* <<G5L.1232>> Video corruption, cont'd.
FILE CORRUPTION FOR THE VISUALLY-ORIENTED, or,
OFF-LABEL USES FOR NETPBM
Version G5L.1232
One thing that we can do with Netpbm that's a lot of fun is artful
corruption of binary data. I used the following method to corrupt a
video file, but it will work for any data.
Let's say you've got a small video file called "video_file.mkv" that
is 5432101 bytes long. You can very easily turn it into an image that
you can edit using GIMP or Krita or any other image editing software
by simply grafting a Netpbm header onto the top of the file. In this
example we will use the PPM format specifically, for a full-color
image with 8-bit RGB pixels.
Before we do anything, we want to look at the hex values of the first
few bytes of the file we'll be corrupting. Let's use 'hexdump' to
read the hex values for the first 16 bytes:
$ hexdump -Cn 16 video_file.mkv
00000000 1a 45 df a3 01 00 00 00 00 00 00 23 42 86 81 01
|.E.........#B...|
00000010
The "00000000" is an index that tells us what part of the file we're
looking at. Obviously, we're at the very beginning. The part that
starts with "1a 45 df a3 ..." is the actual file data, and we'll want
to remember those first few bytes later.
Now, let's concern ourselves with creating a PPM image header to tack
onto our video file. The PPM header format is very simple. It can
just be an ASCII string starting with the letters "P6" followed by a
newline, then the image width and height (in pixels) separated by
whitespace and followed by a newline, then the maximum value of each
color component, again, followed by a newline; like this:
P6
[width] [height]
[pixel_depth]
To determine the width and height to use in your header, divide your
file's size (in bytes) by 3 (each pixel will require three bytes,
remember), then find the square root. For our example video_file.mkv:
√ 5432101 ÷ 3 = 1345.6226563689142. We can only use integer
dimensions though, so discard or round the fractional part. Our image
will be 1345 x 1345 pixels.
For the pixel_depth, or maximum value of each color component, just
use 255 for a the typical eight-bits-per-color RGB format.
So, our PPM header will look like this:
P6
1345 1345
255
Very simple. Now, write that to a file by issuing:
$ echo -e "P6\n1345 1345\n255" > header
It's important that the header ends with a newline character. The
'echo' tool appends a newline by default, but if you decide to edit
the header or create it using a different tool, make sure there's a
newline after the "255".
Next, stick the header onto your video file:
$ cat header video_file.mkv > video_file_image.ppm
Now you can open video_file_image.ppm in your favourite graphics
editor and mangle it. One thing to keep in mind is that video files
will produce interesting glitches just by changing the occasional
byte or two, but changing a lot of bytes will make them unplayable.
After you've tweaked a few pixels, save the image and then remove the
PPM header. How do we do that? Well, this is why we did that hex dump
earlier. Your graphics editor will probably change the format of your
PPM header — GIMP certainly does — so we don't know exactly how
much of the top of the file we need to remove, but we do know what
the beginning sequence for the video portion of the file looks like,
so we can use 'sed' to erase everything before a given sequence of
bytes:
$ sed -ze 's/^.*\?\(\x1A\x45\xDF\xA3\)/\1/'
modified_video_file_image.ppm > corrupted_video_file.mkv
Did you catch that? Mind the part with "\x1A\x45\xDF\xA3". Those are
the first four bytes of the video file: 1A 45 DF A3, remember? I used
four bytes here; in some cases two would be enough, an in other cases
you might need more than four, but in most cases four bytes should be
enough of a pattern for sed to match.
And there you have it! If you haven't destroyed the bitstream too
badly, you should be able to open that file up in your video player
and enjoy its glitchy goodness. Before you can use your glitched file
in any kind of nonlinear video editor, you'll probably want to render
it to a new file using 'avconv' or whatever you like, because the
file that you corrupted might play okay in VLC or mplayer, but an
NLVE will probably not be happy with an error-ridden file.
LÆMEUR
<
[email protected]>
--
Excerpted from:
PUBLIC NOTES (G)
http://alph.laemeur.com/txt/PUBNOTES-G
©2016 Adam C. Moore (LÆMEUR) <
[email protected]>
