UIImage+Grayscale.m

//  UIImage+Grayscale.m
//
//  Created by Andreas Liebschner on 4/13/12.
//

#import "UIImage+Grayscale.h"

@implementation UIImage (grayscale)

typedef enum {
    ALPHA = 0,
    BLUE = 1,
    GREEN = 2,
    RED = 3
} PIXELS;

- (UIImage *)convertToGrayscale {
    CGFloat scale = [[UIScreen mainScreen] scale];
    
    CGSize size = [self size];
    int width = size.width *scale;
    int height = size.height *scale;
    
    // the pixels will be painted to this array
    uint32_t *pixels = (uint32_t *) malloc(width * height * sizeof(uint32_t));
    
    // clear the pixels so any transparency is preserved
    memset(pixels, 0, width * height * sizeof(uint32_t));
    
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    
    // create a context with RGBA pixels
    CGContextRef context = CGBitmapContextCreate(pixels, width, height, 8, width * sizeof(uint32_t), colorSpace, 
                                                 kCGBitmapByteOrder32Little | kCGImageAlphaPremultipliedLast);
    
    // paint the bitmap to our context which will fill in the pixels array
    CGContextDrawImage(context, CGRectMake(0, 0, width, height), [self CGImage]);
    
    for(int y = 0; y < height; y++) {
        for(int x = 0; x < width; x++) {
            uint8_t *rgbaPixel = (uint8_t *) &pixels[y * width + x];
            
            // convert to grayscale using recommended method: http://en.wikipedia.org/wiki/Grayscale#Converting_color_to_grayscale
            uint32_t gray = 0.3 * rgbaPixel[RED] + 0.59 * rgbaPixel[GREEN] + 0.11 * rgbaPixel[BLUE];
            
            // set the pixels to gray
            rgbaPixel[RED] = gray;
            rgbaPixel[GREEN] = gray;
            rgbaPixel[BLUE] = gray;
        }
    }
    
    // create a new CGImageRef from our context with the modified pixels
    CGImageRef image = CGBitmapContextCreateImage(context);
    
    // we're done with the context, color space, and pixels
    CGContextRelease(context);
    CGColorSpaceRelease(colorSpace);
    free(pixels);
    
    // make a new UIImage to return
    UIImage *resultUIImage = [UIImage imageWithCGImage:image scale:scale orientation:UIImageOrientationUp];
    
    // we're done with image now too
    CGImageRelease(image);
    
    return resultUIImage;
}

@end