imgmap.m

#import <AppKit/AppKit.h>
#import <objc/runtime.h>
#import <objc/message.h>

@interface NSBitmapImageRep (Mapping)
- (void)map:(void (^)(NSUInteger[4]))aBlk;
@end
@implementation NSBitmapImageRep (Mapping)
- (void)map:(void (^)(NSUInteger[4]))aBlk
{
    NSUInteger px[4];
    for(int y = 0; y < self.size.height; ++y) {
        for(int x = 0; x < self.size.width; ++x) {
            [self getPixel:px atX:x y:y];
            aBlk(px);
            [self setPixel:px atX:x y:y];
        }
    }
}
@end

int main(int argc, char *argv[]) {
    NSAutoreleasePool *p = [[NSAutoreleasePool alloc] init];

    NSImage *img = [[NSImage alloc] initWithContentsOfFile:@"/Users/fyolnish/Dropbox/Desktop/test.png"];
    NSBitmapImageRep *bm = [img representations][0];
    [bm map:^(NSUInteger *px) {
        if(!(bm.bitmapFormat & NSAlphaNonpremultipliedBitmapFormat)) {
            double factor = 255.0 / px[3];
            px[0] *= factor;
            px[1] *= factor;
            px[2] *= factor;
        }
        px[0] = px[1] = px[2] = px[3] = 255 - (0.299*px[0] + 0.587*px[1] + 0.114*px[2]);
        
    }];
    
    [[bm TIFFRepresentation] writeToFile:[@"~/Desktop/test.tiff" stringByExpandingTildeInPath] atomically:YES];
    
    [p release];
}