Pad images to fit 4 by 6 photo paper bash script using imagemagick

Alec Jacobson

August 16, 2009

weblog/

I was a little dismayed that I couldn't use my previously mentioned applescript to pad images with colors other than black. A shrewd observer will also notice that the Image Events suite resamples when it pads resulting in some annoying re-anti-aliasing. I wrote the following bash script to the same task as the applescript from the command line:

#!/bin/bash

USAGE="Usage: $0 [--replace] [--padcolor=color] ratio imagefile(s)
  ratio              is a float equal to long dimension / short dimension to pad 
                     to fit a 4\"x6\" card, divide 6 by 4 giving ratio = 1.5
  --replace or -f    option to overwrite original images with padded versions,
                     default is to preppend 'pad-[ratio]-' to each naee
  --padcolor= or -c= option to change pad color from default black. Takes a
                     color can be any Imagemagick color or hex color:
                     (See http://www.imagemagick.org/script/color.php)
"
if [ "$#" == "0" ]; then
  echo "$USAGE"
  exit 1
fi

replace=false
if [ "$1" == "--replace" -o "$1" == "-f" ]
then
  echo "Overwriting existing image files..."
  replace=true
  shift
fi

padcolor="black"
if [ `echo "$1" | grep "^\(--padcolor\)\|\(-c\)="` ]
then
  padcolor=`echo "$1" | sed "s/^.*=//"`
  shift
  echo "Using $padcolor as pad color..."
fi

ratio=`echo "$1" | grep "^[0-9]*\.\?[0-9]\+$\|^[0-9]\+\.$"`
if [ "$ratio" == "" ]
then
  echo "$USAGE"
  exit 2
fi
shift
while (( "$#" )); do
  image="$1"
  dir=`dirname "$image"`
  base=`basename "$image"`
  if ! $replace 
  then
    base="pad-$ratio-$base"
  fi
  if wh=`identify -format "%w %h" "$image" 2>/dev/null`
  then
    width=${wh%% *}
    height=${wh#* }
    if [ $height -gt $width ]
    then
      # vertical
      long=`echo "$height/$width > $ratio" | bc -l`
      if [ $long -eq "1" ] 
      then
        # taller than ratio
        newwidth=`echo "$height/$ratio" | bc -l`
        newwidth=`echo "($newwidth+0.5)/1" | bc`
        newheight="$height"
      else
        # shorter than ratio
        newwidth="$width"
        newheight=`echo "$width*$ratio" | bc -l`
        newheight=`echo "($newheight+0.5)/1" | bc`
      fi
    else
      # horizontal
      long=`echo "$width/$height > $ratio" | bc -l`
      if [ $long -eq "1" ] 
      then
        # wider than ratio
        newwidth="$width"
        newheight=`echo "$width/$ratio" | bc -l`
        newheight=`echo "($newheight+0.5)/1" | bc`
      else
        # skinnier than ratio
        newwidth=`echo "$height*$ratio" | bc -l`
        newwidth=`echo "($newwidth+0.5)/1" | bc`
        newheight="$height"
      fi
    fi
    printf "Padding into $base..."
    convert -size "$newwidth"x"$newheight" xc:"$padcolor" .padcanvas.miff
    composite -gravity center "$image" .padcanvas.miff "$dir"/"$base"
    rm .padcanvas.miff
    echo "DONE"
  else
    echo "$image is not a proper image...skipping."
  fi
  shift
done

I have not completely bullet proofed the above but it should get the job done. Here’s the same before and after shots to show what this script does:

Original	Padded with black	Padded with transparent pixels

Look closely at the last column, the images are padded with transparent pixels! Could be useful for web posting.