in reply to Image::Magick resize question

If you die or warn when failing to open the image, you'll get much better diagnostics. The error message you have suggests that fh IMAGE is not being set to anything useful:

      open IMAGE, "< $picture" or die $!;

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will holler as soon as it sees that your file doesn't open. The message will tell you, more or less, what is lacking. I suspect that either file permissions are wrong, or paths are relative to the wrong directory.

To live by: Always Check Status of System Interfaces After Use.

Update: On the error checking and operator precedence issue, you could replace '||' in your $image->Read() statement with &&, but that form is confusing to most people. merlyn's two statement form is more readable to programmers without a strong background. in C.

After Compline,
Zaxo

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Re: Re: Image::Magick resize question
by emilford (Friar) on Jun 08, 2002 at 13:15 UTC
    Yeah, I usually always try to check return values in my code. I actually do catch the return code in my open(image) code like you suggested.

    My actual code is: 
    sub convert_images {

my ($array, $start_directory) = ($_[0], $_[1]);

    foreach my $picture (@$array) {
        # read the image via a filehandle
        my $image = Image::Magick->new;
        open(IMAGE, "<$start_directory\\$picture") || die "Unable to o
+pen image: $!";
        $image->Read(file=>\*IMAGE) || die "Unable to read image: $!";
        close(IMAGE);
    
        # resize the image to the correct width and height
        $image->Resize(geometry=>'450X300') || die "Unable to resize i
+mage: $!";
    
        # write the image back to disk
        $image->Write(filename=>"$picture", compression=>'None') || di
+e "Unable to write image: $!";
    }

}

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    Even when I run this, the program does not die at the open command. If it did, I'd see my error message "Unable to open image: $!" somewhere in the script error message. That means the file is being opened correctly, right? The line that it complains about is the $image->Resize(geometry='450X300'); line. Am I doing something wrong there? Thanks for the response.
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      $image->Read(file=>\*IMAGE) || die "Unable to read image: $!";

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      According to my docs, that should be: 
      { my $result = $image->Read(file => \*IMAGE);
  die $result if $result;
}

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      Perhaps another pass over the manual would be useful, since your code is quite different from anything I've ever seen in the docs. No results are ever returned in $!.

      -- Randal L. Schwartz, Perl hacker

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      Just a question on the notation used for the return value for Image::Magick::Read. I understand from merlyn's post that the Read function does not return a result in the error variable ($!).

      Let's just pretend that it did return a value (even though it doesn't). Even then, wouldn't the line

      $image->Read(file=>\*IMAGE) || die "Unable to read image: $!";

      be better written as

      $image->Read(file=>\*IMAGE) or die "Unable to read image: $!";

      The difference being using or instead of ||. Not that it will do anything in this case, but when checking for return values in $!, is one not always suppose to use the "or" operator rather than "||"? I believe I was once told that using "or" (since it has a higher precedence) is always preferred when dealing with this type of situation. Did I hear wrong or am I right? Or does it depend upon the type of function called?

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        my $y = int rand 10 or die; This will work correctly since or binds losely. 10 is regarded as a parameter to rand which is regarded as a parameter to int; the result will be tested with or and die 1 out of 10 times. my $y = int rand 10 || die; This will never die since || binds tightly and causes 10 || die to be evaluated first. The result of that expression is then passed to rand as a parameter, and so on.

        Sometimes, very rarely, you actually want the tight binding rather than the lose, however.

        Makeshifts last the longest.

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