Trimming exterior empty space with ImageMagick does not work on scanned documents

documentsimage manipulationimagemagickscannerscanning

Given an image that has some actual content inside and usually some unwanted white or black or transparency around that, I would like to trim or crop the exterior parts using ImageMagick.

The following image has been drawn digitally on a computer (on the HTML <canvas>):

canvas.png

The following ImageMagick command is what I tried:

$ convert canvas.png -trim +repage canvas_trimmed.png

And it worked perfectly:

canvas_trimmed.png

So this is exactly what I want. But now I want this to work with scanned documents as well, which are not as "perfect" as computer-generated images, i.e. they have more shades of "white" and "black" and no transparency which would be easier to detect. Sometimes, they even have some black bars around the white background of the paper because the area of the scanner is larger than the paper:

scan.jpg

With this image, I tried the following commands in the given order, each trying to be more aggressive, but none yielding any results – you can't see any difference between the original image and the "trimmed" images, i.e. the trimming or cropping does not work at all:

$ convert scan.jpg -trim +repage scan_trimmed.jpg
$ convert scan.jpg -fuzz 10% -trim +repage scan_trimmed.jpg
$ convert scan.jpg -fuzz 60% -trim +repage scan_trimmed.jpg
$ convert scan.jpg -fuzz 60% -bordercolor white -border 1x1 -trim +repage scan_trimmed.jpg
$ convert scan.jpg -fuzz 60% -bordercolor black -border 1x1 -trim +repage scan_trimmed.jpg

What am I doing wrong here? How can the ImageMagick command that reliably trims computer-generated images be modified so that it trims scanned documents of the style above just as reliably?

Best Answer

You can use -shave and simply shave off the edges and then use the logic you use afterwards to process accordingly.

Note: The amount you shave off (e.g. the argument after "-shave" 40x40 or 10x10, etc. ) is important so be sure to test thoroughly to ensure this setting works universally in your environment for your images.

Example Logic

@ECHO ON

SET Convert="C:\Program Files\ImageMagick\Convert.exe"
%convert% C:\Folder\Circle.jpg -shave 40x40 C:\Folder\ShavedCircle.jpg
<The rest of your logic against C:\Folder\ShavedCircle.jpg now>

Before

enter image description here

After

enter image description here

Further Resources

  • Shave, removing edges from a image

    The reverse of the "-border" or "-frame" operators, is "-shave", which if given the same arguments, will remove the space added by these commands.

    The main thing to keep in mind about these three operators is that they add and remove space on opposite sides of the images, not just one side, or adjacent sides.

    If you want to only remove one edge of an image, then you will need to use the "-chop" operator instead. (See the Chop Examples below).

    As before all the operators "-border", "-frame", and "-shave", only effect the real image on the virtual canvas and not the virtual canvas itself.

    source

Related Question