Creating gradients in PHP


A few days ago I was looking around for methods to calculate color gradients in PHP. Nothing I found seemed to fit (most of them were centered around creating pictures with GD), so I decided to roll my own. You can see the code below.

There is a small test script attached to the end, which generates the image you can see here. It supports multiple color spaces (RGB, YUV and HSV), the idea being that some gradients are more natural in color spaces other than RGB. By support I mean that the gradient is calculated in the given color space, after which it is transformed back into RGB.

Hope you find this useful.

<?php
 class ColorGradient {
    function rgb2yuv($r, $g, $b) {
      # adapted from: http://www.fourcc.org/fccyvrgb.php
   $y = 0.299 * $r + 0.587 * $g + 0.114 * $b;
   return array(
    $y,
    ($b - $y) * 0.565,
    ($r - $y) * 0.713
   );
    }
    
    function rgb2hsv($r, $g, $b) {
      # adapted from: http://mikeheuss.com/scripts/ColorToy/
      $min = min($r, $g, $b);
      $max = max($r, $g, $b);
      
      $v = $max/255;
      $delta = $max-$min;
      
      if ($max != 0)
        $s = $delta/$max;
      else
        return array(-1, 0, $v);
  
      if ($r == $max) $h = ($g-$b)/$delta; 
      else if ($g == $max)  $h = 2+($b-$r)/ $delta;
      else $h = 4+($r-$g)/$delta;       
        
      $h*=60;
      if ($h<0) $h+=360;
        
      return array($h, $s, $v);
    }
    
    function ColorGradient($colors, $minimum = 0.0, $maximum = 1.0, $colorspace = 'rgb', $use_cache = true) {
      $funcStr = '';
      $limits = array_keys($colors); sort($limits);
      $low_limit = array_shift($limits);
      
      $gap = $maximum - $minimum;
      $funcStr .= "$value = $value * 100 / $gap;n";
      
      foreach ($colors as $limit => $color) {
        preg_match('/^#?([0-9a-f]{2})([0-9a-f]{2})([0-9a-f]{2})$/iD', $color, $m);
        $colors[$limit] = array(hexdec($m[1]), hexdec($m[2]), hexdec($m[3]));
        $orig_colors[$limit] = $colors[$limit];
        if ('yuv' == $colorspace)
          $colors[$limit] = $this->rgb2yuv($colors[$limit][0], $colors[$limit][1], $colors[$limit][2]);
        else if ('hsv' == $colorspace) 
          $colors[$limit] = $this->rgb2hsv($colors[$limit][0], $colors[$limit][1], $colors[$limit][2]);
      }
      
      foreach ($limits as $up_limit) {
        $m_start = $colors[$low_limit]; $m_end = $colors[$up_limit];
        
        $funcStr .= "if ($value >= $low_limit && $value <= $up_limit) {n";
        $gap = ($m_end[0] - $m_start[0]) / 100;
        $funcStr .= "  $a = {$m_start[0]} + ({$gap}) * $value;n";
        $gap = ($m_end[1] - $m_start[1]) / 100;
        $funcStr .= "  $b = {$m_start[1]} + ({$gap}) * $value;n";
        $gap = ($m_end[2] - $m_start[2]) / 100;
        $funcStr .= "  $c = {$m_start[2]} + ({$gap}) * $value;n";
        $funcStr .= "}n";
        
        $low_limit = $up_limit;
      }
      
      $funcStr .= "if ($value <= 0) return array({$orig_colors[0][0]}, {$orig_colors[0][1]}, {$orig_colors[0][2]});n";
      $funcStr .= "if ($value >= 100) return array({$orig_colors[100][0]}, {$orig_colors[100][1]}, {$orig_colors[100][2]});n";
      
