You might be interested in Image::Math::Constrain

Update:  Example:

#!/usr/bin/perl

use Image::Math::Constrain;

my $math = Image::Math::Constrain->new(800, 600);

my ($width, $height, $scale) = $math->constrain(1123, 1243);

printf "new width  = %.4f\n",   $width;
printf "new height = %.4f\n\n", $height;

printf "old aspect ratio = %.4f\n", 1123 / 1243;
printf "new aspect ratio = %.4f\n", $width / $height;  # same as old
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The following snippet is taken from Generate thumbnail and small overstruck images for web publication. $thumbX, $thumbY are the desired dimensions. The $portrait stuff allows the rectangle to be rotated to generate a landscape or portrait image depending on the aspect ratio of the original image. You probably want to omit that for your purposes.

  my ($height, $width) = $image->Get ('height', 'width');
  my $portrait = $height > $width;
  my ($x, $y) = ! $portrait ? ($thumbX, $thumbY) : ($thumbY, $thumbX);
  my $scaleX = $x / $width;
  my $scaleY = $y / $height;
  my $scale = $scaleX < $scaleY ? $scaleX : $scaleY;

  $thumb->Resize (width=>$width * $scale+0.5, height=>$height * $scale
++0.5);
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#!/usr/bin/perl

use warnings;
use strict;

my $height = "1000";
my $width = "1234";

my $maxh = "600";
my $maxw = "800";

if ($height > $maxh || $width > $maxw)
{
    my $hratio = $height / $maxh;
    my $wratio = $width / $maxw;

    my $newh = $height / $hratio;
    my $neww = $width / $hratio;

   print "Height is now $newh, Width is now $neww";  
}
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It's not quite as trivial as it might seem at first (most of us who have tried probably got it wrong the first time :)

Just as an example, take a width of 1000 and a height of 100. Your algorithm would compute 6000x600 (W x H), although the expected result would be 800x80 (i.e. max width doesn't exceed 800 and aspect ratio stays at 10:1)

I know how to determine height and width with Image::Info but I need to know how I can HTML resize (yes, just HTML. I know it'll load the whole image anyway but it'll fit better)

Just send either the maximum allowed height or the maximum width along with the image to the browser and the browser will calculate the other length such that the image is shown/resized proportionally.

Note: Even though the above will work, you can save the network and the web client a lot of bandwidth and cpu cycles if you properly resize the picture server side and just transport a downsized picture over the network.

There is an easy way to do it in HTML... If you want a fixed width or a fixed height: - Fixed height (the width will be adjusted automatically keeping the ratio of the picture) <img src="07cat.jpg" height="96" /> - Fixed width (the height will be adjusted automatically keeping the ratio of the picture) <img src="07cat.jpg" width="96" /> I hope this will help too...

A bit of old school HTML. <img src="index.jpg" width="800" height="*"> The asterisk keeps it proportional, no matter the original size. Asterisks also work with table dimensions.

It should also be noted that the following is very useful, dynamic resizing based on the window and element sizes:

<img src="index.jpg" width="90%" height="*">
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