Using pack/unpack on a PNG file

sierpinski has asked for the wisdom of the Perl Monks concerning the following question:

Hello monks,

I've been reading about PNG files and pack/unpack for a little while, and thought I understood it enough to perform a simple operation, but apparently I was mistaken. I created a PNG file with another script, and now I'm trying to get to know pack/unpack so I can read/write the associated binary data. The script doesn't error, but only makes a single byte file with "very short lines (no magic)" as a result. Here is the code:

#!/usr/bin/perl

use strict;
use warnings;

my $ifh = open("<:raw", "./file-bar.png");

my $data = unpack('a*', $ifh);

my $outputfile = "newfile.png";

open (my $ofh, '>:raw', $outputfile) 
    or die "Could not open '$outputfile' $!";

print $ofh pack('a*', $data);
close $ofh;
print("Image created.\n");
[download]

Here are the file types of the initial and created files:

$ file file-bar.png 
file-bar.png: PNG image data, 400 x 300, 4-bit colormap, interlaced
$ file newfile.png 
newfile.png: very short file (no magic)
[download]

I've tried to piece together bits of code from various sources, so that's why it's not of the same style, but feel there's no point in cleaning up code that doesn't work yet. Any help is greatly appreciated.

Comment on Using pack/unpack on a PNG file Select or Download Code

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Re: Using pack/unpack on a PNG file by AnomalousMonk (Archbishop) on May 26, 2015 at 03:12 UTC
`my $ifh = open("<:raw", "./file-bar.png");` Wishful, also quite surprising that Perl seems happy with it even with warnings and strictures enabled. In any event, the "successful" result (1) of open-ing the file is assigned to `$ifh` and this is packed and written (apparently also "successfully") to another file. But it's still junk. `c:\@Work\Perl\monks\sierpinski>perl -wMstrict -MData::Dump -le "my $ifh = open('<:raw', 'junk.txt'); dd $ifh; " 1` [download] Can anyone explain to me how this works — with strictures enabled? Give a man a fish: `<%-(-(-(-<`	[reply] [d/l] [select]
Re^2: Using pack/unpack on a PNG file by choroba (Cardinal) on May 26, 2015 at 07:08 UTC
Documented behaviour: (...) if FILEHANDLE is an expression, its value is the real filehandle. (This is considered a symbolic reference, so use strict "refs" should not be in effect.) `system 'echo abc > 1.txt'; open '<:raw', '1.txt' or die $!; print while readline '<:raw'; unlink '1.txt';` [download] Update: I wouldn't recomend it, though, as it's confusing to a reader :-) The file is still opened with default layers: `print PerlIO::get_layers("<:raw"); # unixperlio` [download] لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ	[reply] [d/l] [select]
Re^3: Using pack/unpack on a PNG file by AnomalousMonk (Archbishop) on May 26, 2015 at 07:24 UTC
This is considered a symbolic reference, so use strict "refs" should not be in effect. Yes, but if strictures are enabled the example still runs (and strictures were also enabled above), so I'm still confused: `c:\@Work\Perl\monks\sierpinski>perl -wMstrict -le "system 'echo abc > 1.txt'; open '<:raw', '1.txt' or die $!; print while readline '<:raw'; " abc` [download] Update: I wouldn't recomend it, though, as it's confusing to a reader ... Darn right! Give a man a fish: `<%-(-(-(-<`	[reply] [d/l] [select]
Re^4: Using pack/unpack on a PNG file by choroba (Cardinal) on May 26, 2015 at 07:30 UTC
Re^5: Using pack/unpack on a PNG file by AnomalousMonk (Archbishop) on May 26, 2015 at 07:35 UTC
Re^4: Using pack/unpack on a PNG file by Anonymous Monk on May 26, 2015 at 09:48 UTC
Re: Using pack/unpack on a PNG file by RichardK (Parson) on May 25, 2015 at 22:49 UTC
unpack takes a string as it's second parameter not a file handle. See the basic debugging checklist for lots of helpful advice on tracking down problems.	[reply]
Re: Using pack/unpack on a PNG file by Anonymous Monk on May 26, 2015 at 00:42 UTC
Also, this is wishful thinking :) `my $ifh = open("<:raw", "./file-bar.png");`	[reply] [d/l]
Re: Using pack/unpack on a PNG file by sierpinski (Chaplain) on May 26, 2015 at 03:03 UTC
Somehow in searching for info, I didn't come across Pack/Unpack Tutorial (aka How the System Stores Data) until now, and it's shining a huge light on the whole thing, so it seems I should be able to figure this out after all.	[reply]
Re^2: Using pack/unpack on a PNG file by AnomalousMonk (Archbishop) on May 26, 2015 at 03:18 UTC
See also perlpacktut. Update: In general, pack/unpack are almost as much fun as regexes! Give a man a fish: `<%-(-(-(-<`	[reply] [d/l]
Re^2: Using pack/unpack on a PNG file by AnomalousMonk (Archbishop) on May 26, 2015 at 03:28 UTC
Also consider: `c:\@Work\Perl\monks\sierpinski>perl -wMstrict -le "my $s = 'some string -- could be any length'; ;; my $up = unpack 'a', $s; $s eq $up or die 'A: not identity operation'; ;; my $p = pack 'a', $up; $s eq $p or die 'B: not identity operation'; ;; print 'a-ok'; " a-ok` [download] Give a man a fish: `<%-(-(-(-<`	[reply] [d/l] [select]