Gradient overlay on transparent png

On my website I have a PHP script that turns this image into a recolored version of the same image based on a $_GET query. An example with the query being 07a is available here.

Here is the PHP script that does this:


    if (isset($_GET['color']) && $_GET['color'] !== "") $color = preg_match('/^([a-fA-F0-9]{3}|[a-fA-F0-9]{6})$/',$_GET['color']) ? $_GET['color'] : '777777';
    else $color = '000000';

    if (strlen($color) === 3) $color = $color[0].$color[0].$color[1].$color[1].$color[2].$color[2];

    $color = str_split($color,2);
    foreach ($color as $k=>$m){ $color[$k] = intval($color[$k],16); }

    $img = imageCreateFromPng('splat-base.png');
    $background = imagecolorallocate($img, 0, 0, 0);
    imagecolortransparent($img, $background);
    imagealphablending($img, false);
    imagesavealpha($img, true);

    imagefilter($img, IMG_FILTER_COLORIZE, intval($color[0] - 255 ,16), $color[1], $color[2]);

    header('Content-type: image/png');

The actual work is done by the imagefilter function, the rest is just for preserving transparency. What I would like to do, is to have 2 color values sent via $_GET, and create a top-to-bottom gradient applied to the image with the given color values. Here is an example with the values being 08f and 0a7 (lighter color will always be first):

gradient splat

One possible solution I thought of was to generate an image of the same size as my splat-base.png (1000 x 1000 px) with the gradient, then with some trickery apply that over the splat image, but I don't know how to go about generating that gradient image or how to apply it to the splat image either.

I don't have the ability to install PHP extensions like ImageMagick, so I must work with the built in functions of PHP and libraries that only use those.


So, I googled how to create an image gradient in php. The idea is that, you define the start and end color, and define a step size of $stepSize = ($end-$start)/($imgHeight). You make a for loop, and in each step of the loop, create a 1 pixel tall rectangle that is as wide the image, and set its color to $start+$i*$stepSize. That idea, and code, I found here:

That code creates a new image of a linear gradient. For your situation, I ran with that basic idea: Shave off 1 pixel rows from your source image, and IMG_FILTER_COLORIZE each row to the color of the gradient.

Here is the code:


//move alpha preserving code to its own function 
//since it is used several times
function preserveAlpha(&$img) {
    $background = imagecolorallocate($img, 0, 0, 0);
    imagecolortransparent($img, $background);
    imagealphablending($img, false);
    imagesavealpha($img, true);

$img = imagecreatefrompng('splat-base.png');
//grab width and height
$width = imagesx($img);
$height = imagesy($img);

//the final image
$final = imagecreatetruecolor($width, $height);


//start and end values of the gradient
$start = $_GET['start'];
$end = $_GET['end'];

//convert 3 color codes to 6
if (strlen($start) == 3) {
    $start = $start[0] . $start[0] . $start[1] . $start[1] . $start[2] . $start[2];

if (strlen($end) == 3) {
    $end = $end[0] . $end[0] . $end[1] . $end[1] . $end[2] . $end[2];

//make sure the lighter color is first

$starthex = intval($start, 16);
$endhex = intval($end, 16);
if ($endhex > $starthex) {
    //then we need to flip
    $temp = $end;
    $end = $start;
    $start = $temp;

//convert from hex to rgb
$s = array(
    hexdec(substr($start, 0, 2)),
    hexdec(substr($start, 2, 2)),
    hexdec(substr($start, 4, 2))
$e = array(
    hexdec(substr($end, 0, 2)),
    hexdec(substr($end, 2, 2)),
    hexdec(substr($end, 4, 2))

//the for loop. go through each 1 pixel row of the image, 
//shave it off, apply a filter, and append it to our new image

for ($i = 0; $i < $height; $i++) {
    //grab the $ith row off of the image
    $pixelRow = imagecreatetruecolor($width, 1);
    imagecopy($pixelRow, $img, 0, 0, 0, $i, $width, 1);
    //calculate the next color in the gradient
    $r = intval($s[0] - ((($s[0] - $e[0]) / $height) * $i));
    $g = intval($s[1] - ((($s[1] - $e[1]) / $height) * $i));
    $b = intval($s[2] - ((($s[2] - $e[2]) / $height) * $i));
    //apply the filter
    imagefilter($pixelRow, IMG_FILTER_COLORIZE, $r - 255, $g, $b);
    //append it to our new iamge
    imagecopy($final, $pixelRow, 0, $i, 0, 0, $width, 1);

header('Content-Type: image/png');

/* Some Cleanup */