Learning more about convert from ImageMagick

As I often scan documents that in the end I want to get in Black & White and cropped, I decided to look at the capabilities of the convert command line tool so I wouldn't have to manually change those images.

With the Gimp I can load the image, crop it to only what I want to keep, then convert the page to black and white. However, to get a valid black and white image, you want to first apply a filter that looks like this:

This is done using the following convert command line option:

-level 55%,71%,1.0

This tells the convert tool to change all colors that have a level of 55% or less to zero (black) and any color that have a level of 71% or more to one (white). Anything else is left alone.

Without that step, we get many artifacts like in the following word:

(click to better see the dots)

You may think that isn't much to worry about. Maybe you're right as in... maybe your fax system will work right with such images. I tried with two different systems, and letters with too many small dots appear really dark / black on the destination fax machine. So... if i want to fax such documents, I've got to apply the level and there is the result:

Now that last entry looks perfect on the destination fax machine.

So the resulting command line with all the processing looks like this:

convert source.png
    -crop 5078x6578+0+0
    -level 53%,71%,1.0
    -monochrome
    destination.png

These specific crop dimensions are for an image scanned at 600 DPI on my old Epson scanner.