Functions for calculating new proportional sizes for images

image_size_math.php

<?php
/*
*  Get a new larger proportional height and width for an image
*  Adapted from http://stackoverflow.com/a/4820929/648844
*  
*  Parameters: original width, original height, minimum width, minimum height
*  Return: null if image exceeds minimums or array with rounded width and height
*/
function image_resize_up($orig_w, $orig_h, $MIN_W, $MIN_H){
    $ratio = $orig_w * 1.0 / $orig_h;

    $w_undersized = ($orig_w < $MIN_W);
    $h_undersized = ($orig_h < $MIN_H);
    
    if ($w_undersized OR $h_undersized)
    {
          $new_w = round( max($MIN_W, $ratio * $MIN_H) );
          $new_h = round( max($MIN_H, $MIN_W / $ratio) );
          return array('width' => $new_w, 'height' => $new_h);
    }
    return null;
}

/*
*  Get a new smaller proportional height and width for an image
*  Parameters: original width, original height, maximum width, maximum height
*  Return: null if image is under maximum or array with rounded width and height
*/
function image_resize_down($orig_w, $orig_h, $MAX_W, $MAX_H){
    $ratio = $orig_w * 1.0 / $orig_h;

    $w_undersized = ($orig_w > $MAX_W);
    $h_undersized = ($orig_h > $MAX_H);
    
    if ($w_undersized OR $h_undersized)
    {
          $new_w = round( min($MAX_W, $ratio * $MAX_H) );
          $new_h = round( min($MAX_H, $MAX_W / $ratio) );
          return array('width' => $new_w, 'height' => $new_h);
    }
    return null;
}