How to add drop shadow to a picture via CLI?

Question

I have a bunch of pictures (mostly screenshots, but it doesn't matter) to which I'd like to add a drop shadow. (Reason for it is to add a nice border, as they have white background and I use them on a white background.)

My current workflow is:

• open the .png file in Inkscape,
• Filters/Shadow and glow/Drop shadow,
• select a 3px blur radius, with a 2px 2px offset,
• export the modified .png.

However, this is done all manually… and it is quite annoying when I have more than… 2 pictures.

I know Inkscape can be used with Command Line Interface (CLI) (I'm on Ubuntu 18.04). I know how to import and export files. However, I don't know how to apply a given filter.

Here's what I've tried, it didn't work:

$ inkscape --verb org.inkscape.effect.filter.ColorDropShadow --verb FileSave --verb FileClose image.png

Hence, could you tell me how to add drop shadow to a picture using CLI?
(This could be done using either Inkscape, or any other open-source software running on Linux)

Answer 1

Rather than using the vector graphic tool Inkscape I would recommend the ultimate command line tool for bitmap manipulations: ImageMagick.

It should be available through your distribution repositories. This tool comes with many effects, including generating a drop shadow.

This is basically done with the convert -shadow option but you then need to combine the generated background image again with the original. This can all be done in a one-line command using -layers merge option:

convert input.jpg \( +clone -background black -shadow 50x10+15+15 \) +swap -background none -layers merge +repage shadow.png

Useful options for the drop shadow are determined with -background <color> and -shadow percent-opacity{xsigma}{+-}x{+-}y{%}. The example above generates a grey drop shadow with 50% opacity, 10% size and x=+15% y=+15% offset:

Source of above information and more options, and different shadow types see:

• Image Magick Manual: Bluring and Sharpening

Answer 2

Based on Takkat's answer, here is the bash function I have added to my .bashrc:

function dropshadow () {
    filename=$(basename -- "$1")
    # extension="${filename##*.}"
    basename="${filename%.*}"
    # we want to enforce png (even if .jpg as input)
    suffix="_shadow.png"
    convert "$1" \( +clone -background black -shadow 50x10+5+5 \) +swap -background none -layers merge +repage "$basename$suffix"
}

• Usage: dropshadow <yourfile.ext>,
• Output: creates <yourfile>_shadow.png in your current working directory.