How do I convert a Color Image to Black and White using ImageMagick?

Question

Background: I performed an Indexed conversion in Gimp of a color image and the result was a nice B&W version of the source Image.

I have tried numerous options in ImageMagick to no avail. I can get close but never quite as clear and crisp as what gimp seems to do effortlessly.

Here is my source:

var bmp = new MagickImage(sourceImage);

bmp.Threshold(new ImageMagick.Percentage(60));

bmp.Resample(200, 200);
bmp.ColorType = ColorType.Bilevel;
bmp.BitDepth(1);
bmp.Settings.Compression = CompressionMethod.Group4;
bmp.Strip();

bmp.Format = MagickFormat.Tiff;

I have been adjusting the Threshold call and have tried various suggestions I have seen online with varying amounts of success.

magickimage has a feature called -monochrome but I have not found how that is achieved in the .net library.

I am sure this is possible but what is the best way to achieve a nice B&W conversion.

Answer 1

Otsu's method will perform better when dealing with texts over a background with slightly gradient colors:

How do I convert a Color Image to Black and White using ImageMagick?

According to @fmw42 's comparison on multiple threshold methods: the Local Adaptive(its equivalent in imagemagick is -lat) algorithm might work best on texts over background:

Also try to combine with connected components processing that was introduced in another answer made by @fmw42 to remove unnecessary edges/dots which might get confused by any further OCR process.

Answer 2

This is possibly the simplest way you can use Greyscale in Magick.NET.

MagickImage image = new MagickImage(imagePath);
image.Grayscale();

string fileName = image.FileName + "_grey.png";
image.Write(fileName);

Instead of image.FileName you can also directly use the image path if you have it. Optionally you can add a PixelIntensityMethod in Greyscale for eventually better results.

Don't forget to call image.RePage() when you want to crop the image.

Answer 3

With ImageMagick 7, you can do Otsu thresholding.

Input:

magick check.png -alpha off -auto-threshold otsu x.png

Result:

There is no built-in equivalent in ImageMagick 6. However I have a script, otsuthresh that will do that At my web site

So in ImageMagick 6, you just have to do simple thresholding.

convert check.png -alpha off -threshold 50% y.png

Result:

I note that your input image has an opaque alpha channel which needs to be removed to get proper results.