      $funcStr .= "n";
      if ($use_cache) {
        $funcStr .= "$c = "$a_$b_$c";n";
        $funcStr .= "if (array_key_exists($c, $cache)) {n";
        $funcStr .= "  $a = $cache[$c][0]; $b = $cache[$c][1]; $c = $cache[$c][2];n";
        $funcStr .= "} else {n";
      }
      
      if ('yuv' == $colorspace) {
        $funcStr .= "$y = $a; $u = $b; $v = $c;n";
    $funcStr .= "$a = $y + 1.403 * $v;n";
    $funcStr .= "$b = $y - 0.344 * $u - 0.714 * $v;n";
    $funcStr .= "$c = $y + 1.770 * $u;n";
      }
      else if ('hsv' == $colorspace) {
        $funcStr .= "$h = $a; $s = $b; $v = $c;n";
        $funcStr .= "$a = $b = $c = 0;n";
        $funcStr .= "if (0 == $s) { $a = $b = $c = $v; }n";
        $funcStr .= "else {n";
        $funcStr .= "  $h /= 60; $i = floor($h); $f = $h-$i; $p=255*$v*(1-$s);n";
        $funcStr .= "  $q=255*$v*(1-($s*$f)); $t=255*$v*(1-$s*(1-$f)); $v*=255;n";
        $funcStr .= "  switch($i) {n";
        $funcStr .= "    case 0: $a = $v; $b = $t; $c = $p; break;n";
        $funcStr .= "    case 1: $a = $q; $b = $v; $c = $p; break;n";
        $funcStr .= "    case 2: $a = $p; $b = $v; $c = $t; break;n";
        $funcStr .= "    case 3: $a = $p; $b = $q; $c = $v; break;n";
        $funcStr .= "    case 4: $a = $t; $b = $p; $c = $v; break;n";
        $funcStr .= "    default: $a = $v; $b = $p; $c = $q; break;n";
        $funcStr .= "  }n";
        $funcStr .= "}n";        
      }
      
      if ($use_cache) {
        $funcStr .= "  $cache[$c] = array($a, $b, $c);n";
        $funcStr .= "}n";
      }
      
      $funcStr .= "n";
      $funcStr .= "return array(intval($a), intval($b), intval($c));n";

      $this->cache = array();
      $this->gradientFunc = create_function('$value,$cache', $funcStr);
    }
    
    function getColorArray($value) {
      return call_user_func($this->gradientFunc, $value, $this->cache);
    }
    
    function getColorWeb($value) {
      $r = $this->getColorArray($value);
      return sprintf('#%02X%02X%02X', $r[0], $r[1], $r[2]);
    }
    
    function getColorGD($value, $image) {
      $r = $this->getColorArray($value);
      return imagecolorallocate($image, $r[0], $r[1], $r[2]);
    }
 }
 
 /*
  * Example code:
  
 $f = new ColorGradient(array(0 => 'FF0000', 100 => '00FF00'), 0.0, 1.0, 'rgb');
 $g = new ColorGradient(array(0 => 'FF0000', 100 => '00FF00'), 0.0, 1.0, 'yuv');
 $h = new ColorGradient(array(0 => 'FF0000', 100 => '00FF00'), 0.0, 1.0, 'hsv');
 
 $im = imagecreatetruecolor(1000, 30);
 for ($i = 0; $i < 1000; $i++) {
    $c = $f->getColorGD($i / 1000, $im);
    imageline($im, $i, 0, $i, 9, $c);    
    $c = $g->getColorGD($i / 1000, $im);
    imageline($im, $i, 11, $i, 19, $c);
    $c = $h->getColorGD($i / 1000, $im);
    imageline($im, $i, 21, $i, 30, $c);
    
    print $i . " -> " . $f->getColorWeb($i / 1000) . ' ' . $g->getColorWeb($i / 1000) . ' ' . $h->getColorWeb($i / 1000) . "n";
 } 
 imagepng($im, 'gradient.png');
 
 //*/


One response to “Creating gradients in PHP”

  1. This is a nice little script. Would it be alright if I used an adaptation of it on my PHP framework? We’re working on adding an image ‘helper’ library as one of the new extensions in a future release, having a feature like this would be a great addition. Let me know!

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