PHP怎么把一张图片透明化

PHP怎么把一张图片透明化？

具体问题：

把一张图片（根据指定的RGB颜色范围）透明化。但是实际处理当中，下面的代码值移除了白色，请教是怎么回事？

$o_pic = '1.jpg';
//要处理的色阶起始值
$begin_r = 215;
$begin_g = 215;
$begin_b = 215;
list($src_w,$src_h,$src_type) = getimagesize($o_pic);// 获取原图像信息
$file_ext = get_ext($o_pic);//获取扩展名
$target_im = imagecreatetruecolor($src_w,$src_h);//新图
if($file_ext == 'jpg') //转换JPG 开始
{
    $src_im = ImageCreateFromJPEG($o_pic);
      
    imagecopymerge($target_im,$src_im,0,0,0,0,$src_w,$src_h,100);
    for($x = 0; $x < $src_w; $x++)
    {
        for($y = 0; $y < $src_h; $y++)
        {
            $rgb = imagecolorat($src_im, $x, $y);
            $r = ($rgb >> 16) & 0xFF;
            $g = ($rgb >> 8) & 0xFF;
            $b = $rgb & 0xFF;
            if($r > $begin_r && $g > $begin_g && $b > $begin_b ){   
                imagecolortransparent($target_im, imagecolorallocate($target_im,$r, $g, $b));                
            }
        }
    }
}

方法：

/**
 * Created by PhpStorm.
 * User: shellus
 * Date: 2016-12-01
 * Time: 23:12
 */
require 'vendor/autoload.php';
// import the Intervention Image Manager Class
use Intervention\Image\ImageManager;
// create an image manager instance with favored driver
$manager = new ImageManager(array('driver' => 'gd'));
$img =  $manager->make('1.jpg');
for ($y = 0; $y < $img->height(); $y++)
{
    for ($x = 0; $x < $img->width(); $x++)
    {
        $c = $img -> pickColor($x, $y, 'array');
        if(abs($c[0] - 205) < 50  && abs($c[1] - 223) < 50 && abs($c[2] - 211) < 50 ){
            $c[0] = $c[1] = $c[2] = 255;
            $img -> pixel($c, $x, $y);

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