Batch crop and resize images to create thumbnails

Question

I have a large set of jpg images for which I want to create thumbnails. The images all have different sizes and resolutions, but I would like all thumbnails to have a standard size, e.g. 120x80px. However, I do not want to stretch the images. So I would like to do something of the following:

1. Crop image to a 1.5 : 1 aspect ratio. Center the cropping area (i.e. cut off an equal amount left and right, or above and below
2. Resize the image to 120 x 80 px.

Is there a linux command to do so? I looked into imagemick convert, but I can't figure out how to do the centered cropping. It seems that you have to manually specify the cropping area for each image?

Answer 1

This works for images larger than 120x80. Not tested on smaller ones, but you should be able to tune it.

#! /bin/bash
for img in p*.jpg ; do
    identify=$(identify "$img")
    [[ $identify =~ ([0-9]+)x([0-9]+) ]] || \
        { echo Cannot get size >&2 ; continue ; }
    width=${BASH_REMATCH[1]}
    height=${BASH_REMATCH[2]}
    let good_width=height+height/2

    if (( width < good_width )) ; then # crop horizontally
        let new_height=width*2/3
        new_width=$width
        let top='(height-new_height)/2'
        left=0

    elif (( width != good_width )) ; then # crop vertically
        let new_width=height*3/2
        new_height=$height
        let left='(width-new_width)/2'
        top=0
    fi

    convert "$img" -crop "$new_width"x$new_height+$left+$top -resize 120x80 thumb-"$img"
done