如何在PHP中获得图像质量

Question

I've been doing some speed optimization on my site using Page Speed and it gives recommendations like so:

Optimizing the following images could reduce their size by 35.3KiB (21% reduction).

Losslessly compressing http://example.com/.../some_image.jpg could save 25.3KiB (55% reduction).

How are the calculations done to get the file size reduction numbers? How can I determine if an image is optimized in PHP?

As I understand it, they seem to base this on the image quality (so saving at 60% in Photoshop or so is considered optimized?).

What I would like to do is once an image is uploaded, check if the image is fully optimized, if not, then optimize it using a PHP image library such as GD or ImageMagick. If I'm right about the number being based on quality, then I will just reduce the quality as needed.

How can I determine if an image is fully optimized in the standards that Page Speed uses?

Answer 1

Chances are they are simply using a standard compression or working on some very simple rules to calculate image compression/quality. It isn't exactly what you were after but what I often use on uploaded images etc etc dynamic content is a class called SimpleImage:

http://www.white-hat-web-design.co.uk/blog/resizing-images-with-php/

this will give you the options to resize & adjust compression, and I think even change image type (by which I mean .jpg to .png or .gif anything you like). I worked in seo and page optimization was a huge part of my Job I generally tried to make the images the size the needed to be no smaller or bigger. compress JS & CSS and that's really all most people need to worry about.