make a grayscale image using CIFilter.

monochro.m

// To Compile: cc -fobjc-arc -framework Cocoa -framework CoreImage monochro.m -o monochro

#import <Cocoa/Cocoa.h>
#import <CoreImage/CoreImage.h>
#include <stdio.h>

void usage()
{
    printf("Usage: argv[0] < INPUTFILE > OUTPUTFILE\n");
    printf("       argv[0] INPUTFILE > OUTPUTFILE\n");
    exit(1);
}

int main(int argc, char *argv[])
{

    @autoreleasepool{
        CIImage *image;
        if(argc == 2){
            if(strcmp("-", argv[1])){
                NSURL *fileURL = [NSURL fileURLWithPath: [[NSString stringWithUTF8String: argv[1]] stringByExpandingTildeInPath]];
                image = [CIImage imageWithContentsOfURL: fileURL];
                if(!image){
                    printf("cannot open file: %s\n", argv[1]);
                    usage();
                }
            }else{
                NSFileHandle *fh = [NSFileHandle fileHandleWithStandardInput];
                NSData *inputData = [fh readDataToEndOfFileAndReturnError: nil];
                image = [CIImage imageWithData: inputData];
            }
        }else if(argc == 1){
            NSFileHandle *fh = [NSFileHandle fileHandleWithStandardInput];
            NSData *inputData = [fh readDataToEndOfFileAndReturnError: nil];
            image = [CIImage imageWithData: inputData];
        }else{
            usage();
        }

/*
//      CIFilter を使いまわす必要がなければ CIFilter をいちいち作るよりも、
//      CIImage#imageByApplyingFilter:withInputParameters: を使う方が簡単。
        CIFilter *filter = [CIFilter filterWithName: @"CIColorMonochrome"];
        [filter setValue: image forKey: kCIInputImageKey];
        [filter setValue: [CIColor colorWithString: @"1.0 1.0 1.0"] forKey: kCIInputColorKey];
//        [filter setValue: @0.5 forKey: kCIInputIntensityKey];

        CIImage *newImage = filter.outputImage; 
*/
//      単純にモノクロにするなら、CIColorMonochrome よりも
//      CIPhotoEffectMono の方が良さそう。
//        CIImage *newImage = [image imageByApplyingFilter: @"CIColorMonochrome" withInputParameters: @{kCIInputColorKey: [CIColor colorWithString: @"1.0 1.0 1.0"], kCIInputIntensityKey: @1.0}];
        CIImage *newImage = [image imageByApplyingFilter: @"CIPhotoEffectMono" withInputParameters: nil];

        NSBitmapImageRep *bitmap = [[NSBitmapImageRep alloc] initWithCIImage: newImage];
        NSData *data = [bitmap representationUsingType: NSBitmapImageFileTypeJPEG
            properties: [NSDictionary dictionary]];
        NSFileHandle *outFH = [NSFileHandle fileHandleWithStandardOutput];
        [outFH writeData: data];
    }
    return 0;
}