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After speaking with my project manager about our mobile site, she said she had read somewhere that there is an image format that allows an image to download in stages so it's quality gets better as the image is loading (it's meant to speed up a site somehow).

Does anyone know what she is on about and if this is possible to do using photoshop and a jpg?

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    While you've got an answer to the question you asked, consider discussing with your manager the possibility of an XY problem, where you're actually trying to make the site load faster perceptually. Perhaps you may find that you can get a faster hosting for your mobile version, or optimize all images to make the total page load time lower. Just something to think about. Sep 27 '16 at 19:05
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    @user1306322 I have gone for lazy loading the images so they don't load initially and only load when they come into view. It has sped up the site somewhat
    – Pete
    Sep 28 '16 at 7:40
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Save as a JPEG and select PROGRESSIVE in the save options. You can also select a number of 'scans' to adjust how low the resolution starts off and therefore how gradually the JPEG will get from low resolution to full resolution.

It's worth noting that this is a very old school method and was only really relevant in the days of slow internet connections. Most internet connections are so fast now that you won't even see the scans, just the final full resolution image.

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    @Pete It also makes the images bigger. PNG also can have progressive scans. Theoretically you could have very big images and stop the loading once you have enough. Ive yous never seen any browser that can do this.
    – joojaa
    Sep 27 '16 at 10:17
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    I disagree about the fact it is old-school. Progressive loading decreases perceived download time. Also, slow internet connections is not a thing of the past (shared wifi, subway mobile internet, developing countries).
    – Diti
    Sep 27 '16 at 14:31
  • Fair point @Diti. I'm probably guilty of over generalising there.
    – Westside
    Sep 27 '16 at 14:46
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Or better still create your design at conception with vector images and deploy the icons and interface as a font on the server... assuming you want your site to load quickly on a mobile. If you need to use a bitmap (hero banner, staff photo, gallery), process these as part of your pre-flight for optimal resolution, size and colour before upload. You really should not need to deploy progressive bitmap files at this stage unless there is a clearly defined requirement with the end user. The comment about bandwidth is fair although if you are concerned about web performance for a demographic on modems or < 1MB wireless you shouldn't be deploying full screen backdrops and large images in the first place.

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