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.

Einhorn

Please post your input image or a link to it. – Mephitic

edited to add a sample image. The light colors of this paper are designed to drop out in a B&W conversion. The pen and background should convert to black as should most words on the page. – Einhorn

Answer 1

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.

Mephitic

Any pointers for doing this exact same process in MagickImage.NET? – Einhorn

Sorry, I do not know MagickImage.NET. Contact the magick.net creator. He might help you since it is coded in IM 7 or convert my IM 6 script. See github.com/dlemstra/Magick.NET – Mephitic

on windows using version 7.1.1 using like convert check.png -alpha off -threshold 50% y.png gave a mostly blank page but 80% gave the desired result – Sandal

@Sandal The threshold % is image dependent and possibly version dependent if a bug in that version (yours or mine). But why are you using convert in Imagemagick 7. You should be using magick in place of convert. – Mephitic

@Mephitic that's easy: using convert since it worked for me but magick with otsu didn't :) – Sandal

Please show the image you tried to use for Otsu thresholding. Otsu thresholding only works if the image is basically two different brightness colors, i.e., two peaks in the histogram. – Mephitic

What was the exact version 7.1.1.x that you are using? – Mephitic

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.

Intolerant

Answer 3

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.

Meekins