Using the C API for ImageMagick(on iPhone?) to convert to monochrome?

You can achieve monochrome conversion, as described here, with MagickQuantizeImage function. I'm not quite familiar with dithering images, but an example may look like the following.

#include <wand/MagickWand.h>int main(int argc, char **argv) {  const size_t number_colors = 2;  const size_t treedepth     = 1;  MagickWandGenesis();  MagickWand *wand = NULL;  wand = NewMagickWand();  MagickReadImage(wand,"source.png");  MagickQuantizeImage(                     wand,            // MagickWand                     number_colors,   // Target number colors                     GRAYColorspace,  // Colorspace                     treedepth,       // Optimal depth                     MagickTrue,      // Dither                     MagickFalse      // Quantization error                 );  MagickWriteImage(wand,"out.png");  if(wand)wand = DestroyMagickWand(wand);  MagickWandTerminus();  return 0;}

This can give you a rather speckled image at times.

Adjusting depth, color-number, and/or disabling dithering may give you results closer to what you would expect from the examples provided.

MagickQuantizeImage(                   wand,            // MagickWand                   number_colors,   // Target number colors                   GRAYColorspace,  // Colorspace                   treedepth,       // Optimal depth                   MagickFalse,     // No-dither                   MagickFalse      // Quantization error               );

Like such...

For iOS

Not much effort is needed to port the example code to iOS. NextStep/Objective-c methods are compatible with MagickWand library. The following example uses a temporary file to store the monochrome image, but I'm sure there is a better way to pass Magick image-data directly to UImage object.

// MyViewController.h#import <UIKit/UIKit.h>#import <wand/MagickWand.h>@interface MyViewController : UIViewController@property (retain, nonatomic) IBOutlet UIImageView *imageView;@property (retain, nonatomic) MagickWand *wand;@end// MyViewController.m#import "MyViewController.h"@implementation MyViewController- (void)viewDidLoad{    [super viewDidLoad];    MagickWandGenesis();    self.wand = NewMagickWand();    [self drawMonochromeImage:@"logo:"];}-(void)drawMonochromeImage:(NSString *)filePath{    // Create temporary file    NSString *tempFilePath = [NSTemporaryDirectory()                                  stringByAppendingPathComponent:@"logo.jpg"];    // Read given image with C-string    MagickReadImage(self.wand,        [filePath cStringUsingEncoding:NSASCIIStringEncoding]    );    // Monochrome image    MagickQuantizeImage(self.wand,2,GRAYColorspace,1,MagickFalse,MagickFalse);    // Write to temporary file    MagickWriteImage(self.wand,        [tempFilePath cStringUsingEncoding:NSASCIIStringEncoding]    );    // Load UIImage from temporary file    UIImage *imgObj = [UIImage imageWithContentsOfFile:tempFilePath];    // Display on device    [self.imageView setImage:imgObj];    [self.imageView setContentMode:UIViewContentModeScaleAspectFit];}-(void)viewDidUnload{    // Clean-up    if (self.wand)        self.wand = DestroyMagickWand(self.wand);    MagickWandTerminus();}@end