ImageMagick exract non-rectangular region with transparent background

Question

Based on the answer to the following question Programmatically divide scanned images into separate images right now I able to extract rectangular region.

Is it possible to modify the following ImageMagick script:

infile="image.png"
inname=`convert -ping $infile -format "%t" info:`
OLDIFS=$IFS
IFS=$'\n'
arr=(`convert $infile -blur 0x5 -auto-level -threshold 99% -type bilevel +write tmp.png \
-define connected-components:verbose=true \
-connected-components 8 \
null: | tail -n +2 | sed 's/^[ ]*//'`)
num=${#arr[*]}
IFS=$OLDIFS
for ((i=0; i<num; i++)); do
#echo "${arr[$i]}"
color=`echo ${arr[$i]} | cut -d\  -f5`
bbox=`echo ${arr[$i]} | cut -d\  -f2`
echo "color=$color; bbox=$bbox"
if [ "$color" = "gray(0)" ]; then
convert $infile -crop $bbox +repage -fuzz 10% -trim +repage ${inname}_$i.png
fi
done

to be able to extract non-rectangular region that exactly matches the black regions in the tmp.png:

For example, add a transparent background for the rest of the rectangular region of the extracted image that is not related to the black region.

Answer 1

Yes, my ImageMagick script can be modified as follows:

infile="image.png"
inname=`convert -ping $infile -format "%t" info:`
OLDIFS=$IFS
IFS=$'\n'
arr=(`convert $infile +repage -blur 0x7 -auto-level -negate -threshold 2% -negate -type bilevel +write tmp.png \
-define connected-components:verbose=true \
-connected-components 8 \
null: | tail -n +2 | sed 's/^[ ]*//'`)
num=${#arr[*]}
IFS=$OLDIFS
for ((i=0; i<num; i++)); do
echo "${arr[$i]}"
color=`echo ${arr[$i]} | cut -d\  -f5`
bbox=`echo ${arr[$i]} | cut -d\  -f2`
echo "color=$color; bbox=$bbox"
if [ "$color" = "gray(0)" ]; then
convert tmp.png -crop $bbox +repage -fuzz 10% -trim +repage -alpha copy -channel a -negate +channel ${inname}_$i.png
fi
done

The following results are transparent, so show white for the outside. But if you download them, you will see that the background is transparent.