How come ShortPixel tells me that I can optimize an image even more if I reupload it?

You may have noticed that when you upload an image a second time to our Online Image Compressor (i.e., when you upload the already compressed image), it suggests that you optimize it further. This may not make sense at first glance, but here's the explanation.

When our servers optimize an image, we use an algorithm that determines the differences between the original and the optimized image before we deliver it to you. And you will get the optimized image if the difference coefficient is smaller than a certain number, because if the difference is too big, it means that the image was compressed too much.

Now let us imagine the following situation:

• A: Original image
• B: Optimized image 1 --> The compressed image from A.
• C: Optimized image 2 --> The compressed image from B.

The difference between A and B is usually smaller than the difference coefficient, and the difference between A and C is likely to be bigger than the difference coefficient. But the difference between B and C will be smaller, and therefore a little more optimization is suggested.

In short, it all depends on what you are comparing the optimized image to. That's why it's important to optimize only once, and that's why the tests you do with our online compressor after you have already optimized your images are irrelevant.