dereferencing problem

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

Greetings and salutions;

I am using a subroutine I found to obtain the dimansions of a gif image. The sub returns a reference to the data but when I attempt to dereference it ie. $$height or $$width I get SCALAR(0xed880) SCALAR(0xed898) instead of the values I'm looking for. I have tried returning the vars $height and $width directly but the data they contain is innacurate when compared to the original image.

Does the expression: return \($width, $height);
require some other syntax to dereference properly?

Should the syntax be: return (\$width, \$height);
to dereference properly?

This is how I am attempting to use the subroutine:

# # # # # # # # # # # # # # # # # # # # # # # # # # # # # # 
#     Display the uploaded file.                          #
# # # # # # # # # # # # # # # # # # # # # # # # # # # # # # 
  $MyFileName = "$ClassPath/classimg/$UpLoadedImg";
  if (-e $MyFileName){
    ($width, $height) = gifdim($MyFileName);
          if ($$height > 150){
            # If the image is too tall for the display area
            # reduce the height to fit and recalculate the width
            $NewHeight = 150;
            $NewWidth = $$width - ($$height - 150);
            }
          elsif ($width > 150){
            # If the image is too wide for the display area
            # reduce the width to fit and recalcualte the height
            $NewWidth = 150;
            $NewHeight = $$height - ($$width - 150);
            }
          # The image will fit in the display area so leave it alone
          else {
            $NewHeight = $$height;
            $NewWidth = $$width;
            }
  }
# more code ...


sub gifdim ($) {
# # # # # # # # # # # # # # # # # # # # # # # # # # # # # #
#        Get the dimensions of a gif                      #
# # # # # # # # # # # # # # # # # # # # # # # # # # # # # # 

    my $filename = $_[0];

    open(GIF, $filename) || return (undef, undef);
    my $buf = '';
    my $n = read GIF, $buf, 10;
    close GIF;

   return (undef, undef) if $n < 10;
    my ($head, $width, $height) = unpack("A6vv", $buf);
    ($head, $width, $height) = unpack("A6vv", $buf);

    return (undef, undef) unless $head =~ /^GIF8[79]a/;
   return \($width, $height);
#    return ($width, $height);
}
[download]

I'm sure I'm missing something obvous but I'm stumped

My thanks to The Monastery
NereDoWell

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Re: dereferencing problem by Mr. Muskrat (Canon) on Mar 14, 2003 at 22:14 UTC
Once suggestion from me... drop that subroutine like a hot potato and use Image::Info for retreiving the image information! `use Image::Info qw(dim); my $MyFileName = "$ClassPath/classimg/$UpLoadedImg"; my ($width, $height); if (-e $MyFileName){ my $info = image_info($MyFileName); ($width, $height) = dim($info); ...` [download]	[reply] [d/l]
Re: dereferencing problem by dga (Hermit) on Mar 14, 2003 at 22:11 UTC
Briefly looking at your code, I would try removing all the \$ and $$ stuff and I think it will be a good start. Use the `return( $width, $height );` and get rid of the other one. Then in the main script you test like so. `if($height > 150 ) { # ... } elsif($width > 150) { # ... stuff here } # ... more stuff here` [download] In this program you dont need any references just the regular values $var etc. Update: You might also try scaling the images by the same percent instead of by the same numeric amount. Say its 200x100 then 150/200 is .75 so make the 100 dimension 75 (100*.75) instead of 50 (100-(200-150)) like the current code would.	[reply] [d/l] [select]
Re: Re: dereferencing problem by NereDoWell (Initiate) on Mar 14, 2003 at 23:04 UTC
dga Many thanks; I got sidetracked to the dereferencing when I got the wrong values for height and width from the subroutine. After scrapping that and going back to using the vars directly I noticed that I was still sending the default image to the sub. There was no way the sub could return the values I was looking for. All is now well. I also took your advice and altered the resizing to a percentage. I would also like to say thank you here to bbfu and Mr. Muskrat for their help and advice. NereDoWell	[reply]
Re: dereferencing problem by bbfu (Curate) on Mar 14, 2003 at 22:12 UTC
Use the line that's commented (`return ($width, $height);`) instead of the one w/ the backslash. There's no reason to return references in this case. (Update: Oh yes, and of course remove all the dereferencing.) If you get inaccurate results then, then it's a problem with the routine and you should scrap it. Some possible alternates include GD and Image::Magick. Update: Oh, and just FYI, the two lines: `return \($width, $height); # and... return (\$width, \$height);` [download] do exactly the same thing. In other words, taking a reference to a list (not an array, mind you) returns a list whose elements are all references to the elements in the original list. bbfu Black flowers blossum Fearless on my breath	[reply] [d/l] [select]
Re: dereferencing problem by thpfft (Chaplain) on Mar 15, 2003 at 17:53 UTC
`use Image::Size;` merlyn's old article has the details.	[reply]
Re: dereferencing problem by Desdinova (Friar) on Mar 14, 2003 at 22:49 UTC
First the others are right there is no real reason to complcate this with references a simple copy by value is fine here, and there is no need to redo what has been done... Now looking at the question I have to say that I grabbed your code and stripped to the most simple form and tested it. It doesn't have the derefeencing problem you mention `($width, $height) = gifdim('test_text'); print "Height = $$height Width = $$width \n"; exit; sub gifdim ($) { my $filename = $_[0]; my $height =200; my $width =300; return \($width, $height); }` [download] output:Height = 200 Width = 300 So im not sure what problem you are having.	[reply] [d/l]
Re: Re: dereferencing problem by NereDoWell (Initiate) on Mar 14, 2003 at 23:19 UTC
Desdinova; Thanks for looking. I have stopped using the reference and I am now returning the vars by value. My true error was in failing to send the proper image to the subroutine. This obvously returned values that did not match the image I was maniplating. Since the original subroutine returned references I thought I might have introduced an error when I changed it. I tried using the references to check this and got sidetracked. Just why $$var gave me SCALAR(0xed880) instead of the value located at \$var I don't know but since the new structure works I'll come back to that issue later. Thanks and Thanks again to all NereDoWell	[reply]