How do I enable progressive encoding for JPG images?
First of all, what IS progressive rendering/encoding? If you've been around the Internet for any length of time, you've seen two types of image loading: baseline and progressive.
Baseline encoding loads the image one row at a time, from top to bottom. It makes it readily apparent that the loading is in progress, and you often can't tell what the image is until most of it has loaded. Sometimes, browsers won't even show anything until the entire image is loaded, to avoid this sort of incomplete image.
Progressive encoding loads the full image frame rather quickly, but with only some of the pixel data present. The effect of this is that you'll see a blurry image to start with, that will get clearer over time as the image completes.
Now, if you're on a fast enough connection, you never notice the difference. However, if you're using your mobile phone, and a page has a lot of images, it becomes very apparent which time of encoding has been used.
Generally, anyone who knows anything about image optimization will tell you to use progressive encoding for your images, because it makes them appear to load faster, and makes for a more pleasant experience for your visitors and/or customers. With progressive encoding, there is a side bonus in that the images will often look just fine before the image has even completed loading. So that makes it possible for the page to "appear" to have finished loading even when it hasn't yet.
Back on track now, how do you enable it? You don't! EWWW Image Optimizer encodes all images using progressive mode automatically. The first reason is the user experience, as we've described. The second reason is that the size is almost always smaller when using progressive rendering. You get the best of both worlds, better user experience AND faster loading images due to reduced size.
Are there any exceptions? If an image is smaller than 10kb, then it doesn't matter if it uses progressive encoding, it will load too fast to notice. Also, progressive encoding tends to struggle on very small images. Lastly, if you're using lossless compression, and EWWW can't make it smaller, then you're stuck with the image as it was. EWWW IO will never save a JPG image that isn't smaller than the original, not even to get the benefits of progressive encoding. Odds are, if your image cannot be further compressed in lossless mode, it's probably already progressively encoded.