sub JPEGData {
    my $self = shift;
    my $pos = 2;
    my $len = length($_[0]);
    $self->{_jepgdata} = $_[0];

    while((my $s=substr($_[0], $pos, 2)) ne "\xff\xc0" and $pos < $len) {
        $pos += 2+unpack('n', substr($_[0], $pos+2,2));
    }
    croak "Can't get the width and height of JPEG data" if $pos>=$len;
    @{$self}{qw/_width _height/} = unpack('nn', substr($_[0], $pos+5,4));
    undef $self->{_jpegfile};
    $self;
}