Multiply a filter using the PHP GD library

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imagecolorat , PNG . , 50% ( (128, 128, 128)) (0, 0, 0, 64) 64, -. .

// bottom layer
$img1 = imagecreatefromjpeg(realpath(__DIR__.'/profilePic.jpg'));

// top layer
$img2 = imagecreatefrompng(realpath(__DIR__.'/border2.png'));
imagealphablending($img2, false);
imagesavealpha($img2, true);

$imagex = imagesx($img1);
$imagey = imagesy($img1);

$imagex2 = imagesx($img2);
$imagey2 = imagesy($img2);

// Prereq: Resize img2 to match img1, cropping beyond the aspect ratio
$w1 = max(min($imagex2, $imagex), $imagex);
$h1 = max(min($imagey2, $imagey), $imagey);

$w_using_h1 = round($h1 * $imagex2 / $imagey2);
$h_using_w1 = round($w1 * $imagey2 / $imagex2);

if ($w_using_h1 > $imagex) {
    fit($img2, $imagex, $imagey, 'HEIGHT', true);
}
fit($img2, $imagex, $imagey, 'WIDTH', true);

// Actual multiply filter
for ($x = 0; $x < $imagex; ++$x) {
    for ($y = 0; $y < $imagey; ++$y) {
        $rgb1 = imagecolorat($img1, $x, $y);
        $rgb2 = imagecolorat($img2, $x, $y);
        $idx1 = imagecolorsforindex($img1, $rgb1);
        $idx2 = imagecolorsforindex($img2, $rgb2);

        // Shift left 8, then shift right 7
        // same as multiply by 256 then divide by 128
        // approximate multiply by 255 then divide by 127
        // This is basically multiply by 2 but, expanded to show that
        // we are adding a fraction of white to the translucent image
        // $adder = ($idx2['alpha'] << 8 >> 7);
        $adder = ($idx2['alpha'] << 1);
        $rmul = min(255, $idx2['red']   + $adder);
        $gmul = min(255, $idx2['green'] + $adder);
        $bmul = min(255, $idx2['blue']  + $adder);

        $color_r = floor($idx1['red'] * $rmul / 255);
        $color_g = floor($idx1['green'] * $gmul / 255);
        $color_b = floor($idx1['blue'] * $bmul / 255);

        $newcol = imagecolorallocatealpha($img1, $color_r, $color_g, $color_b, 0);
        imagesetpixel($img1, $x, $y, $newcol);
    }
}
imagejpeg($img1, __DIR__.'/out.jpg');



/**
 * Fits an image to a $w x $h canvas
 * 
 * @param type $w Target width
 * @param type $h Target height
 * @param int $fit_which Which dimension to fit
 * @param bool $upscale If set to true, will scale a smaller image to fit the given dimensions
 * @param bool $padded If set to true, will add padding to achieve given dimensions
 * 
 * @return Image object
 */
function fit(&$img, $w, $h, $fit_which = 'BOTH', $upscale = false, $padded = true) {

    if (!in_array($fit_which, array('WIDTH', 'HEIGHT', 'BOTH'))) {
        $fit_which = 'BOTH';
    }
    $w0 = imagesx($img);
    $h0 = imagesy($img);

    if (!$upscale && $w0 <= $w && $h0 <= $h)
        return $this;

    if ($padded) {
        $w1 = max(min($w0, $w), $w);
        $h1 = max(min($h0, $h), $h);
    }
    else {
        $w1 = min($w0, $w);
        $h1 = min($h0, $h);
    }
    $w_using_h1 = round($h1 * $w0 / $h0);
    $h_using_w1 = round($w1 * $h0 / $w0);

    // Assume width, crop height
    if ($fit_which == 'WIDTH') {
        $w2 = $w1;
        $h2 = $h_using_w1;
    }
    // Assume height, crop width
    elseif ($fit_which == 'HEIGHT') {
        $w2 = $w_using_h1;
        $h2 = $h1;
    }
    elseif ($fit_which == 'BOTH') {
        if (!$padded) {
            $w2 = $w = min($w, $w_using_h1);
            $h2 = $h = min($h, $h_using_w1);
        }
        else {
            // Extend vertically
            if ($h_using_w1 <= $h) {
                $w2 = $w1;
                $h2 = $h_using_w1;
            }
            // Extend horizontally
            else {
                $w2 = $w_using_h1;
                $h2 = $h1;
            }
        }
    }

    $im2 = imagecreatetruecolor($w, $h);
    imagealphablending($im2, true);
    imagesavealpha($im2, true);

    $transparent = imagecolorallocatealpha($im2, 255, 255, 255, 127);
    imagefill($im2, 0, 0, $transparent);

    imagealphablending($img, true);
    imagesavealpha($img, true);
    // imagefill($im, 0, 0, $transparent);

    imagecopyresampled($im2, $img, ($w - $w2) / 2, ($h - $h2) / 2, 0, 0, $w2, $h2, $w0, $h0);

    $img = $im2;    
}