PHP用GD库生成高质量的缩略图片

<?php
$filename="image.thumb";
// 生成图片的宽度
$resizewidth=400;
// 生成图片的高度
$resizeheight=400;

function resizeimage($im,$maxwidth,$maxheight,$name){
$width = imagesx($im);
$height = imagesy($im);
if(($maxwidth && $width > $maxwidth) || ($maxheight && $height > $maxheight)){
if($maxwidth && $width > $maxwidth){
$widthratio = $maxwidth/$width;
$resizewidth=true;
}
if($maxheight && $height > $maxheight){
$heightratio = $maxheight/$height;
$resizeheight=true;
}
if($resizewidth && $resizeheight){
if($widthratio < $heightratio){
$ratio = $widthratio;
}else{
$ratio = $heightratio;
}
}elseif($resizewidth){
$ratio = $widthratio;
}elseif($resizeheight){
$ratio = $heightratio;
}
$newwidth = $width * $ratio;
$newheight = $height * $ratio;
if(function_exists("imagecopyresampled")){
$newim = imagecreatetruecolor($newwidth, $newheight);
imagecopyresampled($newim, $im, 0, 0, 0, 0, $newwidth, $newheight, $width, $height);
}else{
$newim = imagecreate($newwidth, $newheight);
imagecopyresized($newim, $im, 0, 0, 0, 0, $newwidth, $newheight, $width, $height);
}
imagejpeg ($newim,$name . ".jpg");
imagedestroy ($newim);
}else{
imagejpeg ($im,$name . ".jpg");
}
}

if($_files['image']['size']){
if($_files['image']['type'] == "image/pjpeg"){
$im = imagecreatefromjpeg($_files['image']['tmp_name']);
}elseif($_files['image']['type'] == "image/x-png"){
$im = imagecreatefrompng($_files['image']['tmp_name']);
}elseif($_files['image']['type'] == "image/gif"){
$im = imagecreatefromgif($_files['image']['tmp_name']);
}
if($im){
if(file_exists("$filename.jpg")){
unlink("$filename.jpg");
}
resizeimage($im,$resizewidth,$resizeheight,$filename);
imagedestroy ($im);
}
}
?>

<?php
include('resizeimage.php');
if(!empty($_post)){
echo($filename.".jpg?cache=".rand(0,999999));
}
?>
<form name="test" action="?submit=true" enctype="multipart/form-data" method="post" >
<input type="file" name="image" size="50" value="浏览"><p>
<input type="submit" value="上传图片">
</form>