convert-ico-to-png

#!/bin/sh
set -euf

## Program Tracking ##

program_command="convert-ico-to-png"
program_version="3.0.0"
program_created="2015-05-30"
program_updated="2016-02-14"
program_license="GPL"
program_contact="Joel Parker Henderson (joel@joelparkerhenderson.com)"
program_variant="$program_command version $program_version updated $program_updated"

## Help ##

help(){
cat <<END
convert-ico-to-png:
convert an ICO image file to a PNG image file.

Syntax:

    convert-ico-to-png
      [--input <input ico file name>]
      [--output <output png file name]
      [--size <pixel width>]
      [--srcset]

Example with defaults:

    convert-ico-to-png

Example with specifics:

    convert-ico-to-png --input favicon.ico --output favicon.png --size 16

Options:

  * \`-i\`, \`--input\`: input file name; default is "favicon.ico"
  * \`-o\`, \`--output\`: output file name; default is "favicon.png"
  * \`-s\`, \`--size\`: size as a pixel width; default is 16.
  * \`--srcset\`: generate a source set of icons using sizes 16, 32, 48, 64, and symlines.

## Source set details ##

The \`--srcset\` option generates our preferred image source set,

The goal is to make the files work with this kind of HTML that provides four image scales:

    <img src="foo.png" srcset="foo-1x.png 1x, foo-2x.png 2x, foo-3x.png 3x, foo-4x.png 4x">

We prefer this file naming convention for source set image files:

  * append the base pixel size
  * such as "16x16", "32x32", "48x48", "64x64"

We prefer this file naming convention for source set image symlinks:

  * append the scale multiplier
  * such as "1x", "2x", "3x", "4x".

This tool generates these files:

  * \`foo-16x16.png\`
  * \`foo-32x32.png\`
  * \`foo-48x48.png\`
  * \`foo-64x64.png\`

This tool generates these symlinks:

  * \`foo-16x16-1x.png\` => \`foo-16x16-1x.png\`
  * \`foo-16x16-2x.png\` => \`foo-16x16-1x.png\`
  * \`foo-16x16-3x.png\` => \`foo-16x16-1x.png\`
  * \`foo-16x16-4x.png\` => \`foo-16x16-1x.png\`

If you prefer other conventions, then please let us know, and we'll do
our best to add your conventions as options in this tool.

Pull requests are welcome.

## Requirements ##

This script needs:

  * The \`convert\` command, typically provided by ImageMagick, or GraphicsMagick, or any command-line compatible graphics tool.

Command: $program_command
Version: $program_version
Created: $program_created
Updated: $program_updated
License: $program_license
Contact: $program_contact
END
}

## Essential Functions ##

out () { printf %s\\n "$*" ; }
err () { >&2 printf %s\\n "$*" ; }
die () { >&2 printf %s\\n "$*" ; exit 1 ; }
die_opt_unk() { die "Option $1 is unknown" ; }
die_opt_arg() { die "Option $1 needs an argument" ; }

## Defaults ##

input_file_name="favicon.ico"
output_file_name="favicon.png"
pixel_width=16

## Options ##

while [ $# -gt 0 ]; do
  case $1 in
    -- )
      shift
      break
      ;;
    -h | --help )
      help
      ;;
    -V | --version )
      out $program_variant
      ;;
    -i | --input )
      [ $# -le 1 ] && die_opt_arg $1
      input_file_name=$2
      shift
      ;;
    -o | --output )
      [ $# -le 1 ] && die_opt_arg $1
      output_file_name=$2
      shift
      ;;
    -s | --size )
      [ $# -le 1 ] && die_opt_arg $1
      pixel_width=$2
      shift
      ;;
    --srcset )
      srcset=true
      ;;
    -?*)
      die_opt_unk $1
      ;;
    *)
      break
  esac
  shift
done

## Main ##

if [ "$srcset" = true ]; then
    output_file_base="${output_file_name%.*}"
    for n in 1 2 3 4; do
        size=$(($n * $pixel_width))
        output_file_name_now="${output_file_base}-${size}x${size}.png"
        output_symlink_now="${output_file_base}-${pixel_width}x${pixel_width}-${n}x.png"
        convert "$input_file_name" -thumbnail "${size}x${size}" -alpha on -background none -flatten "$output_file_name_now"
        ln -sfn "$output_file_name_now" "$output_symlink_now"
    done
else
    convert "$input_file_name" -thumbnail "${pixel_width}x${pixel_width}" -alpha on -background none -flatten "$output_file_name"
fi