Re: getting the dimensions of a picture (jpeg)

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Re^2: getting the dimensions of a picture (jpeg) by blazar (Canon) on Feb 17, 2005 at 10:54 UTC
As side notes: `#!/usr/bin/perl -w use strict;` [download] Notice that nowadays (unless you have serious backward compatibility issues) it is recommended to `use warnings;` [download] instead. Since when I made a similar remark somebody popped out asking why I thought the latter was a better alternative, I'll point out in advance that no, I'm not explaining it. perldoc warnings and perldoc perlrun give some clues about it. `my $jpg = shift or die "$!\n";` Huh?!? `$!` is meaningful only after a failure of a system call (well, "system library"). At this point it may be anything... `my $data = get_file($jpg); my @res = sizeJPG($data); print "$jpg width $res[0] height $res[1]\n";` [download] Please, do not misunderstand me: I know that this is a minimal example, but why do you put the return values of your sub into an array only to later use its entries separately? I.e., wouldn't have `my $data = get_file($jpg); my ($h,$v) = sizeJPG($data); print "$jpg width $h height $v\n";` [download] been clearer? Also, in case the regex below is not matched, the return values will be undef, and this will issue a warning in the print statement above. And if you locally disable 'uninitialized' warnings, then you'll get an IMHO inconsistent output. You may want to treat this case in a different way... `sub sizeJPG { return unless $_[0]; my ( $width, $height ); ( $height, $width ) = unpack( "nn", $1 ) if $_[0] =~ /\xFF\xC0...(....)/; return ( $width, $height ); }` [download] I like to do `local $_=shift;` [download] if possible, but that's just a personal preference. As a general remark you can also use the return value of the match operator, but indeed in this case it can be slightly awkward: `my ($width,$height)=unpack "nn", (/\xFF\xC0...(....)/)[0];` [download] and it will also issue a warning if the regex is not matched. So a possibly better solution may be: `my ($width,$height)=unpack "nn", /\xFF\xC0...(....)/ ? $1 : ''; # or "\0" x 4, maybe?` [download] Let's go on... `sub get_file { open FILE, $_[0] or die $!; binmode FILE; local $/; my $data = <FILE>; close FILE; return $data; }` [download] Why not `sub get_file { local $/; open my $fh, '<:raw', shift or die $!; <$fh>; }` [download] instead? (But then I'd probably use a `do` block. In the meantime we continue to wait for Perl6's `.slurp`!!)	[reply] [d/l] [select]
Re^3: getting the dimensions of a picture (jpeg) by zentara (Cardinal) on Feb 17, 2005 at 13:37 UTC
All good points. I was just in a hurry, and saw I had an old snippet which showed it. Last night I realized it could also probably be sped up too, since it prints on each pixel. It probably could be re-written to process chunks at a time, and even regexed chunks at a time. The only problem is making sure the chunk boundaries are on pixel boundaries, so you don't break up the pixel-triples. I'm not really a human, but I play one on earth. flash japh	[reply]
Re^3: getting the dimensions of a picture (jpeg) by squirrel448 (Initiate) on Jun 10, 2008 at 18:51 UTC
I'm trying to play around with all this code you've posted but I can't seem to get past the original script's line 18-- "No such file or directory; it's also got a syntax error, "my $data = ;" crashes the script. Any way all these snippets can be squished together to make a usable script? Replacing that 'shift or die' thing at the top with 'picture.jpg' doesn't run as smoothly as I'd like it to :(	[reply]