Binary string packing/unpacking issues

Discussion in 'Ruby' started by Lucas L., Jun 14, 2008.

  1. Lucas L.

    Lucas L. Guest

    I have a packed string of raw binary pixel data that I want to
    manipulate. However, I'm facing a few problems.
    Firstly, the only way I've managed to unpack the data so far is with
    "pixels.unpack('b*')". It works great, but comes out in one massive
    Is there a way to:
    a)put this into an array with 8 bits in each element, eg. ['00000000',
    b)unpack small sections of a few bites
    The other problem is that I don't know of an efficient way to manipulate
    this data without having to unpack\pack massive strings each time.

    Are there any solutions to these?

    Lucas L., Jun 14, 2008
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  2. Lucas L.

    Dave Bass Guest

    This code:

    "Pixels".each_byte { |b| printf("%08b\n", b) }



    Any help?
    Dave Bass, Jun 14, 2008
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  3. Lucas L.

    Lucas L. Guest

    That's good for the first problem, thanks.
    Now for the more difficult one of efficiently manipulating small
    portions of the packed binary data.
    Lucas L., Jun 14, 2008
  4. With Ruby 1.8 you can easily manipulate individual bytes directly:

    irb(main):006:0> s="abc"
    => "abc"
    irb(main):007:0> s[2]
    => 99
    irb(main):008:0> s[2].to_s 2
    => "1100011"
    irb(main):009:0> s[2] |= 4
    => 103
    irb(main):010:0> s
    => "abg"
    irb(main):011:0> s[2].to_s 2
    => "1100111"

    I don't have a 1.9 handy so I can't tell you how to do it there. But
    the basic lesson should be, that you can leave your data in a String and
    manipulate it directly there.

    Kind regards

    Robert Klemme, Jun 14, 2008
  5. Lucas L.

    Lucas L. Guest

    How can I replace portions of a packed binary string this way?
    I know with a normal string it is easy:
    irb(main):001:0> s = "abcdef"
    => "abcdef"
    irb(main):002:0> s[0..2] = "cba"
    => "cba"
    irb(main):003:0> s
    => "cbadef"

    But I can't figure out how to do it with binary.

    Lucas L., Jun 15, 2008
  6. Lucas L.

    Todd Benson Guest

    Look closely at Robert's irb line number 9.

    A string is just bytes in a row with special characteristics/methods.

    Todd Benson, Jun 15, 2008
  7. Lucas L.

    Lucas L. Guest

    You'll have to bear with me on this, I struggle with this sort of this.
    I don't really know what using | achieves.
    And a string may be bytes normally, but mine is a sequence of bits (well
    is it in my flawed understanding). I'm using Gtk::pixbuf.pixels, if that
    helps at all.

    Sorry for the stupidity.
    Lucas L., Jun 15, 2008
  8. It's the bitwise OR operator.

    irb(main):001:0> 1 | 2
    => 3
    I do not know what Gtk::pixbuf.pixels returns. But if it is a String
    you can manipulate it like was have show before. Can you post what "p
    your_pixbuf.pixels" and "p your_pixbuf.pixels.class" print?

    Kind regards

    Robert Klemme, Jun 15, 2008
  9. Lucas L.

    Lucas L. Guest

    I don't really know why he uses it though.

    Can you post what "p
    Now we're getting somewhere. I was using puts to print pixels, and all I
    got was a bunch of question marks. p returns a massive String
    (pixels.class shows String). The string itself is uncompressed pixel
    data, a byte per channel of RGBA. The output is a massive string of
    "\000\000\000\000\000\000\000\000..." with "\377" where a pixel is
    drawn, which I'm assumming is 0xFF in the alpha channel (why is \377

    However, when I tried
    There was no change.
    So confused:(

    Lucas L., Jun 15, 2008
  10. Because those numbers are in octal and 0377 == 0xFF

    Sebastian Hungerecker, Jun 15, 2008
  11. Lucas L.

    Paul Irofti Guest

    That's because you don't seem to grasp the fundamentals, but eitherway
    you're trying to do more `advanced' stuff based on them.
    Relax a little bit, put your project on hold and start reading on the
    way data is stored, how a bit and a byte are defined, and some Boolean
    Algebra 101 won't hurt you.

    Afterwards you'll not only be more productive, but you'll be able to see
    things more clearly and make design choices a lot smarter.
    Paul Irofti, Jun 15, 2008
  12. irb#1(main):006:0> "\377"
    => "\377"
    irb#1(main):007:0> "\377".length
    => 1
    irb#1(main):008:0> '\377'
    => "\\377"
    irb#1(main):009:0> '\377'.length
    => 4

    Notice something?

    I agree with Paul, you have trouble with the basics. If you try things
    out on a smaller scale in IRB you'll probably see more easily how
    everything works.

    Kind regards

    Robert Klemme, Jun 15, 2008
  13. Lucas L.

    ara.t.howard Guest

    use narray

    a rather complicated example

    the basic starting point is

    width = 800
    height = 600

    na = NArray.to_na raw_pixel_data, NArray::BYTE, width, height

    then to, for example, increase every pixel value by 1

    na += 1


    for more.

    gem install narray

    too, of course.


    a @
    ara.t.howard, Jun 16, 2008
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