How can I add a drop-shadow to an image using PHP?

Question

I am looking for a way to add a drop shadow to an image using PHP. Before you answer or vote to close: I am not looking to do this using CSS or HTML. I want to generate an image file. This is not a duplicate of this question nor this one.

I am looking for a very specific effect. For example, given this input image:

I want to produce the following image:

---

TL;DR: This image above was generated using Photoshop's Drop Shadow effect. I want a look very similar to that. For reference, here's the settings our design team used. Ideally, I'd have similar control in my code for angle, distance, opacity, etc:

I have full access to our debian-linus-based servers, so any GD or ImageMagick solution will work. As will any FOSS linux software solution, although I'd prefer a way to do it with IM or GD as those are already installed and don't require new software to be installed.

The shadow must be able to be placed on a transparent, non-rectangular PNG!

I'm asking the question mainly because the scripts and solutions I have found on the web either only produce rectangular shadows, look like total poo, or just plain don't work at all.

Answer 1

Just for the hell of it (I know it was answered and accepted): a few months ago, in response to a question on graphic design stackexchange about recovering a mask from a PNG where the source file was lost I slapped together something which uses PHP GD functions to extract the alpha channel from a transparent PNG. As Joe in a comment mentioned above, you can use the alpha channel as the drop shadow, merely offset it by x and y pixels, and then apply an image convolution blur filter to it, then copymerge the original on top. Following code is probably SLOW and proof of concept, but it is a start and it is in PHP as you originally requested.

<?php

$im = imagecreatefrompng('./images/alphatest_nolayer.png');
$w = imagesx($im);
$h = imagesy($im);

$om = imagecreatetruecolor($w,$h);

for ($x = 0; $x < $w; $x++) {
    for ($y = 0; $y < $h; $y++) {
        $rgb = imagecolorat($im, $x, $y);
        $colors = imagecolorsforindex($im,  $rgb);

        $orgb = imagecolorallocate($om,$colors['alpha'],$colors['alpha'],$colors['alpha']);
        imagesetpixel($om,$x,$y,$orgb);
    }
}

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

imagedestroy($om);
imagedestroy($im);

?>