Blend transparent gradient with an image using php GD library

Question

I try to render gradient on top of an image it need to be from full color to transparent, here is my code. I get black image and if I put start more then 0 I got white gradient but no image. the output image is 338x100 px but the input image need to be aligned to right if the image is narrower.

function hex2rgb($hex) {
    $rgb[0] = hexdec(substr($hex, 0, 2));
    $rgb[1] = hexdec(substr($hex, 2, 2));
    $rgb[2] = hexdec(substr($hex, 4, 2));
    return $rgb;
}

function int2rgb($color) {
    $result[] = ($color >> 16) & 0xFF;
    $result[] = ($color >> 8) & 0xFF;
    $result[] = $color & 0xFF;
    return $result;
}

if (isset($_GET['start']) && isset($_GET['stop']) && isset($_GET['color'])) {
    $input = imagecreatefrompng('file.png');
    $width = imagesx($input);
    $output = imagecreatetruecolor(338, 100);
    $color = hex2rgb($_GET['color']);
    $fill = imagecolorallocate($output, $color[0], $color[1], $color[2]);


    for ($x=0; $x<$_GET['start']; ++$x) {
        for ($y=0; $y<100; ++$y) {
            imagesetpixel($output, $x, $y, $fill);
        }
    }
    $range = $_GET['stop']-$_GET['start'];
    for ($x=$_GET['start']; $x<$_GET['stop']; ++$x) {
        $alpha = round(255-($x*255/$range));
        $correct_x = $width < 338 ? $x+$width-338 : $x;
        for ($y=0; $y<100; ++$y) {
            $input_color = int2rgb(imagecolorat($input, $correct_x, $y));

            $new_color = imagecolorallocate($output,
                                            (($color[0]-$alpha)*$input_color[0])/255,
                                            (($color[1]-$alpha)*$input_color[1])/255,
                                            (($color[2]-$alpha)*$input_color[2])/255);
            imagesetpixel($output, $x, $y, $new_color);
        }
    }
    if ($_GET['stop']<338) {
        $stop = $width < 338 ? $_GET['stop']+$width-338 : $_GET['stop'];
        imagecopy($input, $output, $stop, 0, $_GET['stop'], 0, 338-$stop, 100);
        header('Content-Type: image/png');
        imagepng($output);
    }
}

I run the script with gradient.php?start=20&stop=200&color=ff0000 and got this instead of red gradient.

How to make that gradient red from full color to full transparent? So it look like this:

Answer 1

If you create a image using imagecreatetruecolor it gets a black background. Using imagefill you can change the background of an image easily. The imagecolorallocatealpha function let's you create a color containing transparency. 127 means fully transparent and 0 not transparent.

It works now and I simplified your code a bit:

function hex2rgb($hex) {
    $rgb[0] = hexdec(substr($hex, 0, 2));
    $rgb[1] = hexdec(substr($hex, 2, 2));
    $rgb[2] = hexdec(substr($hex, 4, 2));
    return $rgb;
}

if (isset($_GET['start']) && isset($_GET['stop']) && isset($_GET['color'])) {
    $color = hex2rgb($_GET['color']);
    $range = $_GET['stop']-$_GET['start'];

    // create input image
    $input = imagecreatefrompng('file.png');


    // create output image
    $height = imagesy($input);
    $width = imagesx($input);
    $output = imagecreatetruecolor($width, $height);

    // put a transparent background on it
    $trans_colour = imagecolorallocatealpha($output, 0, 0, 0, 127);
    imagefill($output, 0, 0, $trans_colour);

    // create the gradient
    for ($x=0; $x < $width; ++$x) {
        $alpha = $x <= $_GET['start'] ? 0 : round(min(($x - $_GET['start'])/$range, 1)*127);
        $new_color = imagecolorallocatealpha($output, $color[0], $color[1], $color[2], $alpha);
        imageline($output, $x, $height, $x, 0, $new_color);
    }

    // copy the gradient onto the input image
    imagecopyresampled($input, $output, 0, 0, 0, 0, $width, $height, $width, $height);

    // output the result
    header('Content-Type: image/png');
    imagepng($input);
}