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

Answer 2

Here's a Python script crop-resize.py that crops, centers, and resizes input images:

usage: crop-resize.py [-h] [-s N N] [-q] [--outputdir DIR]
                      files [files ...]

Resize the image to given size. Don't strech images, crop and center
instead.

positional arguments:
  files               image filenames to process

optional arguments:
  -h, --help          show this help message and exit
  -s N N, --size N N  new image size (default: [120, 80])
  -q, --quiet
  --outputdir DIR     directory where to save resized images (default: .)

The core function is:

def crop_resize(image, size, ratio):
    # crop to ratio, center
    w, h = image.size
    if w > ratio * h: # width is larger then necessary
        x, y = (w - ratio * h) // 2, 0
    else: # ratio*height >= width (height is larger)
        x, y = 0, (h - w / ratio) // 2
    image = image.crop((x, y, w - x, h - y))

    # resize
    if image.size > size: # don't stretch smaller images
        image.thumbnail(size, Image.ANTIALIAS)
    return image

It is very similar to the bash script by @choroba.

Answer 3

Ok I managed create something that works at least for square thumbnails. However, I'm not quite sure how to change this to a 1:5 by 1 thumbnail.

make_thumbnail() {
    pic=$1
    thumb=$(dirname "$1")/thumbs/square-$(basename "$1")
    convert "$pic" -set option:distort:viewport \
      "%[fx:min(w,h)]x%[fx:min(w,h)]+%[fx:max((w-h)/2,0)]+%[fx:max((h-w)/2,0)]"$
      -filter point -distort SRT 0  +repage -thumbnail 80  "$thumb"
}

mkdir thumbs
for pic in *.jpg
do
    make_thumbnail "$pic"
done