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What is progressive JPEG and does support it?

Posted by Kraken Community. Last updated on May 31st 2016 (7 years ago)

Normal JPEG images are represented as top-to-bottom scans of an image at full resolution. Incidentally, that is just how they render, from top-to-bottom, so the whole image needs to have been downloaded before it can be viewed in its entirety. JPEG images encoded in this way are called baseline JPEGs.

This rendering behaviour can be disruptive to the User Experience of a website for three reasons:

  • Users with slower connections must watch the image render line-by-line before he or she obtains any value from it, and
  • Even on faster connections, there can be a distracting "blinds" effect of the image being rendered from top to bottom, which might also have a negative impact on how favourably the site is perceived.
  • It's so nineties!

Progressive JPEG is a way of encoding the entire image using successive scans from a lowest quality scan to a full-detail scan of the image. What that means from a User Experience standpoint is that you do not need to wait for the entire image to download - first the low quality scan is rendered, and quickly replaced with scans containing greater detail until the entire image has arrived.

The obvious advantage to that rendering behaviour is that the area or placeholder for that image on a web page is quickly filled up with some sort of representation of that image, allowing the user to already get some sense of what is being visually represented.

To recap, the entire image is shown right away, albeit in a low-quality, pixellated form, which is successively or "progressively" replaced with scans containing incremental detail until the full-detail representation is displayed once the entire file has arrived. encodes all JPEGs uploaded to the service using the progressive standard, as it tends to be viewed as the better way of encoding JPEG images, and we think so too!