Appending images using ImageMagick convert - graphics

Question

So I have a bunch of images that I wish to append vertically:
convert *.png -append output.png
However I ran into two problems:
Not all the images are of the same width. Hence images smaller in width than the widest image have white space trailing after them because they are left-aligned.
There's no spacing between the images.
How do you center align all the images and specify spacing between them?

Answer 1

Just use ImageMagick's montage utility. Align the images with the -gravity & -extent options, and adjust the spacing with -geometry.
montage fishscales.png circles.png verticalsaw.png \
-tile 1x -geometry +10+10 \
-gravity Center -extent 120 \
out.png

Answer 2

My approach would be a shell script as follows:
1. Run a loop over all your images and find the width of the widest (using ImageMagick `identify`) - say 8 to 10 lines of shell script
2. Create a transparent "spacer image" the same width as the widest image and the height you want for vertically spacing images - one line of shell script
3. Run a loop over all your images first adding the transparent "spacer" to the bottom of the existing output image then compositing images that are narrower than your widest image onto a transparent background the width of your widest image - then appending that to the output image - maybe 15 lines of shell script.
Here is the output with three images:
Red=80px wide
Green=180 px wide
Blue=190px wide
Could that sort of approach work for you - I could probably code it over a day or two between other things!
Here is what I mean:
#!/bin/bash
# User-editable vertical spacing between images
SPACING=10
function centre() {
echo DEBUG: Centering $1
TEMP=$$tmp.png
w=$(convert "$1" -ping -format "%w" info:)
h=$(convert "$1" -ping -format "%h" info:)
convert -size ${MAXW}x${h} xc:"rgba(0,0,0,0)" PNG32:$TEMP
composite -resize '1x1<' -gravity center "$1" $TEMP $TEMP
if [ $2 -eq 0 ]; then
mv $TEMP output.png
else
convert output.png $TEMP -append output.png
rm $TEMP
fi
}
# Step 1 - determine widest image and save width in MAXW
MAXW=0
for i in *.jpg; do
w=$(convert "$i" -ping -format "%w" info:)
[[ $w -gt $MAXW ]] && MAXW=$w
echo DEBUG: Image $i width is $w
done
echo DEBUG: Widest image is $MAXW
# Step 2 - Create transparent spacer image, with width MAXW
convert -size ${MAXW}x${SPACING} xc:"rgba(0,0,0,0)" PNG32:spacer.png
# Step 3 - Build output image
n=0
for i in *.jpg; do
# Add a vertical spacer if not first image
[[ $n -ne 0 ]] && convert output.png spacer.png -append output.png
# Centre image horizontally and append to output
centre "$i" $n
((n++))
done
Another, completely different option would be to add all heights of the N images and create a transparent canvas the width of the widest image and the height of all the images combined plus (N-1)* vertical spacing. Then overlay the images into the canvas at the corrrect place - this involves more maths and less processing and reprocessing of images which could mean that this approach would lose less quality than the one I have suggested. I would do this approach with PerlMagick as it is better for math than bash.