如何创建透明画布,然后向其添加透明PNG?

我需要创建一个透明的图像,然后结合透明PNG到它,同时保持图像质量。

我该怎么做?

imagecreatetruecolor(...);
//processing using imagecopymerge(..);
imagepng(...);

outputs a black background.

谢谢)

这是我的实际代码供参考…

        $d = getimagesize(TMP.$this->files[0]);
    $source_height = $d[0];
    $source_width = $d[1];

    $this->canvas = imagecreatetruecolor($source_width*count($this->files),$source_height);

    imagealphablending($this->canvas, false );

    $i=0;
    foreach($this->files as $f){
        $dst_x = $source_width*$i;
        $im = imagecreatefrompng(TMP.$f);
        imagecopyresampled  (  $this->canvas  , $im  ,
               $dst_x  ,
               $dst_y = 0 , 
               $src_x = 0 ,
               $src_y = 0 ,
               $source_width  ,
               $source_height  ,
               $source_width  ,
               $source_height);

        $i++;
        imagepng($im,TMP.$i.".png");
        if($i>3)break;
    }
    $fn = TMP."stiched_up_$i*$source_width.png";
    imagesavealpha($this->canvas,TRUE);
    imagepng($this->canvas,$fn);