Код:

#!/usr/bin/perl -ws
# jpegrescan by Loren Merritt
# Last updated: 2008-11-29 / 2011-11-01
# This code is public domain.

use File::Slurp;
@ARGV==2 or die "usage: jpegrescan in.jpg out.jpg\ntries various progressive scan orders\n";
$fin = $ARGV[0];
$fout = $ARGV[1];
$ftmp = "$fout-$$.scan";
$jtmp = $fout;
$verbose = $v;
$quiet = $q;
undef $_ for $v,$q;
undef $/;
$|=1;

# convert the input to baseline, just to make all the other conversions faster
# FIXME there's still a bunch of redundant computation in separate calls to jpegtran
open $OLDERR, ">&", STDERR;
open STDERR, ">", $ftmp;
open TRAN, "-|", "./jpegtran", "-v", "-optimize", $fin or die;
write_file($jtmp, <TRAN>);
close TRAN;
open STDERR, ">&", $OLDERR;

$type = read_file($ftmp);
$type =~ /components=(\d+)/ or die;
$rgb = $1==3 ? 1 : $1==1 ? 0 : die "not RGB nor gray\n";

# FIXME optimize order for either progressive transfer or decoding speed
sub canonize {
    my $txt = $prefix.$suffix.shift;
    $txt =~ s/\s*;\s*/;\n/g;
    $txt =~ s/^\s*//;
    $txt =~ s/ +/ /g;
    $txt =~ s/: (\d+) (\d+)/sprintf ": %2d %2d", $1, $2/ge;
    # treat u and v identically. I shouldn't need to do this, but with jpegtran overhead it saves 9% speed. cost: .008% bitrate.
    $txt =~ s/^2:.*\n//gm;
    $txt =~ s/^1:(.+)\n/1:$1\n2:$1\n/gm;
    # dc before ac, coarse before fine
    my @txt = sort {"$a\n$b" =~ /: *(\d+) .* (\d);\n.*: *(\d+) .* (\d);/ or die; !$3 <=> !$1 or $4 <=> $2 or $a cmp $b;} split /\n/, $txt;
    return join "\n", @txt;
}

sub try {
    my $txt = canonize(shift);
    return $memo{$txt} if $memo{$txt};
    write_file($ftmp, $txt);
    open TRAN, "-|", "./jpegtran", "-scans", $ftmp, $jtmp or die;
    $data = <TRAN>;
    close TRAN;
    my $s = length $data;
    $s or die;
    $memo{$txt} = $s;
    !$quiet && print $verbose ? "$txt\n$s\n\n" : ".";
    return $s;
}

sub triesn {
    my($bmode, $bsize);
    my ($limit, @modes) = @_;
    my $overshoot = 0;
    for(@modes) {
        my $s = try($_);
        if(!$bsize || $s < $bsize) {
            $bsize = $s;
            $bmode = $_;
            $overshoot = 0;
        } elsif(++$overshoot >= $limit) {
            last;
        }
    }
    return $bmode;
}

sub tries { triesn(99, @_); }

$prefix = "";
$suffix = "";

if($rgb) {
    # 012 helps very little
    # 0/12 and 0/1/2 are pretty evenly matched in frequency, but 0/12 wins in total size if every image had to use the same mode
    # dc refinement passes never help
    $dc = tries("0: 0 0 0 0; 1 2: 0 0 0 0;",
                "0: 0 0 0 0; 1: 0 0 0 0; 2: 0 0 0 0;");
    # jpegtran won't let me omit dc entirely, but I can at least quantize it away to make the rest of the tests faster.
    $prefix = "0 1 2: 0 0 0 9;";
} else {
    $dc = "0: 0 0 0 0;";
    $prefix = "0: 0 0 0 9;";
}

# luma can make use of up to 3 refinement passes.
# chroma can make use of up to 2 refinement passes.
# refinement passes have some chance of being split (luma: 4%,4%,4%. chroma: 20%,8%) but the total bit gain is negligible.
# msb pass should almost always be split (luma: 87%, chroma: 81%).
# I have no theoretical reason for this list of split positions, they're just the most common in practice.
# splitting into 3 ections is often slightly better, but the total number of bits saved is negligible.
# FIXME: penalize lots of refinement passes because it's slower to decode. if so, then also force overwrite if bigger than the input.
sub try_splits {
    my $str = shift;
    my %n = map {$_ => sprintf "$c: 1 %d $str; $c: %d 63 $str;", $_, $_+1} 2,5,8,12,18;
    my $mode = triesn(2, "$c: 1 63 $str;", @n{2,8,5});
    return $mode if $mode ne $n{8};
    return triesn(1, $mode, @n{12,18});
}

foreach $c (0..$rgb) {
    my @modes;
    my $ml = "";
    for(0..($c?2:3)) {
        push @modes, "$c: 1 8 0 $_; $c: 9 63 0 $_;".$ml;
        $ml .= sprintf("$c: 1 63 %d %d;", $_+1, $_);
    }
    my $refine = triesn(1, @modes);
    $refine =~ s/.* (0 \d);//;
    $ac .= $refine . try_splits($1);
}

$prefix = "";
undef %memo;
$mode = canonize($dc.$ac);
try($mode);
$size = $memo{$mode};
!$quiet && print "\n$mode\n$size\n";
$old_size = -s $fin;
if($size < $old_size) {
        write_file($fout, $data);
}
unlink $ftmp;

Код:

Объявление переменных

#Комментарий - конвертирование файла в baseline, чтобы ускорить все остальные конвертации
jpegtran -v -optimize $file

объявление $type, не знаю, что это, скорее всего определения формата - baseline или progressive
объявление $rgb - цветового пространства (color space), проверяет, является ли формат файла RGB, либо BW, CMYK не пропускает

#FIXME
sub canonize, месиво из регулярок(help)
sub try, запуск jpegtran с параметром -scans file
sub triesn, не понять
sub tries { triesn(99, @_); } - ?

if($rgb) {
какая-то магия, очевидно запуск функции tries с заданными параметрами
$dc = tries("0: 0 0 0 0; 1 2: 0 0 0 0;",
                "0: 0 0 0 0; 1: 0 0 0 0; 2: 0 0 0 0;");
} else не интересует

# комментарий
# с упоминанием
# о дальнейшей работе с битами
не понятный мне код