这是我刚刚写的代码。它获取一个图像，将其裁剪，然后创建一个jpg图像。如果我给它一个jpg文件，它可以很好地工作，但不适用于png（透明）图像。它只是制作了一个黑色背景的空白图像。
我是第一次使用GD库，那么我的错误是什么？
public function crop_thumb($file, $width=150, $height=150){
$file_type = get_file_extension($file);
$file_name = get_file_name($file);
$original_image_size = getimagesize(_dir_uploads_.$file);
$original_width = $original_image_size[0];
$original_height = $original_image_size[1];
if($file_type == 'jpg'){
$original_image_gd = imagecreatefromjpeg(_dir_uploads_.$file);
}elseif($file_type == 'gif'){
$original_image_gd = imagecreatefromgif(_dir_uploads_.$file);
}elseif($file_type == 'png'){
$original_image_gd = imagecreatefrompng(_dir_uploads_.$file);
}
$cropped_image_gd = imagecreatetruecolor($width, $height);
$wm = $original_width / $width;
$hm = $original_height / $height;
$h_height = $height / 2;
$w_height = $width / 2;
if($original_width > $original_height ){
$adjusted_width = $original_width / $hm;
$half_width = $adjusted_width / 2;
$int_width = $half_width - $w_height;
imagecopyresampled($cropped_image_gd ,$original_image_gd ,-$int_width,0,0,0, $adjusted_width, $height, $original_width , $original_height );
}elseif( ($original_width < $original_height) || ($original_width == $original_height) ){
$adjusted_height = $original_height / $wm;
$half_height = $adjusted_height / 2;
$int_height = $half_height - $h_height;
imagecopyresampled($cropped_image_gd , $original_image_gd ,0,-$int_height,0,0, $width, $adjusted_height, $original_width , $original_height );
}else{
imagecopyresampled($cropped_image_gd , $original_image_gd ,0,0,0,0, $width, $height, $original_width , $original_height );
}
$create = imagejpeg($cropped_image_gd, _dir_uploads_.$file_name."-".$width."x".$height.".jpg");
return ( $create ) ? true : false;
}
我认为在使用imagecreatetruecolor创建图像后，需要使用imagesavealpha()保存alpha通道。
$cropped_image_gd = imagecreatetruecolor($width, $height);
imagealphablending($cropped_image_gd, false);
imagesavelalpha($cropped_image_gd, true);