Why do I have blury images in WordPress

The filter you are using only stops jpeg compression. This does not prevent quality loss when resizing an image. Manually you would apply a sharpen filter afterwards to regain some crispyness in the image. WordPress doesn’t do that.

However, WP does have the WP_Image_Editor class for image manipulation. It has some built-in methods, such as resize and rotate, but no sharpen. However, the class also gives a gateway to PHP’s ImageMagick library, which allows you to sharpen images.

To cut a long story short, someone has already done this for you and made it into a plugin.

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