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php优化压缩的图片

Jun 21, 2016 am 08:49 AM
imagepng img null php size

php程序开发中经常涉及到生成缩略图,利用php生成缩略图这个过程本身没难度,但是你知道php能够优化调节生成的缩略图的质量吗?也就是说php能够控制生成缩略图的清晰度以及生成后的缩略图的体积。下面我们就来看看如何利用php优化我们压缩后的图片。

对于如何使用php生成缩略图这里就不介绍了,大家可以参考本站下面这篇文章:

php生成图片缩略图的方法

首先我们来看看用php压缩图片的程序代码:

<?php header('Content-type: image/PNG');
$image=@imagecreatefrompng('http://www.Alixixi.com/images/logo.png');
imagepng($image,'logo.png',0); //注意后面那个数字0,这里即压缩等级,参数范围:0-9*/
imagedestroy($image);
Copy after login

以上 imagepng 函数的第三个参数即是本篇文章所要分析的,这个参数的意思是生成图片的质量等级。这里可以分10个等级(0-9),0等级时即无压缩,图片不会失真,图片最清晰,但图片的体积也最大,随着压缩等级数字不断增大,图片会变得越来越不清晰,但压缩后图片的体积可以减少到原来的50%,压缩比还是挺大的。

下面看一个具体的例子吧,现在一个原图体积是125k,下面是经过不同压缩等级后的测试结果:

imagepng($img,null,0); --> Size = 225K
imagepng($img,null,1); --> Size = 85.9K
imagepng($img,null,2); --> Size = 83.7K
imagepng($img,null,3); --> Size = 80.9K
imagepng($img,null,4); --> Size = 74.6K
imagepng($img,null,5); --> Size = 73.8K
imagepng($img,null,6); --> Size = 73K
imagepng($img,null,7); --> Size = 72.4K
imagepng($img,null,8); --> Size = 71K
imagepng($img,null,9); --> Size = 70.6K

注意以上当压缩等级为0的时候,体积比原图还大的原因是原始图片其实是经过了轻微的压缩,而0等级的压缩就是一点压缩都没有,所以它的体积会大于原始图片。

根据以上结论和实际测试结果,作者得出的结论是在优化图片的时候,一般采用2等级比较合适,图片没有失真,但体积减少了30%,达到优化的目的了。如果采用6、7、8、9等级压缩的话,图片有所失真,而体积减少却不明显。所以不宜采用这些等级进行压缩优化图片。



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