ibood-captcha.sh

#!/usr/bin/env bash

img=$1

blocksize=15

img_width=300
img_height=300

w_steps=$(( img_width / blocksize ))
h_steps=$(( img_height / blocksize ))

colours=()

w_i=0
while (( w_i < w_steps )); do
    h_i=0
    while (( h_i < h_steps )); do
        rgb=$(convert $img -crop $blocksize"x"$blocksize"+"$(( w_i*blocksize ))"+"$(( h_i*blocksize )) -resize 1x1 -format "%[fx:int(255*r+.5)],%[fx:int(255*g+.5)],%[fx:int(255*b+.5)]" info:-)
        IFS=',' read -r -a rgba <<< $rgb
        colour=$((1*${rgba[0]} + 1*${rgba[1]} + 1*${rgba[2]}))
        colours[${#colours[@]}]=$colour" "$w_i"x"$h_i

        h_i=$(( h_i + 1))
    done

    w_i=$(( w_i + 1))
done

IFS=$'\n' sorted=($(sort <<<"${colours[*]}"))

block=${sorted[0]/* /}
b_x=${block/x*/}
b_y=${block/*x/}

x=$(( b_x * blocksize + blocksize / 2 ))
y=$(( b_y * blocksize + blocksize / 2 ))

echo $x","$y