Simple script to rin perfectly good image files.

glitch.rb

#!/usr/bin/env ruby

=begin

Simple script to smack your glitch up.

Call this program with the name of an image file, and two numbers

The first number is used to determine when to mess with a byte in the 
image file.  If you pass in, say 2000, then even 2000th byte gets tweaked.

The second number is a bit-mask value.  Keep it under 255.  It's used to set some
bits in any tweaked byte.

The output file name is derived from the input file name and the two integers you give.

It makes it easier to see to what parameters gave what results.



=end

if ARGV.size < 3
  puts "Usage: #{__FILE__} source-image byte-mod-int bitmask-int"
  exit
end

infile   = ARGV.shift
byte_mod = ARGV.shift.to_i
bitmask  = ARGV.shift.to_i


ext     = File.extname  infile
base    = infile.sub /#{ext}$/, ''
outfile = "#{base}.#{byte_mod}-#{bitmask}#{ext}"  

warn "Glitch #{infile} and write to #{outfile}"

b_counter = 0

File.open(infile, 'rb') do |f|
  File.open(outfile, "w+b") do |ff|  
    f.each_byte do |b|

      b_counter += 1
      b_counter %= byte_mod

      if b_counter == 0
        b = b && bitmask
      end

      # The trick here is that 'print'
      # works with strings and will bork your
      # data in the wrong way, hence `chr`
      ff.print b.chr
    end
  end
end




=begin


Copyright (c) 2013 James Britt / Neurogami

Released under the The MIT License - http://opensource.org/licenses/MIT

   Feed your head

   Hack your world

   Live curious


=end