Clear pixels on a specific color register in a Sixel image.

clear-sixel-reg.rb

#!/usr/bin/env ruby
#
# Usage:
#
#   $ clear-sixel-reg register? files*
#
# The script reads Sixel images, are replaced every pixel using the
# specified color register with holes. The output is always sent to stdout.
#
# The `background selector` parameter of the protocol selector is forced
# to be 1.
#
# If `register` is omitted it is `0`.
# If no files are provided in the command-line, it reads stdin.

if ARGV.size == 0
  STDERR.puts "Usage: #$0 color-register? files*"
  exit 1
end

TARGET_REG = ARGV.first =~ /\A[0-9]+\Z/ ? Integer(ARGV.shift) : 0

def convert(stream)
  parsing_color_reg = false
  current_reg = -1

  if stream.read(2) != "\033P"
    STDERR.puts "Missing DCS."
    return
  end

  loop do
    case stream.getc
    when "q" then break
    when nil
      STDERR.puts "Missing DCS."
      return
    end
  end

  print "\033P0;1;0q"

  stream.each_byte do |byte|
    if byte == 0x23   # "#"
      parsing_color_reg = true
      current_reg = 0
    elsif parsing_color_reg
      if (0x30..0x39).include?(byte)   # "0"-"9"
        current_reg = current_reg * 10 + byte - 0x30
      else
        parsing_color_reg = false
      end
    end

    if (0x3F..0x7E).include?(byte) and current_reg == TARGET_REG
      print "?"
    else
      putc byte
    end

  end
end

if ARGV.empty?
  convert(STDIN)
else
  ARGV.each do |file|
    puts file if STDOUT.tty?
    File.open(file) {|f| convert(f) }
  end
end