Using the C API for Imagemagick (On Iphone) to Convert to Monochrome

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

imagemagick monochrome convert in iPhone

It turns out what I've been looking for is Adaptive Thresholding in my conversion to monochrome.

Chris Greening has created an image processing library that handles this quite well.

So no imagemagick needed.

Clean Monocrome image with Imagemagik in iOS

Finally I was able to use the "ConvertImageCommand()" function of the ImageMagick API for C and then used this code to generate the image with grate results!.

char *argv[] = {"convert","-respect-parenthesis","\(", input, "-set", "-colorspace", "RGB", "-colorspace", "gray", "-type", "grayscale","-colorspace","RGB", "-normalize","\)","\(","-clone", "0","-set","-colorspace", "RGB","-colorspace","gray","-negate", "-lat", "15x15+5%", "-contrast-stretch", "0","\)","-compose","copy_opacity","-composite","-fill","white","-opaque","none","+matte", output, NULL};

I hope this can be useful to someone.

Regards,

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