

    // Return the new grayscale image

    return grayScaleImage;

}

- (NSImage*) imageFromCGImageRef:(CGImageRef)image

{

    NSRect imageRect = NSMakeRect(0.0, 0.0, 0.0, 0.0);

    CGContextRef imageContext = nil;

    NSImage* newImage = nil; // Get the image dimensions.

    imageRect.size.height = CGImageGetHeight(image);

    imageRect.size.width = CGImageGetWidth(image);

    

    // Create a new image to receive the Quartz image data.

    newImage = [[NSImage alloc] initWithSize:imageRect.size];

    [newImage lockFocus];

    

    // Get the Quartz context and draw.

    imageContext = (CGContextRef)[[NSGraphicsContext currentContext] graphicsPort];

    CGContextDrawImage(imageContext, *(CGRect*)&imageRect, image); [newImage unlockFocus];

    return newImage;

}

- (CGImageRef)nsImageToCGImageRef:(NSImage*)image;

{

    NSData * imageData = [image TIFFRepresentation];

    CGImageRef imageRef;

    if(!imageData) return nil;

    CGImageSourceRef imageSource = CGImageSourceCreateWithData((__bridge CFDataRef)imageData, NULL);

    imageRef = CGImageSourceCreateImageAtIndex(imageSource, 0, NULL);

    return imageRef;

}